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SID 5



Research Project Final Report
Note
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SID 5 (Rev. 3/06)
Project identification
HH3609STX
Partial rootzone drying: delivering water saving and
sustained high quality yield into horticulture
Contractor
organisation(s)
Lancaster University
East Malling Research
Dundee University
54. Total Defra project costs
(agreed fixed price)
5. Project:
Page 1 of 28
£
1,117,553.00
start date ................
01 April 2004
end date .................
31 March 2009
6. It is Defra’s intention to publish this form.
Please confirm your agreement to do so. ................................................................................... YES
NO
(a) When preparing SID 5s contractors should bear in mind that Defra intends that they be made public. They
should be written in a clear and concise manner and represent a full account of the research project
which someone not closely associated with the project can follow.
Defra recognises that in a small minority of cases there may be information, such as intellectual property
or commercially confidential data, used in or generated by the research project, which should not be
disclosed. In these cases, such information should be detailed in a separate annex (not to be published)
so that the SID 5 can be placed in the public domain. Where it is impossible to complete the Final Report
without including references to any sensitive or confidential data, the information should be included and
section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No"
answer.
In all cases, reasons for withholding information must be fully in line with exemptions under the
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Executive Summary
7.
The executive summary must not exceed 2 sides in total of A4 and should be understandable to the
intelligent non-scientist. It should cover the main objectives, methods and findings of the research, together
with any other significant events and options for new work.
Against a background of increasingly limited water resources, there is a need to develop efficient
horticultural production systems in which water and nutrients are utilised effectively in line with
principles of sustainable development. Defra Policy Area WU01 addresses the need to optimise
water use by UK agriculture and food production industries. A major objective of this programme
is to identify opportunities for water saving in agriculture and horticulture. Genetic improvement
programmes aimed at identifying Quantitative Trait Loci (QTL) for improved drought tolerance
will eventually facilitate the marker-assisted selection of new lines with improved water use
efficiency (e.g. HH3608TX, WU0107). However, UK horticulture is unlikely to benefit from these
advances for at least a decade. In the meantime, irrigation management techniques that improve
the efficiency of water use in existing crops grown in areas where water resources are most
threatened are an attractive option.
These techniques, which include Partial Rootzone Drying (PRD), Deficit Irrigation (DI)
and Regulated Deficit Irrigation (RDI), involve applying slightly less water than the plant needs
so that mild soil water deficits develop. Roots exposed to the drying soil produce chemical
signals that are transported to the shoots where they influence both vegetative and reproductive
growth. High profile successes in the vineyards of South Australia prompted many growers
worldwide to try these irrigation management techniques, often with only limited success.
Perhaps not surprisingly, more R&D was needed to optimise PRD under different growing
conditions and in different crops. This is the remit of Defra HH3609TX which aims to deliver
both water savings and improvements to crop quality in several UK horticultural sectors by
optimising PRD technology.
Strategic research on long distance chemical signalling
mechanisms and their interaction with variable climate and rooting medium was instigated to
underpin the development of PRD.
Experiments were conducted with seven major crop species (strawberry, raspberry,
runner bean, potato, tomato, lettuce and poinsettia) over 5 years. Container- and soil-grown
plants were treated with PRD, DI or RDI at various percentages of full irrigation. Full irrigation
was determined on the basis of potential evapotranspiration, actual evapotranspiration (in
container-grown plants) or soil moisture monitoring. Physiological measurements were made
throughout the cropping cycle to assist in irrigation scheduling, and crop yield, quality and shelflife potential assessed at harvest.
Physiological responses (increased water use efficiency [WUE], decreased yield)
SID 5 (Rev. 3/06)
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depended on the severity of the deficit irrigation imposed. In strawberry and raspberry, 20-25%
less irrigation did not decrease marketable yields or berry quality indicating considerable scope
for water savings. More severe deficit irrigation (50% less irrigation than fully irrigated crops)
incurred, on average, a 20% yield penalty but increased WUE by 20% in strawberry, runner
bean and lettuce, and approximately 50% in raspberry, potato and tomato. Generally, there
were no detectable yield differences between PRD, DI or RDI plants supplied with the same
irrigation volumes, although in runner beans PRD out yielded DI crops by over 20% when soilgrown in polytunnels, while in substrate-grown strawberry, RDI plants out yielded PRD plants.
RDI had positive impacts on berry and fruit crops such as increased colour, flavour and
altered chemical (e.g. antioxidant) composition. RDI was also used successfully to limit
poinsettia stem height so that retailers specifications were met, despite a 90% reduction in the
use of environmentally unsustainable chemical growth retardants. RDI also improved poinsettia
quality and shelf-life potential. However, even mild deficit irrigation (20% less irrigation) resulted
in unmarketable crops of lettuce (bitter taste) and potato (unacceptable skin finish).
Deficit irrigation (and especially alternating wet and dry parts of the rootzone in PRD)
stimulated biomass allocation towards the roots (which may limit the severity of leaf water deficit)
and fruits. Limitation of crop photosynthesis likely accounted for the yield and quality penalties
sustained.
Plant long-distance signalling was quantified by measuring leaf water relations, and
collecting xylem sap to assay its chemical constituents. Irrespective of crop or sap sampling
methodology, deficit irrigation increased concentrations of abscisic acid, (ABA - a hormone that
regulates water use by closing leaf pores known as stomata) and decreased concentrations of
cytokinins (hormones that regulate shoot growth).
The substrates in which crops were grown generally didn’t influence yield, but affected
long-distance signalling. Simulating leaf xylem sap ABA concentration in response to a
standardised PRD treatment showed that ABA-mediated restriction of whole plant transpiration
may be more readily achieved in mineral substrates. Alternating wet and dry parts of the
rootzone in PRD in highly organic substrates (e.g. peat) caused transient leaf water deficits as it
was difficult to re-wet the dry substrate.
Theoretical analysis showed that the optimum time to alternate the wet and dry sides was
when the dry side has dried to a level where root ABA output started to diminish. Direct ABA
measurement is impractical thus alternative indirect approaches including measurement of soil
moisture and plant responses were evaluated. Soil moisture sensing offers the advantage of
ready automation while plant-based measurements can be time-consuming: practicalities (ease
of grower use; equipment expense or availability) are likely to dominate the choice of an
appropriate scheduling technique.
Project progress, the development of new proposals and plans for technology transfer
were discussed regularly with our industry advisory group to help ensure that our research
continued to meet the needs of the industry. Several trials were conducted on commercial
holdings to facilitate direct technology transfer. Consortium members made numerous
presentations at grower days and contributed articles to the trade press. 35 scientific papers
have already been published (see Section 9) and additional manuscripts are currently being
prepared for submission to international scientific journals. Strong industry support for our work
has meant that a HortLINK project (HL0187) and an HDC-funded project (SF 107) have been
developed directly as a result of this Defra project. Further development of PRD and RDI in UK
horticulture is being delivered via Defra projects WU0110 and WU0118, and the EU-funded
SIRRIMED project (commencing October 2009). Other project proposals are currently being
prepared.
Project Report to Defra
8.
As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with
details of the outputs of the research project for internal purposes; to meet the terms of the contract; and
to allow Defra to publish details of the outputs to meet Environmental Information Regulation or
Freedom of Information obligations. This short report to Defra does not preclude contractors from also
seeking to publish a full, formal scientific report/paper in an appropriate scientific or other
journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms.
The report to Defra should include:
 the scientific objectives as set out in the contract;
SID 5 (Rev. 3/06)
Page 3 of 28






the extent to which the objectives set out in the contract have been met;
details of methods used and the results obtained, including statistical analysis (if appropriate);
a discussion of the results and their reliability;
the main implications of the findings;
possible future work; and
any action resulting from the research (e.g. IP, Knowledge Transfer).
SID 5 (Rev. 3/06)
Page 4 of 28
Introduction
Approximately 70% of world-wide water use is committed to agriculture. Despite this, water shortages
limit food production in many regions. If crop production is to be sustained and even increased in a
changing environment then water must be used more efficiently. In the UK, irrigation is required in most
regions in most years to ensure reasonable crop yields of the required quality. The recent Defra study
on water use in agriculture (WU0102) suggests that climate change will further increase demand for
irrigation in these areas by about 20% by 2020 and 30% by 2050. Consequently, UK growers, Defra
and the Environment Agency are becoming increasingly concerned about the future availability of
abstracted water for irrigation. New legislation designed to safeguard these resources (The Water Act
2003) will limit future water use and growers will have to demonstrate efficient use of available water
before time-limited abstraction licences are renewed. Mains supplies will also be limited and expensive.
These unavoidable issues have recently focussed attention on irrigation techniques that allow
more efficient use of water. Increasingly, higher value horticultural crops are grown in the UK under
protected cropping. This increases yields and quality but also provides opportunities both to save water
and nutrient resources and to modify supplies of these variables to regulate growth and development.
This ‘natural’ growth regulation has the potential to replace the widespread use of growth regulating
chemicals, an expensive, undesirable and increasingly unsustainable component of crop production.
Partial Rootzone Drying (PRD) is a deficit irrigation technique designed to enhance crop water
use efficiency (WUE) by exploiting the plant’s long-distance signalling mechanisms that modify plant
growth, development and functioning as the soil dries. The novel science behind these mechanisms
has been revealed largely by the members of this consortium (Davies and Zhang 1991; Davies et al.
2005; Dodd 2005, Dodd and Beveridge 2006; Jia and Davies 2007). Exploitation of this science has
been actively pursued by the South Australian wine industry and an EU-funded consortium
(IRRISPLIT) co-ordinated by Lancaster. Although these projects have delivered substantial water
savings and significantly added value in terms of increased yield quality, labour saving and crop
scheduling in Mediterranean (climatic) environments, when this project was conceived there had been
little development of PRD (or deficit irrigation more generally) in the UK, representing a potential
“missed opportunity” for UK growers, retailers and consumers.
The main project objective was to develop the potential to deliver substantial savings of
irrigation water in different horticultural sectors while maintaining or improving crop quality by optimising
PRD technology. While many PRD trials have delivered very positive results, this is not always the
case and we sought to understand why this happens. Since crop responses to deficit irrigation are a
cumulative response to dynamic changes in plant long-distance signalling, much more information is
needed on the nature and origin of these signals and how they impact on plants grown under different
environmental conditions and rooting media. Furthermore, signalling impacts that may be desirable in
some crops (e.g. limitation of vegetative growth in over-vigorous grape vines may save pruning costs)
may be undesirable in others (e.g. lettuce where vegetative growth is essential) thus it seems likely that
different regimes / approaches need to be developed for different crops under different production
systems. A key aspect on which the success (or otherwise) of deficit irrigation may be judged is
consumer perceptions of product quality: increased quality may be highly sought after (as evidence by
consumer willingness to pay a premium) in certain crops (e.g. strawberry) yet attract little market share
in others (e.g. lettuce). For these reasons, the responses of a wide variety of crops (strawberry,
raspberry, runner bean, potato, tomato, lettuce, poinsettia) to deficit irrigation were investigated.
Scientific objectives
1. To quantify potential water saving under deficit irrigation and the impact of these treatments on
yield and WUE
2. To determine and quantify signalling mechanisms under deficit irrigation
3. To determine the effects of PRD on resource partitioning and root and leaf functioning
4. To determine the environmental conditions and substrates that maximise the benefits of PRD
5. To determine the optimal scheduling of deficit irrigation in the field
6. To determine the 'quality' of deficit-grown produce
7. To ensure effective technology transfer
Progress in relation to stated objectives
Objective 1 was fully met. Experiments with a number of crops showed it was possible to design
appropriate deficit irrigation strategies to save water without compromising yield. Generally, there were
no detectable yield differences between PRD and DI plants, although RDI-grown strawberries
outyielded PRD-grown strawberries while PRD-grown runner beans outyielded DI-grown runner beans.
SID 5 (Rev. 3/06)
Page 5 of 28
Published outputs: See Reference 5 in Section 9
A compilation paper from the entire consortium is in preparation
Objective 2 was fully met. Various hydraulic (leaf water potential -leaf) and chemical signals (xylem
sap pH, and the plant hormones abscisic acid (ABA), indole-3-acetic acid (IAA) and various cytokinins
(CKs)) were quantified. Experiments with poinsettia and strawberry showed that deficit irrigation
increased delivery rates of ABA, but decreased delivery rates of CKs, from the root system. PRD
experiments showed similar changes in hormone concentrations in xylem sap collected from tomato
leaves, and that alternating the wet and dry parts of the root system increased xylem sap ABA
concentration. Modelling leaf xylem sap ABA concentration as a function of ABA contributions from the
wet and dry parts of the root system showed that total soil water availability determined whether DI or
PRD elicited a higher ABA concentration.
Published outputs: See References 9, 17, 19, 20, 21 in Section 9
Objective 3 was fully met. Deficit irrigated crops partitioned more of their resources into the root
system, negatively impacting on yield in lettuce but with no apparent effect in tomato (presumably since
fruit growth was maintained at the expense of vegetative growth).
Published outputs: See Reference 4 in Section 9
Objective 4 was fully met. Extensive data sets in several crops showed that stomatal responses to PRD
were more pronounced at high atmospheric vapour pressure deficit. Substrate did not affect yield or
stomatal responses to PRD in runner bean, but had different effects on raspberry yield in two
consecutive years. Each substrate generated unique relationships between soil matric potential and the
fraction of sap flow from roots of PRD plants in drying soil, and root ABA concentration. Incorporation of
these relationships into a model showed that the increase in shoot xylem ABA concentration in
response to a defined PRD treatment varied with substrate.
Published outputs: See References 27, 28, 33, 34 in Section 9
Objective 5 was fully met. Adoption of automatic soil moisture based irrigation scheduling in RDI-grown
poinsettia successfully produced high quality plants while obviating the need for undesirable sprays of
chemical growth retardants. Various methods of plant-based irrigation scheduling (leaf water potential,
porometric and thermal camera measurements of stomatal conductance) were compared in a number
of crops. Empirical assessment of the effects of different frequencies of alternating wet and dry sides of
PRD plants was made on runner bean stomatal responses and yield.
Published outputs: See References 7, 8, 24, 25, 26, 30, 31, 32 in Section 9
Objective 6 was fully met. Detailed studies of crop quality were made in experiments with strawberry
(including biochemical analysis of flavour volatile production and antioxidant concentrations) and
poinsettia (including plant height specifications and post-production quality and shelf-life tests). Visual
assessments of crop quality were made in potato (tuber size distribution and skin quality), runner bean
(pod curling assessments) and lettuce (colour and taste).
Published outputs: See Reference 23 in Section 9
Objective 7 was fully met. Throughout this project, we held several Advisory Group meetings with
industry representatives working with all crop species. Several of our trials were conducted on
commercial holdings. A number of follow-on studies (funded by Defra, HortLINK and HDC) have been
developed from this project. A full list of technology transfer activities is included as Appendix 1.
Methodology
Conventionally, irrigation scheduling has aimed to meet full crop evapotranspiration (ET), determined
from direct measurements or micrometeorology-based estimations of ET, since the relationship
between ET and crop yield is near-linear at suboptimal water supply. In contrast, deficit irrigation (DI)
applies less water than full crop ET. This project compared two forms of DI, which varied the placement
SID 5 (Rev. 3/06)
Page 6 of 28
of irrigation within the rootzone. In conventional DI, less water was applied to the entire rootzone. In
PRD, only one side of the crop row (or one soil compartment in containerised plants) was irrigated at a
time and the other was allowed to dry the soil. Maintenance of the same wet and dry parts of the
rootzone throughout the growing season (fixed PRD) contrasts with studies where the wet and dry
parts of the rootzone are regularly alternated (alternate PRD). Unless stated otherwise, all mentions of
PRD in this report relate to alternate PRD, although the duration of switching between wet and dry
sides varied with the different crops and production systems.
An alternative irrigation management technique is Regulated Deficit Irrigation (RDI), which aims
to exploit the differential sensitivity of yield-determining processes to water deficits. RDI has been used
extensively in tree crops and vines, where water deficits have been imposed at a particular stage(s) of
crop development, rather than throughout the growing season. When crop phenology is sensitive to
water deficit (e.g. during fruit cell division), 100% ET is supplied but when crop phenology is less
sensitive to water deficit (e.g. during vegetative growth prior to fruit development and late fruit
expansion) less irrigation is supplied. Since many of our experiments compared irrigation placement
(PRD versus DI) rather than timing, the term RDI is used in this report only in relation to work with
strawberry and poinsettia.
Experiments were conducted with seven major crop species grown with conventional deficit
irrigation and PRD, with various percentages of full irrigation. Several approaches were adopted by the
three partner institutions (Dundee, East Malling Research [EMR], Lancaster) to determine full irrigation:
- potential evapotranspiration (ETp) measured using a Skye Evaposensor (EMR)
- direct weighing of pot-grown plants to determine actual evapotranspiration (Lancaster and
EMR)
- soil moisture monitoring to maintain soil moisture at ≥ 95% of field capacity (Dundee) or at pot
capacity (EMR)
- automatic scheduling according to daily radiation load (Lancaster) or substrate moisture content
EMR)
Various physiological measurements (leaf elongation to assay vegetative growth, stomatal
conductance to determine restriction of water loss, pre-dawn and midday leaf water potential to
determine plant water status, soil water status) were made to determine effects of irrigation treatments
during crop growth. Assessments of yield and quality parameters (see Objective 6) relevant to the
different crops were made at harvest / market date.
Although experiments were replicated over multiple years (a minimum of 2 years for each
species), substrates (3 different substrates for containerised raspberry and runner beans) and varieties
(June bearers and ever-bearers in strawberry; a traditional variety and an F1 hybrid in tomato), each
crop was trialled under conditions approaching those used commercially by only one partner. Similarly,
each partner used a different type of irrigation scheduling (as discussed above) thus it is not possible to
separate scheduling from species effects. However, adequate replication within each experiment
allows great confidence within the limitations of each experimental system.
A more detailed description of the methodology adopted for each crop can be found in
Appendix 2
Results
Objective 1
To quantify potential water saving under deficit irrigation and the impact of these
treatments on yield and WUE
Introduction
Following the high profile successes in Southern Australia with grapevine (Dry et al., 1996), PRD has
been trialled in many countries and has been shown to improve water use efficiency (WUE) in many
crops in both in glasshouse studies and field trials. A meta-analysis of these data will shortly be
published (Dodd 2009). In 2004, Defra commissioned this research project to assess the potential of
PRD to deliver water saving into several UK horticulture sectors. The potential benefits associated with
PRD were compared with those of another widely used irrigation management technique, Regulated
Deficit Irrigation (RDI) to determine the most appropriate water saving strategy for the seven different
horticultural sectors.
In assessing the yield responses of plants to deficit irrigation generally, or PRD in particular, two
questions were framed: (1) What is the relationship between yield and irrigation volume ? (this
determines the potential water savings and/or yield penalties – Figure 1.1-1.3) (2) Does PRD
differentially influence yield compared to DI or RDI ? (Figure 1.4). For six of the seven crops assessed
(it was not appropriate to quantify yield in poinsettia as quality attributes are key to market value – see
SID 5 (Rev. 3/06)
Page 7 of 28
Objective 6), relative yield data were summarised (Figure 1.1-1.3) to gain an overview of the responses
of a range of crops.
Results and Discussion
The yield penalty sustained was dependent on the severity of deficit irrigation imposed (see Figure
1.1). For example, in container-grown lettuce, decreasing irrigation by 15% and 30% significantly
decreased shoot fresh weight (=yield) by 13% and 29% respectively. Such close correspondence
between yield and crop evapotranspiration suggests that very little water can be saved without
incurring a yield penalty. For the container-grown strawberry crop in 2004, decreasing irrigation (PRD
and RDI data combined) by 20% and 40% decreased class I berry yield by 16% and 38% respectively,
even though the yield reductions were not statistically significant. In contrast, for the strawberry crops
grown in 2005 and 2006, the link between yield and crop evapotranspiration was not so apparent, with
a 40% decrease in irrigation incurring a 25% yield penalty (Figure 1.1). Of more importance was the
finding that a 20% decrease in irrigation only incurred a significant yield penalty in one (2005) of three
years, suggesting a potential water saving under deficit irrigation with minimal impact on yield; this work
is discussed further below. Crops grown with unrestricted root systems in polytunnels (raspberries in
2005 and 2006, potatoes in 2005) suffered no significant yield penalty despite a 25% decrease in
irrigation. Thus, in several crops a potential water saving of 20-25% can be made without significant
negative impacts on crop yield, with maintenance of crop quality in some (raspberry, strawberry) but
not all (potato) crops – see Objective 6. There does not seem to be any potential for water saving in
lettuce.
Since the original concept of PRD envisaged watering only half the rootzone at each irrigation,
several projects including ours (e.g. IRRISPLIT, the South Australian winegrape project) have imposed
a 50% decrease in irrigation in some trials. Even though the analysis above indicates this will likely
incur some yield penalty, it was informative to compare results obtained under UK conditions.
Decreasing irrigation by 50% decreased yield (relative to fully irrigated crops) by an average of 20%
(Figure 1.2). A notable exception was lettuce, where leaf growth (responsible for the harvested crop
portion) was especially sensitive to soil drying,
1.0
Strawberry 2004
Strawberry 2005
Strawberry 2006
Lettuce 2004
1.0
containers
unrestricted
0.9
Yield ratio (Deficit / Full irrgation)
Yield Ratio (Deficit / Full Irrigation)
1.1
0.9
0.8
0.7
0.8
0.7
*
*
*
*
*
0.6
0.5
0.4
0.3
0.2
0.1
0.6
0.0
50
60
70
80
90
100
110
120
130
Irrigation (% of crop evapotranspiration)
2
3
4
5
6
Crop
Figure 1.1. Crop yield ratio of deficit irrigation to full
irrigation (a ratio of 1 indicates that yield with both
techniques is equivalent) for different crops irrigated
with various percentages of full irrigation. Each point
is the mean response of 16 (lettuce) and 12
(strawberry) plants. Significant (P < 0.05) differences
between deficit irrigation and full irrigation are
arrowed. The dotted line indicates the 1:1 relationship
for irrigation percentages < 100%, and no yield
limitation at percentages > 100%.
SID 5 (Rev. 3/06)
1
Figure 1.2. Crop yield ratio of deficit irrigation (mean
of PRD and DI) to full irrigation (a ratio of 1 indicates
that yield with both techniques is equivalent). The
fraction of full irrigation was 50% except for crops 1
(60%), 3-unrestricted (66%) and 6 (75%). Columns
are means  SE of the number of experiments /
seasons given in parentheses at the base of each
column. Shaded columns denote pot experiments
where the root system was unrestricted. Significant (P
< 0.05) differences between deficit irrigation and full
irrigation are indicated with an asterisk (*). Studies are
numbered thus: (1) Strawberry - Fragaria ananassa
cv. Elsanta (2) Raspberry - Rubus idaeus cv. Glen
Ample (3) Runner Bean - Phaseolus coccineus cv.
Emergo (4) Potato - Solanum tuberosum cv. Maris
Page 8 of 28
Piper (5) Tomato - Solanum lycopersicum cv. Ailsa
Craig (6) Lettuce - Lactuca sativa cv. Rex.
such that a 29% yield penalty was incurred when 30% less irrigation was applied. In crops that
were grown with both restricted and unrestricted root systems (raspberry and runner bean), there was
apparently greater scope for saving water in crops grown with unrestricted root systems. When 50%
less irrigation was applied, the yield penalty incurred was less for crops grown with unrestricted, rather
than restricted, root systems (Figure 1.2), presumably since crops with unrestricted root systems could
access water deeper in the soil profile, thus decreasing their reliance on surface-supplied irrigation.
Water use efficiency can be defined as the ratio of crop yield to irrigation applied (applied water
use efficiency) or water used (intrinsic water use efficiency). In our experiments, the latter could only be
determined in containerised crops that were weighed directly. Thus applied water use efficiency was
calculated, and was increased by deficit irrigation in all crops (Figure 1.3). Although the magnitude of
the increase was roughly 20% in strawberry, runner bean and lettuce, it was approximately 50% in
raspberry, potato and tomato. In runner beans, the increase in WUE was independent of whether the
root systems were constrained or not; but in raspberries grown in unconstrained soil, the WUE almost
doubled for the deficit irrigation treatments. While increased WUE was achieved in all crops, these
gains must be tempered against yield losses in some crops (cf. Figure 1.2)
Generally, there were no detectable yield differences between PRD and DI plants (Figure 1.4).
There were two exceptions: yield of PRD-grown strawberries was 12% less than RDI-grown
strawberries while PRD-grown runner beans outyielded DI-grown runner beans by over 20% when
grown in polytunnels with unrestricted roots.
1.5
containers
unrestricted
1.4
containers
unrestricted
1.9
Yield ratio (PRD / Deficit irrgation)
Water use efficiency ratio (Deficit / Full irrgation)
2.0
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.3
*
1.2
1.1
1.0
0.9
*
0.8
0.7
0.6
1.1
0.5
1
1.0
1
2
3
4
5
6
2
3
4
5
6
Crop
Crop
Figure 1.3. Water use efficiency ratio of deficit irrigation
(mean of PRD and DI) to full irrigation (a ratio of 1
indicates that water use efficiency with both techniques
is equivalent). The fraction of full irrigation was 50%
except for crops 1 (60%), 3-unrestricted (66%) and 6
(75%). Columns are means  SE of the number of
experiments / seasons given in parentheses at the
base of each column. Shaded columns denote pot
experiments where the root system was unrestricted.
Studies are numbered thus: (1) Strawberry - Fragaria
ananassa cv. Elsanta (2) Raspberry - Rubus idaeus cv.
Glen Ample (3) Runner Bean - Phaseolus coccineus
cv. Emergo (4) Potato - Solanum tuberosum cv. Maris
Piper (5) Tomato - Solanum lycopersicum cv. Ailsa
Craig (6) Lettuce - Lactuca sativa cv. Rex.
Figure 1.4: Crop yield ratio (and WUE since both
techniques received the same irrigation volumes) of
PRD to DI. A ratio of 1 indicates that yield or WUE
with both techniques is equivalent. The fraction of full
irrigation was 50% except for crops 1 (60%), 3unrestricted (66%) and 6 (75%). Columns are means
 SE of the number of experiments / seasons given in
parentheses at the base of each column. Shaded
columns denote pot experiments where the root
system was unrestricted. Significant (P < 0.05)
differences between deficit irrigation and full irrigation
are indicated with an asterisk (*). Studies are
numbered thus: (1) Strawberry - Fragaria ananassa
cv. Elsanta (2) Raspberry - Rubus idaeus cv. Glen
Ample (3) Runner Bean - Phaseolus coccineus cv.
Emergo (4) Potato - Solanum tuberosum cv. Maris
Piper (5) Tomato - Solanum lycopersicum cv. Ailsa
Craig (6) Lettuce - Lactuca sativa cv. Rex.
PRD versus RDI - Strawberry (EMR)
Although it is often possible to improve WUE with deficit regimes, in crops like strawberry marketable
yields must be maintained if the techniques are to be commercially viable. In this type of work, the
most relevant expression of WUE is the weight of class 1 fruit produced per unit volume of irrigation
water applied. Perhaps not surprisingly, the success of deficit regimes can vary (Pudney and
SID 5 (Rev. 3/06)
Page 9 of 28
McCarthy, 2004); in studies on strawberry, deficit irrigation reduced fruit size and number, resulting in
losses of marketable yield (Kirnak et al., 2003; Yuan et al., 2004). We conducted polytunnel
experiments with the ‘June-bearer’ ‘Elsanta’ and the ever bearer ‘Flamenco’ to determine if PRD or
RDI regimes could be developed that delivered substantial water savings while maintaining or
improving yields and quality of class 1 fruit.
Results
Irrigation scheduling and water savings
A novel method of scheduling irrigation to substrate-grown strawberries was developed in this project.
Daily plant water use was estimated using an Evaposensor (Figure 1.5) and Evapometer and cultivarspecific calibration factors were calculated weekly to account for increases in canopy area over each
season. Using this system, run-off from well-watered plants following irrigation events was avoided
because plant demand for water was matched exactly with supply. Deficit irrigation treatments were
applied as a percentage of potential evapotranspiration (ETp) at 50% full bloom in each season to try to
ensure that berry numbers were not reduced in deficit-grown plants.
Cumulative irrigation volumes applied to each plant under the seven regimes were calculated
for both 60-day and main season plants and compared to current industry ‘best practice’
recommendations (Figure 1.6). Substantial water savings were achieved by irrigation scheduling, a
total of 12 L water per plant over both seasons. This would equate to a saving of 700 m 3 of water per
hectare of substrate-grown strawberries in commercial production. Further water savings were
delivered by the different severities of the PRD and RDI regimes; in the most severe regimes (60%
ETp) only 57% of the volume of water that would have been used in a commercial crop was applied.
Yields of class 1 fruit
Yields of class 1 fruit from well-watered plants receiving 120% of daily ETp averaged 230 g and 500 g
per plant in 60-day and main season, respectively (Figure 1.7). Yields were significantly reduced by
the PRD 80% and 60% regimes and by the RDI 60% regime in the first year but there were no
significant treatment differences in the main cropping year. Over three seasons; marketable yields were
20
200
*
150
Avergae yield of class 1 fruit per plant (g)
Cumulative irrigation volume (L)
A) 60-day crop
Commercia regime
Well-watered 120%
PRD & RDI 100%
PRD & RDI 80%
PRD & RDI 60%
30
60-day season 2006
10
0
0
20
40
60
Main season 2007
40
30
20
*
*
100
50
0
B) main season crop
B) main season crop
600
400
200
10
0
0
0
20
40
60
WW
80
Days after treatments applied
PRD
100
PRD
80
PRD
60
RDI
100
RDI
80
RDI
60
Irrigation regime
Figure 1.6. Irrigation volumes applied to 60-day and
to main season plants (WW = 120%; PRD and RDI
regimes of 100%, 80%, 60% of ET p). The solid line
represents the current recommended value for
substrate-grown strawberries.
Figure 1.7. Effects of irrigation regimes on yields of
class 1 fruit per plant in A) 60-day and B) main season
plants. Results are means of 12 replicate plants;
asterisks indicate statistically significant differences (P
≤ 0.05) from WW values.
consistently reduced by the most severe PRD and RDI regimes (60 % ETp) but yield penalties were
also incurred with 100% and 80% PRD regimes during the hot summer in 2006. Reductions in yield
SID 5 (Rev. 3/06)
Page 10 of 28
were always due to treatment effects on berry size, not berry number.
Water use efficiency
Water use efficiency can be expressed as the yield (mass) of class 1 fruit produced per unit volume of
irrigation water and a higher value indicates a greater WUE. The irrigation volumes used to produce
the yields of class 1 fruit recorded for 60-day (2006) and main season (2007) were combined to give an
estimate of the overall WUE over both cropping seasons (Table 1.1). The 80% and 60%.PRD and RDI
regimes improved WUE significantly, compared to WW values, in both cropping seasons. However, as
noted above, yields were reduced by some of these treatments. Values of WUE for the conservative
120% ETp WW regime were similar to those calculated for a typical commercial crop.
Discussion
Current guidelines for irrigating
Table 1.1: Water use efficiency (kg class 1 fruit produced per cubic
substrate-grown
strawberries
metre of irrigation water applied) for ‘Elsanta’ strawberry plants
recommend volumes of between 0.5
under deficit irrigation regimes for two seasons. Estimated values
and 0.7 L of water per plant per day
for current commercial practice in substrate-grown strawberry
production are presented for comparison.
(ADAS, 2003). Values of WUE for
WW plants were similar to those
WUE (kg class 1 fruit per m3 H2O)
Irrigation regime
estimated for a typical commercial
(% of ETp)
60-day
Main season Two seasons
crop since we used a rather
conservative approach in which we
WW 120
5.5
11.1
8.4
supplied 120% of estimated daily ETp
PRD 100
5.8
12.3
9.1
to account for leaks, run-off and
6.2
17
11.7
PRD 80
evaporation from the substrate
7.3
25.4
16.5
PRD 60
surface. We have since shown in
RDI 100
5.8
12.3
9.1
WU0110 that 100% of ETp is
7.5
20.3
14.0
RDI 80
adequate to maintain yields of WW
8.4
24.7
16.7
RDI 60
plants and that effective scheduling
SED
0.41
2.27
1.35
can improve WUE by a further 25%
Commercial crop*
3.4-4.8
10.7-13.6
7.1 - 9.2
(WU0110 SID 4 2009).
* Assuming average class 1 yields of 250 g and 550 g per plant and daily
We have shown that if RDI is
irrigation volumes of 500 ml and 700 ml per plant in 60-day and main season
plants, respectively
applied judiciously, substantial water
savings can be made, without
compromising yields of class 1 fruit. WUE, expressed in terms of kg fruit produced per cubic metre of
water applied, was increased from 8.4 in well-watered plants to 14 in RDI-treated plants (Table 1.1).
RDI also improved several components of fruit quality (see Objective 6). The potential to use irrigation
scheduling and RDI to improve WUE, nutrient use efficiency and fruit quality in substrate-grown
strawberry production will be developed further in our new HDC-funded project (SF 107).
Although increases in WUE were obtained with PRD, compared to well-watered values,
reductions in yield of class 1 fruit occurred with most of the PRD regimes tested. Our attempts to
overcome this shortcoming by modifying the ways in which the irrigation was switched were
unsuccessful. The loss in productivity was presumably due to plant water deficits that developed
during the irrigation switching events inherent in all PRD regimes. This problem is likely to be
exacerbated in hot weather and therefore PRD may be a risky strategy to use in commercial
strawberry production. However, loss of turgor during switching may only occur in containerised plants
where root growth, and therefore access to water, is restricted. We are currently testing the potential
of PRD to deliver water savings while maintaining marketable yields and improving fruit quality and
shelf-life potential in field-grown strawberries (HL0187).
Our preliminary results also suggested that if PRD and RDI regimes applied to 60-day plants
were continued during the flower initiation phase (Sept-Oct), the number of trusses produced in the
second cropping year (2006) was increased. Despite the greater number of fruit, berry size was not
reduced and so marketable yields were increased by up to 40% in the second cropping year. Although
the increases in yields were not always statistically significant (e.g. Figure 1.7B), our data suggest that
deficit irrigation offers the potential not only to improve WUE and berry quality, but also to increase
yields of class 1 fruit. Clearly, this result has very important economic implications and, to our
knowledge, has not been reported before. This aspect of the work is currently being progressed in
WU0110.
We also conducted experiments with the ever-bearer ‘Flamenco’ over two cropping seasons
(HH3609TX SID 4 2005-2006). Our results suggested that although vegetative growth could be limited
SID 5 (Rev. 3/06)
Page 11 of 28
by both PRD and RDI, yields of class 1 fruit were reduced by both techniques. This may be due to the
detrimental effects of the deficit regimes on flower initiation which occurs continuously throughout the
season in ever-bearers. More applied work is needed to determine whether irrigation scheduling and
deficit regimes can be applied during specific stages of development to restrict vegetative growth
without compromising fruit size or number. We are currently developing such a concept note for
submission to the HDC Soft Fruit panel later this year.
Objective 2 To determine and quantify signalling mechanisms under deficit irrigation
Introduction
Partial Rootzone Drying is a deficit irrigation technique designed to enhance crop WUE by exploiting
the plant’s long-distance signalling mechanisms that modify plant growth, development and functioning
as the soil dries. It is an adaptation of laboratory split-root experiments (Blackman and Davies 1985;
Gowing et al. 1990; Sobeih et al. 2004) that utilises plant root-to-shoot chemical signals to influence
shoot physiology. It can be operated in drip- or furrow-irrigated crops where each side of the row is
watered independently. When the crop is irrigated, only one side of the row receives water and the
other is allowed to dry the soil. The root system senses soil drying and produces chemical signals that
are transmitted to the shoots to close the stomata (decreasing water loss) and limit vegetative growth,
thus improving WUE (Gowing et al. 1990; Dry et al. 1996; Davies et al. 2000). Although there is
evidence that the plant hormone ABA is one of the components involved in the control of stomatal
conductance as the soil dries (reviewed in Davies and Zhang 1991; Dodd 2005; Dodd et al. 2008a),
more information is needed on how PRD affects the outputs of root-derived signals (in integrating
chemical information from both wet and dry parts of the rootzone) and whether other chemical signals
fluctuate during deficit irrigation treatments.
-2
-1
Stomatal conductance (mmol H20 m s )
Experiments on Strawberry (EMR)
We have shown that deficit regimes can be used to limit transpirational water loss, slow leaf extension
and reduce canopy area; other benefits include improved flavour volatile production and enhanced
bioactive content (see Objective 6). These beneficial effects are thought to be controlled by chemical
signals (e.g. plant hormones) synthesised in response to drying soil (Davies et al., 2000). However, the
signals produced in response to deficit irrigation that regulate shoot responses in strawberry or any
other member of the Rosaceae are not known, although ABA is presumed to be involved (Stoll et al.,
2000; Dodd et al. 2006)
The aim of this work was to identify the signals in strawberry that initiate leaf responses to help
minimise water loss from leaves of deficit-grown plants. To help establish cause from effect, it was
important to be able to detect any minor changes in hormone flux from the roots immediately before the
leaf responses were initiated. The precise control
over environmental conditions afforded by the
WW
GroDome enabled us to predict when the leaf
PRD 80% ETp80
responses to PRD and RDI first occurred and we were
RDI 80% ETp
600
able to target sap collection times accordingly. The
*
hormone profile in these many sap samples was then
*
*
400
established using definitive quantification by GC-MS*
*
SIM. Hormone delivery rates were calculated by
multiplying concentrations by sap flow rates to ensure
200
that any changes in the output of root-sourced
chemical signals were not simply artefacts of reduced
0
transpirational water flow (Else et al., 1995).
0
1
2
3
4
5
6
Transpiration bioassays were used to try to establish
Days after start of treatment
whether changes in hormone output were
Figure 2.1. Effects of PRD and RDI on stomatal
physiologically significant.
conductances measured on a fully expanded
strawberry leaf in a GroDome experiment. Results are
means of six replicate plants; asterisks indicate
statistically significant differences (P ≤ 0.05).
Results
Transpiration rates and stomatal conductances of
PRD- and RDI-treated plants began to diverge from
WW values three days after the beginning of the experiment and were significantly lower by the
afternoon of day 4 (Figure 2.1). Leaf elongation rate (LER) was not affected by the deficit regimes in
SID 5 (Rev. 3/06)
Page 12 of 28
-1
Xylem sap hormone delivery rate (ng h )
these short-term experiments.
Leaf water
pH
ABA
potentials remained unchanged in the PRD- and
50
7.2
RDI-treated plants suggesting that stomatal
40
7.0
closure was regulated by chemical rather than
hydraulic signals. ABA delivery rates from PRD30
6.8
and RDI-treated roots increased as the
20
substrate dried (Figure 2.2) and were
6.6
significantly higher in RDI-treated plants on days
10
6.4
4 and 6; similar increases were detected in
PRD-treated plants although the results were
0
6.2
0.1
0.2
0.3
0.4
0.5
0.1
0.2
0.3
0.4
0.5
not statistically significant due to variability
between plants. No consistent effects on xylem
Z
ZR
WW
PRD 80% ETp
300
sap pH were detected over the six-day
RDI 80% ETp
30
experiment but the deliveries of zeatin (Z) and
zeatin riboside (ZR) decreased as the substrate
200
dried (Figure 2.2) and were significantly lower in
20
PRD-treated plants on day 4.
Export of
dihydrozeatin and dihydrozeatin riboside were
100
10
low and were not affected by deficit regimes.
The output of indole-3-acetic acid (IAA) from
PRD- and RDI-treated roots was reduced within
0
0
0.1
0.2
0.3
0.4
0.5
0.1
0.2
0.3
0.4
0.5
two days and remained lower than WW values
Substrate moisture content (m3m-3)
thereafter.
To test the causal status of the
increased ABA delivery, the concentrations of Figure 2.2. Effects of substrate drying during PRD and RDI
regimes on the delivery of abscisic acid (ABA), pH, zeatin
ABA detected in xylem sap from WW, PRD- and riboside (ZR) and zeatin (Z) from pressurised detopped
RDI-treated plants were used in bioassays strawberry root systems. Delivery rates were calculated from
where detached leaves were fed with a range of multiplying sap flow rates by hormone concentrations
synthetic ABA solutions. These tests suggested quantified by GC-MS-SIM. The horizontal dashed line in
that the changes in ABA output from PRD- or each figure indicates the deliveries at which significant
RDI-treated roots were not sufficient to initiate reductions in stomatal conductances first occurred.
stomatal closure (Figure 2.3). Although this may indicate that other signals are involved, differences in
the sensitivity of the transpiration bioassay to ABA compared to intact leaves may also be a factor.
-2 -1
Transpiration rate (mmol H2O m s )
Discussion
The changes in root-sourced chemical signals
produced in response to deficit irrigation have been
2.0
quantified in strawberry. Increased output of ABA and
decreased delivery of Z and ZR from roots exposed to
drying soil coincided temporally with the onset of
1.5
Transfer
stomatal closure in RDI-treated plants. Delivery of
*
*
ABA was also increased from PRD-treated roots at the
*
*
time stomata began to close although the change was
0 mol m ABA
20 mol m ABA
1.0
not statistically significant in this experiment. In other
10 mol m ABA
40 mol m ABA
experiments, increased output of ABA from PRD0.0
treated roots was followed within hours by the onset of
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
stomatal closure. The reduction in cytokinin export
Clock time
from roots exposed to PRD and to RDI may also have Figure 2.3. Effects of different concentrations of
increased stomatal sensitivity to ABA.
synthetic ABA on rates of water loss from detached
Xylem sap ABA concentrations in planta at the leaves. All leaves were transferred from buffer to
time stomata first began to close were approximately experimental solutions at 11:00. Results are means of
30 µmol m-3 but concentrations above 40 µmol m-3 six replicate leaves; asterisks indicate statistically
were needed to prompt stomatal closure in detached significant differences (P ≤ 0.05).
leaf transpiration bioassays. A lower sensitivity of
stomata to ABA in these bioassays is often observed and on-going work in our HortLINK project
(HL0187) suggests that increased ethylene production in detached leaves may reduce the extent of
ABA-induced stomatal closure (see Tanaka et al., 2005).
The lack of a leaf growth response to PRD and RDI was always observed during these shortterm GroDome experiments. This was unexpected since a slowing of LER is usually one of the first
SID 5 (Rev. 3/06)
Page 13 of 28
-3
-3
-3
-3
detectable responses to soil drying. In our polytunnel experiments, LER didn’t begin to diverge from
WW values until 18 days after the beginning of the deficit irrigation treatments. This implies that either
different signals regulate stomatal and leaf growth responses or that the responses are triggered by
different intensities of the same signal.
These studies have greatly improved our understanding of how the plant’s internal signalling
system changes in the first few days of deficit irrigation. Although this work is a very important first
step, for a crop like strawberry where the aim is to maintain marketable yields and improve fruit quality,
more work needs to be done on optimising the delivery of these root-sourced chemicals over the
cropping season. A lowered photosynthetic rate imposed by continuous and intensive signalling from
roots will also limit production of important precursors to key flavour components such as sugars and
flavour volatiles. In strawberry, partial rather than complete stomatal closure could help to maintain
photosynthesis and the production of essential sugar and flavour volatile precursors while limiting water
loss. Optimising the production and transport of the chemical signals that interact to control water loss
and photosynthetic rate over the cropping season is a key component of HL0187.
Very little is known about the signals that regulate the production of antioxidants in fruit. Fruit
ripening is a developmental event where oxidative processes take place and a variety of reactive
oxygen species (ROS) accumulates (Jimenez et al., 2002). However, in addition to this developmental
control, mechanisms must exist that are able to stimulate further antioxidant production. Our results
with PRD support the notion that the perception of stress can lead to an elevated ascorbic acid (AsA)
and ellagic acid (EA) content (see Objective 6); however, the response was not consistent across all
deficit treatments. The mechanism behind stress-induced increases in antioxidants is unclear,
although methyl jasmonate (MeJA) may be involved (Wang 1999). To increase fruit antioxidant content
reliably and consistently, we need to understand more about the signals that regulate fruit AsA and EA
production. This work is currently being carried out in WU0110.
-2
-1
Stomatal conductance (mmol H2O m s )
Experiments on Poinsettia (EMR)
Both PRD and RDI were imposed on a commercial poinsettia crop to try to control plant height without
reliance on undesirable and costly plant growth regulators (see Objective 6). Both techniques
effectively limited height but RDI was deemed to be the easier technique to implement on a commercial
scale using flood-and-drain irrigation systems that many of the larger growers use. Therefore, work
from 2007 onwards concentrated on developing RDI regimes that could be used on a commercial
scale. The effects of RDI on root-to-shoot signalling were investigated to try to identify the signals that
regulated poinsettia stem and shoot responses to deficit
irrigation.
400
CC
WW
RDI
*
Results
300
Stomatal conductance was significantly reduced within
*
*
seven days of imposing RDI and remained lower than
200
*
well-watered controls throughout the five-week deficit
*
*
regime.
During this period, diurnal measurements
100
showed that stomatal conductances were similar in all
treatments at the beginning of the photoperiod (08:00 in
0
late October) but values were reduced in RDI-treated
09:10
11:12
13:15
15:05
16:45
plants during the middle of the day and remained lower
Clock time
than in well-watered controls for the rest of the
photoperiod (Figure 2.4). When sampled in the same Figure 2.4. Effects of RDI on stomatal conductances
week as the diurnal data were collected, xylem sap measured on a fully expanded poinsettia leaf
delivery of ABA from detopped, pressurised roots was compared to values from commercial control (CC)
increased 2.5-fold by RDI (Figure 2.5). This increased and well-watered (WW)) plants. Results are means
of 12 replicate plants; asterisks indicate statistically
output of ABA presumably triggered and maintained significant differences (P ≤ 0.05) from CC values.
stomatal closure, but reduced cytokinin deliveries may
also have contributed (Figure 2.5). After the RDI regime was ended, output of ABA returned to prestress levels and was similar in all treatments by simulated market date (25 November). Deliveries of
root-sourced IAA were unaffected by the RDI regime.
SID 5 (Rev. 3/06)
Page 14 of 28
Hormone delivery rate (ng h-1)
160
CC
WW
RDI-GP1
140
ABA
IPAR
1.4
ZR
3
*
1.2
120
1.0
100
2
0.8
80
0.6
60
*
1
0.4
40
20
0.2
0
0.0
26/09
27/10
25/11
*
0
26/09
27/10
25/11
26/09
27/10
25/11
Date of measurement
Figure 2.5. Effects of RDI on deliveries of ABA and the cytokinins isopentenyl-adenosine (IPAR) and zeatin
riboside (ZR) in xylem sap collected from pressurised detopped poinsettia root systems. Delivery rates were
calculated from multiplying sap flow rates by hormone concentrations quantified by GC-MS-SIM. Results
are means of sap samples collected from six replicate plants; asterisks indicate statistically significant
differences (P ≤ 0.05) from CC values.
Discussion
Poinsettia stem height was effectively reduced by RDI (see Objective 6) but the ways in which deficit
irrigation limits stem extension in poinsettia, or in other ornamental crops, is not yet known. Although
ABA was traditionally viewed as a growth inhibitor, more recent research suggests that it can help to
maintain shoot growth in plants exposed to drying soil by limiting the growth-inhibiting effects of
ethylene. However, foliar ethylene production was not increased in RDI-treated plants so the reduced
internode lengths in plants exposed to mild soil drying must be regulated in other ways. A reduction in
gibberellin (GA) biosynthesis and/or transport, increased metabolism of active GAs to inactive
metabolites or altered signal transduction are likely lo be involved. The regulation of GA biosynthesis
and transduction in deficit-grown plants will be investigated in future work.
Plants previously exposed to mild soil drying by imposing RDI during the period of rapid shoot
extension have an improved shelf-life potential and tolerance to chilling temperatures when tested
several months later (see Objective 6). These results imply that exposure to a mild form of stress (e.g.
soil drying) can pre-condition plants so that they are more tolerant of stressful conditions encountered
later. Our analyses showed that root-to-shoot signalling had returned to pre-stress levels by market
date in plants previously exposed to RDI and it remained unchanged at the end of the shelf-life tests.
The mechanisms that bestow this enhanced stress tolerance are not yet known but may arise from an
ABA-mediated increase in the production of reactive oxygen species (ROS) (e.g. hydrogen peroxide)
and associated changes in antioxidant capacity during RDI. When stress is next encountered, the
plant’s antioxidant system may then be able to scavenge ROS more efficiently than previously
unstressed plants. Experiments designed to test this hypothesis form part of a concept note that we
recently submitted to the HDC Protected Crops panel and to the HortLINK programme.
Experiments on Tomato (Lancaster)
Experiments aimed to determine signals in xylem sap collected from detached leaves, thus avoiding
concerns that hormone concentrations are altered between exiting the root system and arrival at sites
of action in the leaves. Various hydraulic (leaf water potential -leaf) and chemical signals were assayed
(xylem sap pH, and the plant hormones ABA, IAA, and various CKs) based on their perceived
physiological relevance. The effects of alternating wet and dry sides of PRD plants was also
investigated, to assist in determining when such alternation events should occur (see also Objective 5).
Results and Discussion
Deficit irrigation of tomato (50% of ET applied as PRD) decreased soil water potential (soil) by 0.4
MPa, leaf by 0.08 MPa, whole plant transpiration rate by 22%, whole plant leaf area by 25%, and
increased leaf xylem ABA concentration ( [X-ABA]leaf ) 2.5-fold. Although PRD caused no detectable
SID 5 (Rev. 3/06)
Page 15 of 28
Bulk leaf cytokinin concentration (ng g FW)
40
-1
a
-1
Xylem cytokinin concentration (ng mL )
change in xylem CK concentration, it decreased the CK concentration of fully expanded leaves by 46%
(Figure 2.6). Maintenance of xylem CK concentrations (rather than an increase) while transpiration
decreased suggests that CK loading into the xylem also decreased as the soil dried. Since leaf CK
concentration did not decline proportionally with CK delivery, other mechanisms such as CK
metabolism may also influence leaf CK status of PRD plants (Kudoyarova et al. 2007).
For deficit irrigated tomato grown as above (50% of ET applied as PRD), alternation of wet and dry
sides (PRD-A) increased [X-ABA]leaf up to 2-fold above that of plants where the wet and dry sides were
35
30
25
20
15
10
5
0
PRD WW
Day 2
PRD WW
Day 4
20.0
b
17.5
15.0
12.5
10.0
PRD WW
Day 5
7.5
5.0
2.5
0.0
PRD WW
Day 2
PRD WW
Day 4
PRD WW
Day 5
Figure 2.6. Changes in xylem sap (a) and bulk leaf CK concentration (b) on 3 measurement occasions (Days 2, 4 and 5
after initiating PRD) for the compounds zeatin (hollow bars), zeatin riboside (cross-hatched bars) and zeatin nucleotide
(filled bars). Xylem sap data are means of 8 replicates (comprising 2 samples per treatment per day from all four
experiments) while bulk leaf data are means of 3 replicates. Further details of these experiments are published in
Kudoyarova, Vysotskaya, Cherkozyanova & Dodd 2007. Journal of Experimental Botany 58, 161-168.
xylem [ABA] (nM)
 leaf (MPa)
-1
soil water content (g g )
fixed (PRD-F). Thus alternation further decreased stomatal conductance (Dodd et al. 2006).
Differences in [X-ABA]leaf were detected within an hour of alternation but did not persist beyond the
photoperiod of alternation (Figure 2.7c). [XABA]leaf increased linearly as whole pot soil
water content (pot) and leaf water potential
0.60
a
(leaf) declined, but the difference in [X0.45
ABA]leaf between the two sets of PRD plants
0.30
was not due to differences in either pot or
0.15
leaf (Figure 2.7a, b). In PRD-F plants, the
0.00
-0.4
unwatered part of the root system
b
-0.5
contributes proportionally less to the
-0.6
-0.7
transpiration
stream
as
the
soil
Fixed PRD
-0.8
Alternate PRD
progressively dries (see Figure 2.8 below).
-0.9
Well Watered Control
-1.0
In PRD-A plants, we hypothesise that re800
c
700
watering the dry part of the root system
600
allows
these
roots
to
contribute
500
400
proportionally more to total sap flux, thus
300
200
liberating a pulse of ABA to the transpiration
100
0
stream as the root ABA pool accumulated
10
11
12
13
14
15
16
17
18
during soil drying is depleted. Since the
Time of Day (h)
enhancement of [X-ABA]leaf caused by
PRD-A increased as pot and leaf declined,
Figure 2.7. Whole pot soil water content (a) leaf water potential (b),
and xylem ABA concentration (c) of WW (), PRD-F () and PRD-A
an optimal frequency of alternation to
(▼) plants for which irrigation was alternated at 1000 h. Each point
maximise the cumulative physiological
represents a single plant and regression lines were fitted to each
effects of this ABA pulse must consider
treatment. Further details of these experiments published in Dodd,
possible negative impacts of leaf water
Theobald, Bacon & Davies 2006. Functional Plant Biology 33, 1081deficit as soil water status declines.
1089.
SID 5 (Rev. 3/06)
Page 16 of 28
Fractions of initial sap flow
Experiments on “Two root, one shoot” grafted plants (Lancaster)
Further work with pot-grown tomatoes showed that fixed PRD either increased or decreased [X-ABA]leaf
compared to deficit irrigated plants depending on total soil water availability (Dodd 2007). In attempting
to explain these divergent responses, it was considered necessary to assess the contribution of
different parts of the root system to [X-ABA]leaf. This was achieved by developing a novel grafting
procedure where a single shoot was grafted onto the root systems of two plants, and xylem sap
collected from both detached leaves and each individual root system. Since particular roots can supply
particular leaves in the canopy in so-called “sectorial” plants, approach grafting was avoided. Some
work used sunflower as its cylindrical stems allowed good contact between the stem and sap flow
sensors (necessary to assess the sensitivity of sap flow to soil drying). We hypothesised that
decreasing sap flow from roots in drying soil would limit ABA export to the shoot during PRD, such that
soil moisture heterogeneity would influence the relationship between leaf xylem ABA concentration and
a
b
1.00
1.00
"wet"
"dry"
0.75
0.75
0.50
0.50
0.25
0.25
0.00
0.00
0
240
480
720
960 1200 1440 1680 1920 2160 2400 2640 2880
0
1
2
3
4
soil water content (g g-1)
Time since start of experiment (minutes)
SID 5 (Rev. 3/06)
Page 17 of 28
-1
leaf xylem [ABA] (nM) root xylem [ABA] (nM)
total soil water availability.
Results and Discussion
Initial work used a peat-based substrate with high
water holding capacity to ensure that the irrigated root
system of PRD plants was adequately supplied with
water (soil > - 1 kPa). During PRD, once  decreased
below a threshold, the fraction of sap flow from drying
roots decreased linearly with soil water content (Figure
2.8b). Root xylem ABA concentration increased in both
DI and PRD plants as  declined. Although [X-ABA]leaf
increased in DI plants as  declined, in PRD plants [XABA]leaf actually decreased within a certain  range
(Dodd et al. 2008b). A simple model that weighted the
ABA contributions of wet and dry root systems to [XABA]leaf according to the sap flow from each, better
predicted [X-ABA]leaf of PRD plants than either the root
xylem ABA concentrations derived from wet or dry root
systems, or their mean. This model revealed that for
the same whole pot soil water content, simulated [XABA]leaf was higher in PRD plants than DI plants with
moderate soil drying, but continued soil drying (such
that sap flow from roots in drying soil ceased) resulted
in the opposite effect (Figure 2.9). That total soil water
availability determined whether DI or PRD elicited a
greater [X-ABA]leaf provides a possible physiological
explanation for differences in plant yield between DI
and PRD plants supplied with the same irrigation
volumes (Figure 1.4).
All modelling thus described shows if the wet
part of the root system is adequately watered, there is
an optimal dry part soil water content to maximise ABA
Fractions of sap flow soil water content (g g )
Figure 2.8. Fractions of initial sap flow from wet (blue) and dry (red) parts of the root system during a typical PRD
experiment (a). Arrows are when the wet side was watered. Relationship between soil water content and fraction
of initial sap flow from the dry part of the root system (b). Each point in (b) is derived from a single experiment as
in (a). Further details published in Dodd, Egea & Davies 2008. Plant, Cell and Environment 31, 1263-1274.
3.5
a
3.0
2.5
2.0
PRD - wet
PRD - dry
DI
1.5
1.0
0.5
1.0
b
0.8
0.6
0.4
0.2
0.0
100
c
10
21
d
DI
PRD
18
15
12
9
6
0
2
4
6
8
10
12
14
16
Time (days)
Figure 2.9. Simulated changes in soil water content
surrounding (a), fractions of sap flow from (b) and root
xylem ABA concentration of (c) wet () and dry () parts
of the root system of PRD and DI plants. Leaf xylem ABA
concentration (d) of DI plants equalled root xylem ABA
concentration, and of PRD plants is modelled Further
details published in Dodd, Egea & Davies 2008. Plant,
Cell and Environment 31, 1263-1274.
signalling from the entire root system (Dodd 2008; Dodd et al. 2008b; c). If the soil dries beyond this
threshold, the wet and dry parts of the rootzone should be switched. However, this approach has yet to
be trialled in the field, and in many field experiments with PRD, partial drying of the irrigated roots
occurs if irrigation is infrequent (Kirda et al., 2004) and it is important to assess the implications for ABA
signalling. Soil water status of the irrigated pot affected the relationship between the fraction of sap flow
through the dry part of the root system and soil water content: although soil of the irrigated pot
determined the threshold soil at which sap flow from roots in drying soil decreased, the slope of this
decrease was independent of the wet pot soil (Dodd et al. 2008c). Further modelling predicted that the
specific dry to maximise ABA signalling from the entire root system will vary according to wet (Dodd et
al. 2008c), suggesting flexibility in dry set points if the water status of the irrigated pot varies. Irrigation
scheduling based on ABA modelling of PRD plants may inform and complement other plant- or soilbased methods of scheduling irrigation, but further work is required to validate this approach (Dodd et
al. 2009).
Objective 3 To determine the effects of PRD on resource partitioning and root and leaf functioning
Experiments at Lancaster with glasshouse-grown tomato plants with roots split between 2 x 5 L pots
revealed that total biomass did not differ between PRD and DI plants after four cycles of PRD, but PRD
increased root biomass by 55% as resources were partitioned away from all shoot organs. When the
crop was allowed to fruit on six trusses and ripe tomatoes were harvested over an eight week period,
fruit yield was statistically similar between PRD and control plants, suggesting that biomass allocation
within the shoot altered to maintain fruit yield at the expense of stem and leaf biomass (Mingo et al.
2004).
Further
experiments
showed that after two cycles of
PRD, the promotion of root
0.4
End of 1st cycle of PRD
End of 2nd cycle of PRD
biomass in PRD plants was
associated with the alternation of
0.3
A
A
A
A
wet and dry compartments, with
0.2
increased
root
biomass
occurring
in
the
re-watered
0.1
compartment
after
previous
0.0
exposure to soil drying (Figure
DI plants
DI plants
PRD plants
PRD plants
3.1). Promotion of root biomass
in field-grown PRD plants may
Figure 3.1. Root biomass of DI and PRD plants after one (a) and two
allow the root system to access
(b) drying cycles. Black bars indicate soil drying. Data are means  S.E.
water and nutrients that would
of 5-6 replicate plants, with different letters above the bars indicating
otherwise be unavailable to
significant differences. Further details published in Mingo, Theobald,
Bacon, Davies & Dodd 2004. Functional Plant Biology 31, 971-978.
control
plants.
This
may
contribute to the ability of PRD
plants to maintain a similar leaf water potential to conventionally irrigated plants, even when smaller
irrigation volumes are supplied.
While these deficit irrigation-induced changes in biomass allocation (leaf growth inhibition and
increased biomass allocation to the roots and fruits) may be advantageous in some crops, this would
be a major disadvantage in lettuce where the shoot is harvested. When containerised (5 L pots) lettuce
was subjected to four irrigation treatments (115%, 100%, 85% & 70% of measured ET in the
greenhouse, increased root biomass correlated with decreased irrigation. Despite this, marketable
heads (> 150g within an eight week growing period) were also achieved with the 85% irrigation
treatment.
Decreased vegetative growth (biomass allocation to the leaves) was advantageous in other
crops. In poinsettia, decreased vegetative growth (with no effect on reproductive growth) resulted in
more compact and robust marketable plants (see Objective 6). In strawberry, decreased vegetative
growth allowed easier crop harvesting and increased the penetration of light to developing fruit. Deficit
irrigation during the flower initiation phase (Sept-Oct) increased the number of trusses produced in the
second cropping year. Despite the greater number of fruit, berry size was not reduced and so
marketable yields were increased by up to 40% in the second cropping year.
B
B
B
C
Root dry weight (g)
0.5
SID 5 (Rev. 3/06)
Page 18 of 28
To summarise, deficit irrigation (and especially alternating wet and dry parts of the rootzone in
PRD) stimulated biomass allocation towards the roots (which may limit the severity of leaf water deficit)
and fruits. The latter may have quality implications, as discussed below.
SID 5 (Rev. 3/06)
root xylem ABA concentration (nM)
Sap Flow (% of original)
Objective 4 To determine the environmental conditions and substrates that maximise the benefits of
PRD
Since the action of long distance signals of soil drying (Objective 2) is more effective at high
evaporative demand (Loveys et al. 2004), the low evaporative demand commonly experienced under
UK conditions may limit the effectiveness of PRD. An extensive data set on grapevine water use made
available to us by the Adelaide group shows that PRD only provides tangible benefits (in terms of water
saving) at atmospheric VPDs typically > 2 kPa. In contrast, the high humidity (RH 60 – 80%) and
moderate temperatures (16-25C) maintained in commercial tomato glasshouses in the UK sets the
range of achievable VPD between 0.4 and 1.3 kPa. Thus benefits of PRD are likely to occur only at
limited times of the year when cloud cover is low and radiation load is high, so permitting the internal
glasshouse environment to equilibrate more closely with the external when glasshouse vents are
opened fully. Thus analysis of stomatal responses in our experiment with tomato grown on rockwool
blocks showed that significant effects of PRD on stomatal conductance were typically restricted to
periods of higher VPD.
The effect of imposing deficit irrigation (both PRD and DI) in a range of different substrates (a
peat-based medium, a 3:1 sand: peat mix and a coir-based medium) on the physiology and yield of
pot-grown raspberry and runner beans was investigated over two seasons with each crop. Irrigation
treatments were based on maintaining the media of control plants at or near pot capacity, with DI and
PRD receiving 50% of control irrigation. No significant differences between media were found for yield
(or any other traits) in the runner bean experiments. In contrast, raspberry yield varied with substrate in
each year. Averaged across irrigation treatments (since there was no interaction between irrigation
treatment and substrate), raspberry yield was 42% lower in coir in 2005, and 39% lower in sand in
2006. The inconsistent effects of different substrates on raspberry yield from year-to-year negated
attempts at explaining these responses in terms of either substrate moisture holding capacity and/or
moisture release curves.
The influence of different substrates on the contribution of different parts of the root system to
total sap flow and root xylem ABA concentration (see discussion in Objective 2) was assessed using
“two root, one shoot” grafted plants as described above. During PRD, one pot (“wet”) was watered and
other (“dry”) was not. While each substrate gave a unique relationship between soil matric potential of
the dry pot (dry) and the fraction of photoperiod sap flow from roots in drying soil (F dry) (Figure 4.1a) ,
Fdry decreased with root water potential (root) similarly in all substrates. Likewise, each substrate gave
a unique relationship between soil matric potential (soil) and root xylem ABA concentration ( [ABA]root )
(Figure 4.1b) but [ABA]root increased similarly with decreasing root in all substrates. Across all
substrates, whole plant transpiration was most closely correlated with the mean soil of both pots, and
then with [X-ABA]leaf. A model of leaf xylem ABA concentration, which weighted ABA contributions of
each root system according to the sap flow from each, showed that in some substrates the increase in
[ABA]root from roots in drying soil was partially offset by a decrease in sap flow from roots in drying soil,
such that in response to
defined PRD treatment, the
increase in shoot xylem ABA
100
200
concentration ( [ABA]shoot )
a
b
Sand
varied
from
1.3-3.8-fold
Clay
80
according to the substrate.
Peat
150
Loam
Thus a desired outcome of
60
PRD
(ABA-mediated
100
restriction of whole plant
40
transpiration) may be more
Sand
readily achieved in certain
50
Clay
20
substrates.
Peat
Loam
To summarise, choice
0
0
of substrate in containerised
0.1
1
10
100
0.1
1
10
100
plants will determine the
substrate matric potential (-kPa)
substrate
matric
potential
(-kPa)
precision with which desired
deficit irrigation treatments
can be imposed.
Figure 4.1 Relationship between substrate matric potential and fraction of initial sap
flow from the dry part of the root system (a) and root xylem ABA concentration (b).
Page 19 of 28
04
/0
9/
06
18
/0
9/
06
02
/1
0/
06
16
/1
0/
06
30
/1
0/
06
13
/1
1/
06
27
/1
1/
06
Stomatal conductance (mmol/m2/s)
Objective 5 To determine the optimal scheduling of deficit irrigation in the field
The majority of PRD trials conducted to date have used fairly arbitrary decisions as to when to alternate
the side of the root system that is irrigated. For example, in this project, in pot-grown runner beans, the
impact of alternating every week versus every two weeks was compared. Although there was no
difference in the number or quality of pods produced or in vegetative growth, stomatal conductance of
PRD plants alternated every week was 30% lower than PRD plants alternated every two weeks. This
suggests that root signal output decreased after the first week of root drying, indicating additional scope
for saving water if the frequency of alternation is optimised.
A theoretical analysis (now substantiated by results described in Objective 2 – see Figure 2.9)
suggests that the optimum time would be when the dry side has dried to a level where root signal
output starts to diminish. Direct measurement of this signal (if practical) could provide a good indicator
for scheduling the switch. As this is likely impractical, alternative approaches need to be considered.
These include direct measurement of soil moisture (ideally with a prior calibration of soil moisture
against signal intensity) and through the detection of other plant responses such as leaf wilting or
stomatal closure (reviewed by Jones 2004a; b). These plant responses potentially provide a good
indirect measure of the intensity of the drought signal although product quality may already be
jeopardised if wilting has occurred.
Soil moisture sensing offers the advantage of ready automation, and once sensors are installed,
data can be downloaded continuously by wireless data-link by the irrigation manager. Although such
sensors are increasingly used in production systems, expense usually limits the number of sensors that
can be installed in a given crop thus selection of “average” plants becomes important if appropriate
management decisions are to be
made. Work at EMR with RDIgrown poinsettia used a GP1 data
600
logger (Delta-T Devices, Burwell,
CTL
50% PRD
UK) to trigger irrigation based on
50% RDI
500
soil moisture sensors (e.g. SM
200 probes) to provide a series of
dry down cycles of various
400
intensities. The soil moisture
threshold at which irrigation was
300
triggered
was
adjusted
throughout the growing season to
200
regulate stem extension, allowing
the RDI-treated plants to remain
within
commercial
height
100
specifications.
A
consistent
relative
0
response
of
stomatal
conductance to PRD is frequently
observed (Figure 5.1). However,
a difficulty with using stomatal
Figure 5.1. Temporal changes of stomatal conductance of glasshouse-grown
conductance
to
schedule
Phaseolus coccineus cv. Emergo in response to different irrigation treatments.
irrigation, at least under many UK
Fluctuations in mean conductance on different days were primarily related to
environments, is that stomatal
differing environmental conditions, with low conductance on days of very low
aperture is sensitive not only to
soil
drying
and
root-shoot humidity/high temperatures.
signalling but also to environmental conditions; and large day-to-day variation in mean conductances
can be attributed to differences in environmental conditions. It follows that irrigation scheduling using
stomatal measurements is most likely to be successful where one has a well-irrigated control as a
reference.
Direct measurement of stomatal conductance can also be time-consuming (even with a
transient-time porometer which can make a reading every 30 seconds) if measurements are required
over a large spatial scale. An alternative is to use thermal imaging to measure leaf or canopy
temperature which is directly related to stomatal conductance (higher temperatures indicate stomatal
closure). Approaches to estimate stomatal conductance using canopy temperature combined with the
use of wet and/or dry reference surfaces or measurements of environmental variables (humidity, air
SID 5 (Rev. 3/06)
Page 20 of 28
temperature, radiation and windspeed) have been summarised by Leinonen et al. (2006) and Guilioni
et al. (2008).
To summarise, results from this study show several successful methodologies to schedule
irrigation (when to water, and when to switch) although practicalities (ease of grower use; equipment
expense or availability) are likely to dominate over physiology in choice of an appropriate technique.
These methods are currently being used to schedule irrigation to field-grown strawberries (HL0187).
Objective 6
To determine the 'quality' of deficit-grown produce
Introduction
Although irrigation management strategies such as PRD or RDI can deliver large water savings, these
techniques will only be taken up by the UK horticultural industry if yields and product quality are
maintained or improved. The potential to use these water saving strategies without incurring yield
penalties is summarised under Objective 1 for six of the seven crops studied. In addition to yields,
product quality is also crucial because producers must continue to meet the expectations of retailers
and consumers with regard to product quality and shelf-life.
At the project outset (2004), both the Adelaide experiments and IRRISPLIT work with grapes
and tomato had shown that if PRD was managed carefully, fruit quality could be improved resulting in
substantial added value. Unfortunately, effects of PRD on aspects of crop quality are either not
reported or are assessed only cursorily and so rigorous scientific analysis of quality attributes in deficitgrown plants is sparse. Having established that PRD or RDI regimes could be imposed in some crops
without reducing yields, we quantified the effects of these techniques on various components of
product quality, including those most commonly used by the industry and retailers.
Experiments on Strawberry (EMR)
Retailers and consumers expect high quality fruit of good shape, colour and size, with a good aroma,
sugar acid balance and shelf-life. These quality criteria can be difficult to meet in some seasons and
reduced consumer confidence in UK-produced berries can impact on return sales and limit the overall
profitability of the sector. The potential of PRD and RDI to deliver consistent improvements in berry
quality was determined over three cropping seasons.
Ethyl hexanoate
Concentration (ng g-1 FW)
Results
Appearance and texture
Both PRD and RDI regimes affected fruit colour;
the brightness was reduced and the chroma (or
intensity) was increased. These changes are
indicative of a deeper, more-intensely red,
coloured berry which may be more appealing to
consumers. Fruit firmness was increased by the
60% PRD and RDI regimes and by the 80%
PRD regime but this response was not
consistent over the three cropping seasons.
80
Ethyl butanoate
30
60
20
40
10
20
0
0
WW PRD PRD PRD RDI RDI RDI
120 100 80 60 100 80
60
WW PRD PRD PRD RDI RDI RDI
120 100 80 60 100 80 60
Treatment
Treatment
Flavour and bioactive compounds
Berry soluble solids content (SSC – [BRIX]) was Figure 6.1. Effects of PRD and RDI on the production of two
unaffected by PRD and RDI treatments and important strawberry flavour volatiles.
concentrations of sugars (sucrose, fructose, glucose) and organic acids (citric, malic, oxalic and
ascorbic) were also maintained. However, RDI increased the production of several important flavour
volatiles but some aroma concentrations were reduced by PRD (Figure 6.1).
Berry concentration of several antioxidants (e.g. phenols, ASA, EA) were increased by some
PRD and RDI regimes and these differences were not due simply to less dilution caused by a reduced
water content of the PRD- and RDI-grown fruit. Berry total antioxidant capacity was also increased by
the 60% PRD regime.
Discussion
Texture and colour are important components of fruit quality that influence consumer appeal and SSC
is used by retailers as a proxy measure of berry sweetness. Each of these attributes was either
maintained or enhanced by PRD and RDI and the all-important ratio of sugars to acids was also
unaffected. A recent report that strawberry quality could be significantly improved by RDI should be
SID 5 (Rev. 3/06)
Page 21 of 28
viewed with caution; berry size was reduced by 35% due to the severity of water deficits imposed
(Terry et al., 2007) and so any differences were likely to be artefacts due to dilution effects.
Our work showed that the production of important flavour volatiles was significantly increased
by RDI but these volatiles were either maintained or reduced by PRD. This was not due simply to a
greater supply of precursors (carbohydrates) resulting from reduced vegetative growth since canopy
areas were reduced to a similar extent by PRD and RDI. Many flavour volatiles are derived from
products of photosynthesis and partial stomatal closure may limit their availability. The increase in
berry antioxidant concentration in deficit-grown fruit has important implications both for human health
and for improved berry shelf-life. We are currently trying to exploit the differential responses to PRD
and RDI by developing a new deficit irrigation technique to promote both berry flavour and bioactive
content without incurring yield penalties (WU0110). The potential to use deficit irrigation to improve fruit
quality in field-grown strawberry production is being determined in our on-going HortLINK project
(HL0187).
Plant height (cm)
Experiments on Poinsettia (EMR)
Unlike the industry sectors listed under Objective 1, delivering water savings into the protected
ornamentals sector is not a high priority since water use by this industry is generally very low compared
to others. Some growers use efficient flood-and-drain systems that recycle irrigation water and so their
WUE is already high. However, growers using drip irrigation and mains water would benefit from
irrigation management strategies that reduce water inputs and losses. All growers add fertiliser to the
irrigation water (fertigation) and although exempt from NVZ regulations, the reduction in fertiliser inputs
associated with effective scheduling and deficit regimes would help to offset the rising costs of
fertilisers.
Control of plant height is a major issue for growers of protected potted ornamental plants. This is
currently achieved by frequent applications of plant growth regulators (PGR’s) but these are costly and
environmentally undesirable and are at risk of being withdrawn under EU legislation. Alternative, nonchemical methods of height control are needed to counter these legislative changes. Therefore, the
aim of these experiments was to use deficit irrigation techniques to try to control plant height in
poinsettia and reduce the reliance on PGR’s. The potentially beneficial effects of RDI on plant quality,
and shelf- and home-life potential were also determined. Conditions during distribution and retailing
were quantified and the potential of RDI to
improve tolerance to stresses encountered
Irrigation regimes
Staplehurst
applied
Control
during the distribution chain was also
30
WW
determined.
Investigations into the
RDI
25
chemical signals that impact on plant
growth and shelf-life in deficit-grown
20
poinsettia plants are described under
15
Objective 2.
10
Irrigation regimes
ended
Results
5
Plant quality at simulated market date
Week 35 36 37 38 39 40 41 42 43 44 45 46 47
Experiments
over
three
seasons
0
confirmed that a six-week RDI regime
25/07/06
22/08/06
19/09/06
17/10/06
14/11/06
imposed during the period of rapid growth
Date
successfully reduced stem extension so
Figure 6.2. RDI effectively controlled stem extension so that all
that plant heights were well within
plants were within height specifications at market date, despite
specifications at simulated market date
receiving 90% fewer sprays of PGR’s than the controls. Results
(mid-late November) (Figure 6.2). RDIare means of 12 replicate plants with associated standard errors.
treated plants received only one PGR
spray shortly after pinching, compared to the nine or ten sprays needed to control height in the
commercial controls (CC) and well-watered (WW) plants. Plant widths were unaffected by the
treatments but the stage of cyathia (flower) development at simulated market date was delayed in RDItreated plants. This delayed anthesis may have contributed to the improved overall quality of these
plants during home-life tests (see below). In each season, overall plant quality at market date was
maintained, and sometimes improved slightly compared to commercial controls, by the irrigation
scheduling and RDI regimes (Figure 6.3).
SID 5 (Rev. 3/06)
Page 22 of 28
Shelf-life potential
The deterioration in plant quality during the six-week home-life test was slowed by both RDI regimes,
compared to CC and WW plants (Figure 6.4A). RDI-treated plants remained in good condition
throughout l the home-life tests which were ended in mid January. Greatly reduced rates of leaf
abscission (Figure 6.4B), bract abscission and leaf and bract paling contributed to the higher quality of
RDI-treated plants.
Effects of distribution on shelf-life potential
Poinsettia are notoriously prone to chilling injury and temperatures below 10 ºC can lead to severe
defoliation and leaf and bract necrosis, which greatly reduces plant quality. Therefore, care is taken
during distribution to ensure that exposure to chilling temperatures is minimised. Heated lorries are
used for dispatch and the temperature in each lorry is checked immediately on arrival at depot.
Ambient temperatures encountered by our experimental plants fluctuated at the different stages during
distribution but were maintained above 12 ºC until plants arrived in-store. Despite clear labelling on the
boxes (‘store at ambient temperature’), our experimental plants were placed in a cold-store (4 ºC) until
they were retrieved by EMR staff later that morning. During the subsequent shelf-life test at EMR, leaf
abscission was reduced by 33% in RDI-treated plants compared to CC plants. Our attempts to repeat
this work in 2008 were unsuccessful because the experimental plants were sold in error when they
arrived in store.
SID 5 (Rev. 3/06)
RDI-GP1
4
3
2
1
0
03/12/07
Cummulative leaf drop per plant
Discussion
Our experiments demonstrated unequivocally that
plant height could be effectively controlled by RDI
imposed during the period of rapid stem extension.
Importantly, stringent plant specifications imposed
by retailers at market date were met consistently,
despite a 90% reduction in PGR use. Overall
quality of RDI-treated plants at market date was
maintained or improved and cyathia development
was delayed slightly, compared to CC and WW
plants. Similar results were obtained over three
seasons (2006-2008) using the flood-and-drain
benches at Staplehurst Nursery and irrigation
management strategies such as RDI now need to
be developed for a range of crops to help reduce
this industry’s reliance on PGR’s (see ‘Future
work’).
The shelf- and home-life of RDI-treated
plants was also consistently improved. The
improved shelf-life potential of RDI-treated plants
was presumably bestowed by changes in the
metabolism of deficit-grown plants. Since the
Plant quality score (1-5)
Effects of ethylene on shelf-life potential
Anecdotal evidence suggests that poinsettia are very susceptible to ethylene accumulation and that
exposure to low concentrations during distribution can limit plant quality during shelf- and home-life.
Our measurements in 2006 suggested that whole-plant ethylene production rates both at simulated
market date and during shelf-life tests were very low (0.005 parts per million - ppm). Although these
concentrations are far too low to impact on plant quality, it is likely that plants could encounter very
high ethylene concentrations in depot and during transport to store that may be sufficient to trigger leaf
drop and reduce quality.
In 2007, we were granted access to the Asda Erith depot to determine whether accumulations
of ethylene during distribution subsequently limited shelf-life potential and reduced quality. Ambient
ethylene concentrations were fairly low (< 1 ppm) at Staplehurst Nursery before dispatch, and in-store.
However, ambient ethylene concentrations reached 5 ppm in the depot, presumably due to the
presence of climacteric fruit (e.g. bananas, apples) and other ethylene- producing goods such as
Christmas trees. But in dose-response tests at
A)
CC
EMR, exposure to concentrations of up to 20 ppm
5
WW
for 12 h failed to alter any aspect of plant quality.
RDI
10/12/07
17/12/07
24/12/07
31/12/07
07/01/08
14/01/08
B)
40
30
20
10
0
10/12/07
17/12/07
24/12/07
31/12/07
07/01/08
14/01/08
Date of measurement
Figure 6.3. Effects of the irrigation treatments on A)
overall plant quality and B) cumulative leaf drop during
a six-week shelf life test at EMR. In A) a score of 5
represents excellent quality, 3 is marketable while 1
represents very poor quality. Results are means of six
Page 23 ofreplicate
28
plants with associated standard errors.
deficit irrigation treatment and shelf-life tests were three months apart, RDI must have somehow
‘conditioned’ the plants so that leaf and bract abscission was reduced. Our work with RDI on other
crops such as strawberry indicates that antioxidant levels can be improved in deficit-grown plants. We
will determine whether the total antioxidant capacity is increased in RDI-treated poinsettia and whether
this increased capacity to neutralise ROS generated during exposure to chilling temperatures underlies
the improved shelf-life potential.
Ambient ethylene concentrations of up to 5 ppm were detected in the depot en route to the
store. However, in tests, ethylene concentrations four times higher than this failed to impact on plant
quality.
Other crops
Potatoes were grown in a field on a commercial holding next to a traditionally managed crop. PRD and
RDI treatments received 50% of the amount of water to maintain soil moisture at ≥ 95% of field
capacity. Although the commercially managed crop received approximately two-fold more water
(including rainfall), yield of all experimental treatments was only about 10% less, with no differences
between PRD and RDI. However tuber quality of the conventional crop was high, while the
experimental crop had a high incidence of common scab and poor skin finish. This (micro)biological
response limits the applicability of deficit irrigation to potato, since the soil moisture content required to
preserve tuber quality was far greater than that which would trigger a physiological drought response.
However, we anticipate that substantial water savings can still be delivered by improving irrigation
scheduling and targeting delivery of water more efficiently to help maintain tuber quality and reduce
water and nutrient losses to the environment. This work is being carried out in field trials at EMR
(WU0118).
Experiments with pot-grown (5L) lettuce imposed four irrigation treatments (nominally 115%,
100%, 85% & 70% of measured ET), with the latter two treatments decreasing ET by an average of
11% and 23%. There was no difference in ET between the two most generous watering regimes
despite differences in soil water content at harvest, indicating wasteful water application in the highest
treatment. The 100% irrigation treatment maximised marketable shoot fresh weight, but marketable
heads were also achieved with the 85% irrigation treatment. The severest irrigation treatment resulted
in dark-green, unmarketable heads, due to chlorophyll accumulation and bitter taste.
To summarise, while deficit irrigation had positive (or neutral) impacts on berry and fruit crops,
significant risk of unmarketable crops exists in lettuce and potato subjected to deficit irrigation.
Objective 7 To ensure effective technology transfer
To enable grower interaction and facilitate technology transfer, several trials were conducted on
commercial holdings: poinsettia throughout the project (Staplehurst Nurseries, Kent), potatoes in 2006
(Hilton of Fern, Angus) and tomatoes in 2005 (Flavourfresh, Southport, Lancashire). The research at
the first two enterprises is discussed in Objective 6, while that at Flavourfresh is briefly discussed
below.
A trial was conducted with rockwool-grown tomatoes in a commercial greenhouse near
Lancaster. Transplants were placed atop paired rockwool slabs, allowing equal root establishment
between both slabs. Significant effects of PRD on stomatal conductance (up to 30%) and total sap flow
(22% at mid-day) were typically restricted to periods of higher VPD when cloud cover was low and
radiation load high. Although PRD did not significantly affect individual fruit fresh weight and mean
weekly yield through the season, fruit SSC values increased by 16% and dry matter allocation to the
fruit increased by 20%. Despite these advantages, higher setup costs likely will prevent adoption of
PRD by the industry.
We have already transferred the technology and knowledge from this project to the UK soft fruit
industry (HL0187, SF 107) and we have continued to interact closely with key representatives of the
protected potted ornamental sector during the development of our HortLINK concept note. While
industry representatives have been informed of our results with other crops, the outcomes of these
trials does not, at present, seem to justify rapid technology transfer to industry.
A complete list of technology transfer activities can be found in Appendix 1
SID 5 (Rev. 3/06)
Page 24 of 28
Future work
Six new projects have been developed, based on the successful delivery of the project objectives:
 HortLINK HL0187: Improving water use efficiency and fruit quality in field-grown strawberry,
2007-2012 (EMR)
 WU0110: Developing novel water-saving irrigation strategies to produce fruit with more
consistent flavour and quality and an improved shelf-life, 2007-2012 (EMR)
 WU0118: Improving water use efficiency and tuber quality in potato production by optimising
irrigation scheduling, 2008-2011 (EMR)
 SF 107: Managing water, nitrogen and calcium inputs to optimise flavour and shelf-life in soilless strawberry production. 2009-2012 (EMR)
 EU-funded SIRRIMED project comprising deficit irrigation of tomatoes and potatoes (Lancaster
- commencing October 2009)
 A concept note on improving energy, water, and nutrient use efficiencies and reducing pesticide
use in protected pot plant production has recently been submitted to the HortLINK programme
(EMR)
Main implications of the findings
During the course of this project, we sought to identify opportunities to deliver substantial water savings
while maintaining marketable yields and improving product quality in several sectors of UK horticulture.
Five new research projects have already been developed as a direct result of the progress made in this
project. These strategic and applied projects will assist Defra in meeting the objectives in Policy Area
WU01 and will also deliver into WQ01 since precise and targeted irrigation will also reduce fertiliser
inputs that should, in turn, help to reduce diffuse pollution. These approaches can be expected to
deliver significant economic gains and more efficient production systems that optimise the use of
valuable resources such as water, fertiliser and labour and reduce waste during production. Novel
irrigation strategies developed in this project are also transferable to many other irrigated crops and
new proposals on potted ornamental plants, raspberry and potted living herbs are currently being
developed.
In terms of the potential to deliver water savings, PRD, DI and RDI are all very similar.
However, our results indicate that specific irrigation strategies should be used with some cropping
systems to ensure that yields and product quality are maintained or improved. For example, although
PRD may lead to yield losses in substrate-grown plants compared to DI or RDI, both techniques may
be suitable for field-grown crops. The severity of the deficit irrigation regimes is also critical to success;
although the more severe regimes may use less water, crop yields and quality are nearly always
reduced. Our work with strawberry has shown that substantial water savings can be made simply by
scheduling irrigation effectively. This represents a low risk strategy for growers with none of the
inherent risks associated with managing deficit irrigation. Tools for effective irrigation scheduling have
been developed and tested in this project and, combined with efficient and targeted delivery of irrigation
water and fertiliser, these offer immediate opportunities for improving WUE and nutrient use efficiency
(NUE) in many growing systems (e.g. potato).
Of all of the crops studied, potato is the most important in terms of its water use. The potato
sector uses at least 56% all water abstracted for irrigation in England, and 25% of all water used in
agriculture; about half of this water is used to control ‘common scab’ (Defra WU0101). However,
potatoes are largely grown in eastern regions where water resources are most threatened
(Environment Agency, 2007) and abstraction rates are predicted to rise by a further 30% by 2050
(Defra WU0102). Although our results suggest that deficit irrigation can reduce potato tuber quality
(skin finish) leading to losses in marketable yield, there are other opportunities for delivering water
savings. Genetic improvement programmes are already underway in potato (e.g. HH3615SPC,
HP0218) and can be expected to produce new commercial varieties with improved water use efficiency
(WUE) within 15-20 years. This approach will eventually help to lessen the impact of the predicted
climate change scenarios of dry summers, especially in Southern England (UKCIP08, Jenkins et al.,
2007). In the short-term, improved irrigation scheduling and high-precision delivery methods will help to
secure more immediate water savings without impacting on yields and tuber quality. These approaches
are curremtly being developed in WU0118.
The large number of technology transfer activities undertaken by the project consortium has
also helped to draw attention to issues of water availability and water use efficiency among growers in
SID 5 (Rev. 3/06)
Page 25 of 28
many different sectors, especially in regions such as the south and east where resources are most
threatened. In addition, our work on deficit irrigation has highlighted the potential benefits on crop or
plant quality that can be achieved with effective irrigation management strategies.
References
ADAS (2003) Irrigation Best Practice Grower Guide – Top and Soft Fruit;
Blackman PG, Davies WJ (1985) Journal of Experimental Botany 36, 39-48.
Davies WJ, Bacon MA, Thompson DS, Sobeih W, Gonzalez Rodriguez L (2000) Journal of
Experimental Botany 51, 1617-1637.
Davies WJ, Kudoyarova G, Hartung W (2005) Journal of Plant Growth Regulation 24, 285-295.
Davies WJ, Zhang J (1991) Annual Review of Plant Physiology and Plant Molecular Biology 42, 5576.
Dodd IC (2005). Plant and Soil 274, 251-270.
Dodd IC (2007) Functional Plant Biology 34, 439–448.
Dodd IC (2008) Acta Horticulturae 792, 225-231.
Dodd IC (2009) Journal of Experimental Botany in press
Dodd IC, Beveridge CA (2006) Journal of Experimental Botany 57, 1-4
Dodd IC, Davies WJ, Safronova VI, Belimov AA (2008a) Acta Horticulturae 792, 233-239.
Dodd IC, Egea G, Davies WJ (2008b) Plant, Cell and Environment 31, 1263-1274.
Dodd IC, Egea G, Davies WJ (2008c) Journal of Experimental Botany 59, 4083-4093.
Dodd IC, Egea G, Davies WJ (2009) Acta Horticulturae in press
Dodd IC, Theobald JC, Bacon MA, Davies WJ (2006) Functional Plant Biology 33, 1081-1089.
Dry PR, Loveys BR, Botting D, During H (1996) Proceedings of the 9th Australian Wine Industry
Technical Conference, 126-131
Else MA, Hall KC, Arnold GM, Davies WJ, Jackson MB (1995). Plant Physiology 107, 377-384.
Gowing DJG, Davies WJ, Jones HG (1990) Journal of Experimental Botany 41, 1535–1540.
Guilioni L, Jones HG, Leinonen I, Lhomme JP (2008) Agricultural and Forest Meteorology 148,
1908-1912
Jia W, Davies WJ (2007) Modification of leaf apoplastic pH in relation to stomatal sensitivity to root
sourced ABA signals. Plant Physiology 143, 68-77.
Jimenez A, Creissen G, Kular B, Firmin J, Robinson S, Verhoeyen M, Mullineaux P (2002). Planta
214, 751-758.
Jones HG (2004a) Journal of Experimental Botany 55, 2427-2436.
Jones HG (2004b) Advances in Botanical Research Incorporating Advances in Plant Pathology 41,
107-163.
Kirnak H, Kaya C, Higgs D, Bolat I, Simsek M, Ikinci A (2003). Australian Journal of Experimental
Agriculture 43, 105-111.
Kudoyarova GR, Vysotskaya LB, Cherkozyanova A, Dodd IC (2007) Journal of Experimental
Botany 58, 161-168
Leinonen I, Jones, HG (2004) Journal of Experimental Botany 55, 1423-1431.
Loveys BR, Stoll M, Davies WJ (2004) In ‘Water use efficiency in plant biology’ (Ed MA Bacon) pp.
113-141. (Blackwell: Oxford, UK)
Mingo DM, Theobald JC, Bacon MA, Davies WJ, Dodd IC (2004) Functional Plant Biology 31, 971978.
Pudney S, McCarthy MG. (2004). Acta Horticulturae 664, 567-573.
Sobeih WY, Dodd IC, Bacon MA, Grierson D, Davies WJ (2004) Journal of Experimental Botany 55,
2353-2363.
Stoll M, Loveys B, Dry P (2000) Journal of Experimental Botany 51, 1627-1634.
Tanaka Y, Sano T, Tamaoki M, Nakajima N, Kondo N, Hasezawa S (2005). Plant Physiology 138,
2337-2343.
Wang SY (1999). Journal of Plant Growth and Regulation 18, 127-134.
Yuan BZ, Sun J, Nishiyama S (2004). Biosystems Engineering 87, 237-245.
SID 5 (Rev. 3/06)
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References to published material
9.
This section should be used to record links (hypertext links where possible) or references to other
published material generated by, or relating to this project.
1. Bacon MA (2004) Water use efficiency in plant biology Blackwell Publishing, Sheffield, pp. 1-22.
ISBN 1-4051-1434-7.
2. Loveys BR, Stoll M, Davies WJ (2004) In Bacon, M.A. (Ed) Water Use Efficiency of Plants.
Blackwell. Oxford. pp113-141. ISBN 1-4051-1434-7.
3. Davies WJ, Hartung W (2004) In, New directions for a diverse planet: Proceedings of the 4th
International Crop Science Congress Brisbane, Australia, www.cropscience.org.au.
4. Mingo D, Theobald JC, Bacon MA, Davies WJ, Dodd IC (2004) Functional Plant Biology 31,
971-978
5. Grant OM, Stoll M, Jones HG (2004) Journal of Horticultural Science and Biotechnology 79, 125130.
6. Jones HG (2004) In Water use efficiency in plant biology. (ed. M.A.Bacon), Blackwell Publishing,
Sheffield, pp. 27-41. ISBN 1-4051-1434-7.
7. Leinonen I, Jones, HG (2004) Journal of Experimental Botany 55, 1423-1431.
8. Jones HG (2004) Journal of Experimental Botany 55, 2427-2436.
9. Sobeih W, Dodd IC, Grierson D, Bacon MA, Davies, WJ (2004) Journal of Experimental Botany
55, 2365-2384.
10. Jones HG (2004) Advances in Botanical Research Incorporating Advances in Plant Pathology 41,
107-163.
11. Davies WJ, Kudoyarova G, Hartung W (2005) Journal of Plant Growth Regulation 24, 285-295.
12. Dodd IC, Davies WJ (2005) In: Plant Hormones: Biosynthesis, Signal Transduction, Action! P.J.
Davies ed. Kluwer Academic Publishers, Dordrecht, The Netherlands. pp 493-512
13. Dodd IC (2005) Plant and Soil (Root physiology : from gene to function Special Issue) 274, 257275.
14. Pospisilova J, Dodd IC (2005) In Handbook of Photosynthesis 3rd edition (Ed: M Pessarakli)
Marcell Dekker, pp 811-825.
15. Dodd IC, Beveridge CA (2006) Journal of Experimental Botany 57, 1-4
16. Davies WJ (2006) In Plant Growth and Climate Change. (Eds. JIL Morison and M Morecroft)
Blackwells, Oxford. pp 96-117.
17. Dodd IC, Theobald JC, Bacon MA, Davies WJ (2006) Functional Plant Biology 33, 1081-1089.
18. Grant O, Chaves MM, Jones HG (2006) Physiologia Plantarum 127, 507-518.
19. Dodd IC (2007) Functional Plant Biology 34, 439–448.
20. Kudoyarova GR, Vysotskaya LB, Cherkozyanova A, Dodd IC (2007) Journal of Experimental
Botany 58, 161-168
21. Jia W, Davies WJ (2007) Plant Physiology 143, 68-77.
22. Morison, JIL, Baker NR, Mullineaux PM, Davies WJ (2007) Philosophical Transactions of the
Royal Society 363, 639-658.
23. Dodds PAA, Taylor JM, Else MA, Atkinson CJ, Davies WJ (2007) Acta Horticulturae 744, 295302.
24. Jones HG (2007) Journal of Experimental Botany 58, 119-130.
25. Grant OM, Tronina L, Jones HG, Chaves MM (2007) Journal of Experimental Botany 58, 815825.
26. Stoll M, Jones HG (2007) Journal International des Sciences de la Vigne et du Vin 41, 77-84
27. Dodd IC, Egea G, Davies WJ (2008) Plant, Cell and Environment 31, 1263-1274.
28. Dodd IC (2008) Acta Horticulturae 792, 225-231.
29. Dodd IC, Davies WJ, Safronova VI, Belimov AA (2008) Acta Horticulturae 792, 233-239.
30. Jones HG (2008) Acta Horticulturae 792, 391-403
31. Loveys BR, Jones HG, Theobald JC, McCarthy MG (2008) Acta Horticulturae 792, 421-427.
32. Guilioni L, Jones HG, Leinonen I, Lhomme JP (2008) Agricultural and Forest Meteorology 148,
1908-1912
33. Dodd IC, Egea G, Davies WJ (2008) Journal of Experimental Botany 59, 4083-4093.
34. Dodd IC, Egea G, Davies WJ (2009) Acta Horticulturae in press
35. Dodd IC (2009) Journal of Experimental Botany in press
SID 5 (Rev. 3/06)
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