Water use efficiency in agriculture
Bill Davies
The Lancaster Environment Centre, UK
Summary
• Introduction and definitions
• Impacts of stomata, environment and leaf
metabolism on WUE
• Estimating WUE and modifications through
breeding
• Impact of agronomy on WUE
Global agriculture now accounts for 70% of the amount of
water used on Earth at a time when global population
continues to spiral
Environmental Limitation
Area affected (%)
Drought
25.3
Shallowness
19.6
Cold
16.5
Wet
15.7
Alkaline soils
2.9
Saline or no soil
4.5
Other
3.4
None
12.1
After Boyer, (1982)
Crop losses due to water loss out weigh crop losses due
to any other causes
After Boyer, (1982)
There is a large different between record and average
yields suggested a large amount of variation due to the
environment
Some definitions of WUE
CO2 assimilated/Water lost
harvest index or total biomass/applied water
harvest index or total biomass/water transpired
harvest index or total biomass/water uptake
harvest index or total biomass/water available
WUE can be defined either in terms of an instantaneous
measurement of the efficiency of carbon gain for water
loss; or an integral of this efficiency over time (as a ratio
of water use to biomass or yield accumulation). Important
to define terms precisely.
Yd/El (× 103)
C3 plants:
Cereals
Other Poaceae
Alfalfa
Pulses
Sugar beet
Native plants
1.47 - 2.20
0.97 - 1.58
1.09 - 1.60
1.33 - 1.76
2.65
0.88 - 1.73
C4 plants:
Cereals
Other Poaceae
Other C4
2.63 - 3.88
2.96 - 3.88
2.41 - 3.85
After Briggs & Shantz (1914)
A landmark study was carried out by Briggs and Shantz
(1914) in Akron, Colorado, leading to the development of
the Akron series.
6
Yield (t ha-1)
5
4
3
2
1
0
0
100
200
300
400
Water use (mm)
There is a clear relationship between the amount of water
transpired and yield across a diverse range of crop
species – water loss is an inescapable trade-off for carbon
gain.
Instantaneous WUE is defined as the ratio of CO2
assimilation into the photosynthetic biochemistry (A) to
water lost, via transpiration through the stomata (T).
A and T are regulated by stomatal conductance (gs) and
the respective concentration gradients of water and CO2
between the inside (wi and ci) and the outside of the leaf
(wa and ca).
lower ci
The driving force for CO2 uptake and water loss are
independent.
Therefore increasing ca (the external
concentration of CO2) will increase instantaneous WUE,
as the driving force for water loss will remain
unchanged, while that for CO2 uptake will increase.
A plant can lower ci and achieve a lower ci/ca by closing
its stomata (to limit CO2 diffusion into the leaf); increasing
photosynthetic capacity; or both.
The relationship between water loss and stomatal
conductance is effectively linear meaning that a reduction
in gs will generally result in a reduction in water loss,
while partial stomatal closure does not always reduce
carbon gain
But, there are issues when plants grow in communities
And an issue with changing leaf temperature. Stomata close,
reduce transpiration which causes the leaf to heat up and this drives
transpiration harder
This difference in water use efficiency between C3 and C4
species is a result of the increased driving force for CO2
uptake generate by C4 biochemistry and tissue structure
This difference in water use efficiency between C3 and C4
species is a result of the increased driving force for CO2
uptake generate by C4 biochemistry and tissue structure
Consequently, C4 species can achieve comparable
assimilation rates at lower stomatal conductances and
lower ci, significantly enhancing their WUE.
Traditional breeding programmes have tried to introduce
C4 characteristics into C3 plants, but with little success. It
seems tat biotechnology may achieve some increase in
WUE when C4 traits are introduced into C3 plants
CAM (species that show Crassulacean acid metabolism)
plants have enhanced WUE (due to temporal shifts in CO2
fixation), but their low rate of biomass accumulation
makes introduction of CAM traits into crop species
undesirable.
Carbon isotope discrimination has been used to assess
the genetic variability in the driving force for CO2 uptake.
Two stable isotopes of carbon are found in molecular CO2
(13C and 12C) with a ratio of 1:99 in atmospheric air. Plant
tissues contain considerably less 13C. They are said to
discriminate against it. This is a function of its larger
molecular mass, which slows its rate of diffusion.
A lower ci/ca results in decreased discrimination (∆13C)
against 13C as the driving force for CO2 diffusion is
increased.
Consequently, carbon isotope signatures
demonstrating low ∆13C are diagnostic of a CO2 fixation
environment with a relatively low ci/ca.
Water Use Efficiency (g DM Kg -1 H20)
5.5
4.5
3.5
19
20
13C
21
discrimination (103 X ∆)
There is a strict relationship between the degree of ∆13C
and WUE in crop species, such that tissue with low ∆13C
exhibits, enhanced instantaneous WUE. In other words
low ∆13C is indicative of a low Ci/Ca (Page 2450 of Condon
et al. (2004) JXB 55: 2449).
Decreased ∆13C could however be the result of decreased
stomatal conductance or high mesophyll photosynthetic
capacity (or both).
Page 2449 of Condon et al. (2004) JXB 55: 2449
HIGH MESOPHYLL ACTIVITY
PARTIAL STOMATAL CLOSURE
Does the theory translate into practice?
1
Relative ∆ 13C
L. esculentum
F1 hydrid
L. pennellii
0
2.4
2.5
2.6
2.7
2.8
WUE (g DM Kg-1 H2O)
What is clear is that WUE is heritable – you can breed it
into and out of crops, using conventional breeding
strategies.
Rebetzke, G.J., Condon, A.G., Richards, R.A. and Farquhar, G.D. (2002)
Selection for reduced carbon isotope discrimination increases aerial
biomass and grain yield of rainfed bread wheat. Crop Science 42, 739745.
This has recently led to the commercial production of new wheat
varieties known as ‘Drysdale’ and ‘Rees’ with high WUE, quality and
level of disease resistance.
Greatest yield advantages are realised in locations with low yields – due
to drought? See JXB p.2454-2455 Condon et al. (2004).
Concerns over use of carbon isotope discrimination (e.g. slower growth
rates associated with increased photosynthetic efficiency).
Probable combination of small decrease in conductance and small
increase in photosynthetic capacity, neither of which has a major
individual negative effect
When water evaporates from the sub-stomatal cavity, the
leaf becomes enriched with H218O, due to its slower rate
of diffusion. As the driving force for water loss increases
(due to increased stomatal aperture), this enrichment
decreases.
The ability of tissues to demonstrate discrimination
between water molecules containing the two stable
isotopes of oxygen (18O and 16O) may also be of use.
Consequently dual screening for low ∆13C and low ∆18O
would
identify highly WUE individuals which have
exhibited high levels of transpiration (i.e. enhanced WUE
is more likely to be the result of enhanced mesophyll
photosynthetic capacity).
A gene for WUE?
• Recently the Farquhar group reported the
identification of ERECTA, the first gene that
regulates transpiration efficiency. ERECTA and
other members of the gene family are involved in
the control of guard cell density and mesophyll
structure.
Masle, Gilmour and Farquhar 2005, Nature 436,
866-870
Shpak et al., 2005, Science 309, 290-293 .
WUE and Agronomy
• The major agronomic way of increasing
transpiration efficiency is to maximize the growth
of crops during periods of low vapour-pressure
deficits. Thus in Mediterranean-type climates
autumn sowing rather than spring sowing has a
major influence on transpiration efficiency as a
greater proportion of the autumn-sown crop's life
occurs during the period of low vapour-pressure
deficits in winter (Fischer, 1981 ; Singh et al.,
1997 ; Richards et al., 2002 ).
Seasonal evaporation from soil (mm)
200
150
100
50
0
0
20
40
60
80
100
Fractional area of shaded soil at flowering (%)
Rapid leaf development in annuals has also been
suggested to contribute to the efficient use of soil water.
By establishing maximal leaf area quickly, evaporation
from the soil is minimised and stored soil water is
conserved.
The relationship between transpiration efficiency of wheat and pan evaporation for various
months in the southern hemisphere (left of the line) and the northern hemisphere (right of the
line) (adapted from Fischer, 1981, and Richards et al
Turner, N. C. J. Exp. Bot. 2004 55:2413-2425; doi:10.1093/jxb/erh154
Copyright restrictions may apply.
Changes with time in decadal wheat yields from 1860 to 2000 with explanations for the trends
(from Angus, 2001, with permission from CSIRO Publishing)
Turner, N. C. J. Exp. Bot. 2004 55:2413-2425; doi:10.1093/jxb/erh154
Copyright restrictions may apply.