1
Root and shoot growth by seedlings of annual and perennial medic, and annual and
perennial wheat.
P. R. Ward, J. A. Palta and H. Waddell
Text pages:
18
Tables:
1
Figures:
3
Running title:
Root:shoot ratio in perennial seedlings
Corresponding author: Dr. Phil Ward
CSIRO Plant Industry
Private Bag No 5
Wembley WA 6913
Australia
Phone +61 8 9333 6616
Fax +61 8 9387 8991
Email [email protected]
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1
Root and shoot growth by seedlings of annual and perennial medic, and annual and
2
perennial wheat.
3
P.R. Ward1, J.A. Palta1 and H.A. Waddell1,2
4
1
CSIRO Plant Industry, Private Bag No 5, Wembley WA 6914, Australia.
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2
Current address: School of Plant Biology, University of Western Australia, 35 Stirling
6
Highway, Nedlands WA 6009, Australia.
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Abstract
8
Perennial plants such as lucerne are now widely acknowledged as one means of
9
controlling the expansion of dryland salinity in southern Australia. However, their
10
inclusion in farming systems is limited by poor seedling vigour, thought to be associated
11
with greater allocation of biomass to perennating organs in roots, and poor adaptation to
12
some soils and climatic conditions in the region. For this reason, interest in other
13
perennial options such as perennial wheat is increasing. In this research we compared
14
early growth and root:shoot ratios for annual and perennial medics (Medicago truncatula
15
and M. sativa), and for annual and perennial wheat. For the medics, the annual produced
16
more root and shoot biomass, but there was no difference in root:shoot ratio or depth of
17
root growth. For wheat, there were no differences in root growth, shoot growth, or
18
root:shoot ratio between the annual and perennial lines. The poor competitive
19
performance of M. sativa seedlings was not due to changed allocation of biomass to
20
shoots, but was probably related more to seed size. This does not seem to occur to the
21
same extent in perennial wheat lines, suggesting that their seedling performance may be
22
more competitive.
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1
Key words
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Alfalfa, perennial crops, seedling vigour, secondary salinity
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Introduction
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Dryland salinity is one of the major limitations for agriculture in south-western Australia.
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Currently, about 1.0 Mha is severely salinised (about 10% of the available cropping area),
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and 2.8-4.4 Mha is at high risk of developing dryland salinity (McFarlane et al. 2004).
7
The spread of dryland salinity in the region has been linked to changes in the
8
hydrological balance associated with the removal of native deep-rooted vegetation and
9
replacement with annual species which are comparatively shallow-rooted (George et al.
10
1999; McFarlane and Williamson 2002). Evapotranspiration from annual vegetation over
11
the whole year is less than from native vegetation (Cocks 2001), resulting in higher rates
12
of deep drainage below the root zone. The subsequent rise in the water table leads to the
13
mobilisation and accumulation of salts in the root zone (McFarlane and Williamson
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2002).
15
Recent research has indicated that the spread of dryland salinity can be controlled through
16
the incorporation of perennial plants into annual cropping rotations (Dunin et al. 1999;
17
Ward et al 2006), allowing agroecosystems to more closely mimic natural ecosystems
18
(Hatton and Nulsen 1999). Presently the most widespread herbaceous perennial species
19
for this purpose is lucerne (Medicago sativa), which has a deeper rooting system in
20
comparison to annual species (Dolling et al. 2005a; Ward et al. 2002, 2006). Typically,
21
lucerne has a dimorphic root system with most of the root biomass occurring in the top
22
0.05-0.1 m of the soil profile (Denton et al. 2006) but roots at depths of more than 4 m
3
1
have been reported (Ward et al. 2003; Fillery and Poulter 2006). Lucerne is able to
2
extract more water from deeper in the soil in comparison to annual pastures or crops,
3
creating a buffer zone in which to store water which may otherwise become groundwater
4
recharge (Halvorson and Reule 1980; Latta et al. 2001; Dunin et al. 2001; Ward 2006).
5
Lucerne is known to be poorly adapted to waterlogged (Cocks 2001; Bell 2005), and
6
acidic soils (Pijnenborg et al. 1990; Humphries and Auricht 2001). In south-western
7
Australia, these factors combined with low and variable summer rainfall have
8
substantially limited the areas suitable for lucerne production (Hill 1996; Dolling et al.
9
2005b). Even in regions identified as suitable for lucerne production, good lucerne
10
pastures have been difficult to establish, due to poor seedling vigour (Taylor 1987;
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Humphries and Auricht, 2001) and associated competition from weeds and susceptibility
12
to insect attack. Poor seedling vigour in many perennials is often attributed to the
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supposed large proportion of resources allocated by perennials to roots rather than shoots
14
during early growth (DeHaan et al. 2005; Glover 2005). Evidence supporting this
15
hypothesis for lucerne is equivocal. For instance, Joost and Hoveland (1986) measured
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root growth of lucerne and Lespedeza (Lespedeza sericea), and concluded that Lespedeza
17
produced more root growth per unit of shoot growth than lucerne, but their units of
18
measurement (cm of visible roots in a glass-walled root box per gram of shoot matter)
19
make comparisons with other research difficult. Zebian and Reekie (1998) found that
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over the first six days of growth after germination, lucerne allocated around 50% of its
21
carbon to roots, but they did not continue measurements for a longer period. Neither of
22
these studies compared lucerne seedling root growth with seedling root growth of a
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closely-related annual species.
4
1
Difficulties associated with lucerne establishment and early growth in south-western
2
Australia have focussed attention on the use of other perennial species to prevent the
3
spread of dryland salinity. In particular, perennial cereal crops could be attractive, as they
4
would fit easily into current farming systems (DeHaan et al. 2005; Glover 2005; Cox et
5
al. 2006; Bell et al. 2008). Research into perennial wheat has been ongoing in the USA
6
(Suneson et al. 1963; Cox et al. 2006; Glover et al. 2010) for many years, in an attempt
7
to improve its grain yield. Although much developmental work still needs to be
8
undertaken, perennial grain crops are showing some promise for future farming systems.
9
However, the competitive ability of seedlings of perennial wheat, and root:shoot ratios
10
relative to those for traditional wheat varieties, are not known. This study compares the
11
early root and shoot growth patterns, and root:shoot ratios, of seedlings of annual and
12
perennial wheat and annual and perennial species from the Medicago genus, with the aim
13
of determining whether seedlings of perennial plants allocate more resources to roots than
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seedlings of closely-related annual plants in two widely different genera.
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Materials and methods
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Plant material and experimental design
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Barrel medic (Medicago truncatula cv. A17), lucerne (M. sativa cv Sceptre), wheat
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(Triticum aestivum cv. Wyalkatchem), and a perennial line of wheat (Triticum x
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Agropyron line PI 550713, obtained from The Land Institute, USA) (Suneson et al.
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1964), were chosen as treatments for the study, representing an annual and a perennial
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medic, and an annual and perennial wheat. Four replicate pots of each species were
22
arranged in a randomised block design in a naturally-lit glasshouse, with a photoperiod of
5
1
about 14 hours. Day and night temperatures were maintained at 20°C and 14°C
2
respectively.
3
The pots used were the same as those described by Liao et al. (2006). Briefly, the pots
4
were approximately rectangular in cross-section (0.24 x 0.1 m), with a depth of 1.0 m.
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One wall of each pot was made of clear Perspex, to allow root growth to be observed
6
directly. The bottom 0.1 m of each pot was filled with gravel, and the remainder of the
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pot (to within about 0.1 m of the top of the pot) was filled with a yellow sandy soil
8
(Arenic Yellow-Orthic Tenosol: Isbell 2002) obtained from the Wongan Hills Research
9
Station in Western Australia. The soil was dried, put through a 2 mm sieve, and packed to
10
a bulk density in the pots of approximately 1530 kg/m3.
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Plant growth and nutrition
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Annual medic, perennial medic, and annual and perennial wheat seedlings were
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germinated in a petri dish for two days prior to being transplanted into the pots. Eight
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seedlings were planted into each pot. At the time of planting, fertiliser was mixed into the
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top 0.1 m of soil at a rate equivalent to 100 kg N/ha as urea, and 35 kg P/ha as
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superphosphate for the cereal treatments; and 12 kg K/ha as potash and 7 kg P/ha as
17
superphosphate was added for the legume treatments. A granular Group F commercial
18
rhizobium (ALOSCA Technologies Pty Ltd) was added to the legume treatments at 1 day
19
after sowing (DAP). At 9 DAP, seedlings were thinned to four plants per pot. Additional
20
fertiliser as nutrient solution (containing nitrogen as nitrate 3.3 mg/l, nitrogen as urea
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23.5 mg/l, nitrogen as ammonium 2.9 mg/l, phosphorous 6.1 mg/l, potassium 9.9 mg/l,
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sulphur 0.24 mg/l, iron 0.2mg/l, magnesium 0.6 mg/l, manganese 0.04mg/l, zinc 0.02
6
1
mg/l, boron 0.006 mg/l, copper 0.006 mg/l, and molybdenum 0.002 mg/l), was added at 9
2
and 23 DAP, with each pot receiving about 0.1 l. All plants were watered as required,
3
typically 2-3 times per week.
4
Measurements
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At 4 DAP roots were traced onto transparent film as described by Liao et al (2006), and
6
thereafter root growth assessments continued twice per week except for the week of 24-
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30 December 2006 (13-19 DAP). Mapping ceased at 29 DAP, when the first cereal roots
8
reached the bottom of the pot. Plants were harvested at 31 DAP by cutting the shoots
9
from the roots at the crown. Each plant was partitioned into stems (including petioles)
10
and leaves, with the stem of wheat defined as the area from the crown to the first leaf
11
sheath. Leaf area was measured for each pot using a Li-Cor LI-3100 leaf area meter. The
12
above-ground biomass was determined by drying samples at 70°C for 48 hours before
13
weighing.
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Immediately after the shoots were harvested, the soil in the pot was divided into 0.1 m
15
increments. Roots were recovered from each soil layer using a 2.0 mm sieve as described
16
by Palta and Fillery (1993). Root length and diameter were measured as described by
17
Liao et al. (2006). Briefly, roots were stored in plastic containers at 4ºC until they were
18
stained with 0.1% (w/v) methylene blue and scanned at a resolution of 600 dots per inch
19
using a ScanJet Iic, Hewlett Parkard scanner. Images were analysed using ROOTEDGE
20
software (Iowa State University and United States Department of Agriculture). Dry
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weight of roots was determined after scanning by drying at 70°C for 48 hours.
7
1
Statistical analysis
2
Comparisons among the treatments were performed with Genstat Version 10.1, using
3
repeated measures analysis of variance, where depth was treated as the repeated measure.
4
The analysis was performed this way because measurements at a specific depth are not
5
necessarily independent of measurements at the depths above and below. Differences
6
were considered significant at the 5% level.
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Results
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Root growth
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There were differences in the visual patterns of root growth between the wheat lines and
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the medics (Figure 1). Both the annual and perennial wheat lines grew roots to a depth of
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1.0 m within the duration of the experiment (29 days), whereas the two medics only
12
reached a visual depth of 0.4 to 0.5 m. Furthermore, the wheat lines exhibited
13
considerably more lateral root growth. As expected from these visual observations, there
14
were significant differences in total root length and root dry weight between wheat lines
15
and medics (P = 0.008). The interaction between plant group (wheat versus medic) and
16
plant type (annual versus perennial) was not significant.
17
Within the two wheat lines, there were no significant differences in total root dry matter
18
or total root length (Table 1). However, the annual medic produced significantly more
19
total root dry matter and total root length than the perennial medic. All plant types
20
produced the majority of their roots close to the soil surface (Figure 2). Within a soil
21
depth, there were no significant differences between the two wheat lines in root weight,
8
1
root length or root diameter. For the medics, the annual medic produced significantly
2
more root biomass at depths of 0.0 to 0.1 m and 0.1 to 0.2 m, and significantly more root
3
length at 0.1 to 0.2 m. All other comparisons between root growth of the two medics were
4
non-significant (Figure 2). Medic roots were significantly thinner in diameter than wheat
5
roots at the same soil depth, but there were no significant differences between annual and
6
perennial plants.
7
Rate of root growth, as measured by change in maximum visible root depth with time,
8
was significantly faster for the two wheat lines than the medics (Figure 3). However, the
9
difference between annual and perennial root growth rate was not significant for either
10
wheat (P = 0.080) or medic (P = 0.553).
11
Shoot growth
12
At the conclusion of the experiment, the annual wheat reached Zadoks growth stage 23,
13
and the perennial wheat reached growth stage 21. The annual and perennial medic
14
reached 6-leaf and 4-leaf stages respectively. The wheat lines produced significantly
15
greater shoot biomass (0.26 g/plant), greater leaf weight (0.20 g/plant) and greater leaf
16
area (57.5 cm2) than the medics (0.063 g/plant, 0.039 g/plant, and 14.0 cm2 respectively).
17
The interaction term and the annual versus perennial comparison were not significant in
18
any of these tests. A significant interaction was observed for specific leaf area, with
19
means of 287 cm2/g for the annual wheat, 287 cm2/g for the perennial wheat, 348 cm2/g
20
for the annual medic, and 384 cm2/g for the perennial medic.
21
Within the wheat lines, there were no significant differences between the annual and the
22
perennial for total shoot weight, leaf weight, leaf area or specific leaf area (Table 1).
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1
Significant differences were observed within the medics, with the annual medic
2
producing more biomass and more leaf area than the perennial medic, but having a lower
3
specific leaf area (Table 1).
4
Root:shoot ratio
5
Root:shoot ratios were not significantly different in any of the one-way (Table 1) or two-
6
way (legume versus cereal P = 0.20) ANOVAs.
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Discussion
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Medics
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Lucerne (or alfalfa), the perennial medic used in this study, is known to be a poor
10
competitor as a seedling (Humphries and Auricht 2001). One of the aspects emphasised
11
by agronomists in southern Australia is that weed control during the establishment phase
12
is critical for success. Many agronomists have assumed that lucerne seedlings, being
13
perennial, direct more of their photosynthate below ground (compared with other annual
14
pasture legumes), to ensure a strong root system which will enable plants to survive the
15
hot and dry summer (eg Thomas 2003). The resulting smaller allocation of resources to
16
shoots was thought to reduce seedling vigour. The results presented here indicate that
17
under the conditions used in this study, the poor competitive ability of lucerne is not
18
related to a higher proportion of root growth at the expense of shoot growth. The
19
root:shoot ratio of perennial medic seedlings was not significantly different to that for a
20
closely-related annual medic. However, total root and shoot production was significantly
10
1
lower for the perennial medic compared with the annual medic, indicating an inherently
2
low competitive ability.
3
Humphries and Auricht (2001) suggest that lucerne’s slow growth might be related to
4
seed size. In a survey of several lucerne varieties, Bolanos-Aguilar et al. (2002) measured
5
average lucerne seed weight of between 1.9 and 2.2 mg. By contrast, M. truncatula seeds
6
weigh about 3.5 mg (Gallardo et al. 2006). The differences in seed weights are consistent
7
with differences in total root and shoot growth observed in this study.
8
Although seedlings of the annual medic produced more shoot and root biomass than the
9
perennial medic in this study, the maximum depth of root growth was similar for both
10
medics. Providing relatively more resources to deeper roots at the seedling stage could be
11
one way in which the perennial medic has adapted to increase its chances of long-term
12
survival.
13
For lucerne seedlings, Esechie et al. (2002) measured root:shoot ratios of between 0.6
14
and 0.9 depending on the level of irrigation water salinity. Similarly, Bolinder et al.
15
(2002) measured root:shoot ratio for lucerne of about 0.7 for the first year of growth. Our
16
values of around 0.4 are considerably lower, and may reflect the role of photoperiod on
17
root development. Although temperatures were maintained close to those expected under
18
normal growing conditions, our experiment was conducted with a day length of around
19
14 hours, whereas seedlings of lucerne would normally be grown in south-western
20
Australia with a day length of 10-12 hours. Noquet et al. (2003) showed that in mature
21
lucerne plants, nitrogen partitioning to roots approximately halved when day length was
22
changed from 8 hours to 16 hours. Similar findings for carbon allocation in seedlings
11
1
would result in the lower root:shoot ratios measured in the current study, but further
2
research is necessary to confirm this.
3
Wheat
4
The perennial wheat used in this research was derived from open crosses between
5
Triticum aestivum and Agropyron species (Suneson et al. 1963). The specific aim of the
6
wheat breeding program was to develop perennial wheat lines with high yield, and
7
reasonable grain quality. Preliminary trials at Floreat in Western Australia indicated that
8
the line PI550713 as used in this study was perennial (for at least 3 years) under local
9
conditions provided that watering was maintained during the summer months. As with
10
the medics, there was no difference in seedling root:shoot ratio between the annual and
11
perennial cereals, indicating that the perennial plant does not allocate any extra resources
12
to produce a larger root system at the seedling stage. Gregory et al. (1996) and Liao et al.
13
(2006) described similar root:shoot ratios of around 0.3 for many different wheat
14
genotypes, and it is interesting that perennial wheat also has a root:shoot ratio close to
15
this value.
16
In terms of the suitability of perennial wheat for incorporation into farming systems,
17
much work still needs to be done (Bell et al. 2008). As noted above, the perennial habit
18
was only demonstrated under well-watered conditions, which is unrealistic for the vast
19
majority of the southern Australian cereal-producing areas. Furthermore, the potential
20
role for perennial wheat in combating dryland salinity, through deeper root systems, is
21
neither confirmed nor refuted by this research, as any differences in root growth between
22
annual and perennial genotypes is likely to occur after the early tillering stage. Although
12
1
it is difficult to extrapolate field results from pot experiments under controlled conditions,
2
the fact that perennial wheat allocated a similar proportion of photosynthate below
3
ground compared with traditional wheat varieties, and grew at a similar rate, indicates
4
that it might be a better competitor against weeds than lucerne during the establishment
5
phase.
6
Conclusions
7
Neither perennial wheat, nor perennial medic, allocated a higher proportion of biomass to
8
root production during early growth than their annual counterparts. Rates of root and
9
shoot growth were similar for the wheat lines, but the annual medic produced more
10
biomass both above and below ground than the perennial medic. Despite this, depth of
11
root growth was similar for the two medics. Results presented here suggest that lucerne’s
12
(the perennial medic) poor competitive ability as a seedling is not due to allocation of
13
photosynthate to perennating organs, but might be simply related to poor vigour due to
14
seed size. In contrast, perennial wheat was approximately equally competitive compared
15
with traditional wheat varieties, and could be as easy to establish as traditional wheat
16
varieties.
17
Acknowledgements
18
Thanks to Stan Cox and Greg Rebetzke for supplying the perennial wheat seeds. Heidi
19
Waddell was funded through the Australian Pastoral Research Trust.
13
1
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18
1
1
Table 1. Early root and shoot growth, leaf area and specific leaf area of annual and perennial wheat, and annual and perennial medic.
Wheat
2
3
Medic
Annual
Perennial
p value
Annual
Perennial
p value
Root biomass (g/plant)
0.107
0.082
0.55
0.036
0.017
0.03
Root length (m/plant)
25
18
0.46
6.7
3.1
0.05
Root:Shoot ratio
0.33
0.40
0.39
0.42
0.43
0.90
Shoot biomass (g/plant)
0.32
0.21
0.23
0.08
0.04
0.02
Leaf area (cm2/plant)
67
48
0.32
19
9
0.03
Specific leaf area (cm2/g)
287
287
0.95
348
384
<0.01
1
1
Captions for Figures
2
Figure 1. Example patterns of root growth for annual wheat, perennial wheat, annual
3
barrel medic, and perennial lucerne, as observed from the clear face of the pot.
4
Figure 2. Root length (a and b), root dry matter (c and d), and root diameter (e and f)
5
distributions for annual and perennial wheat (a, c and e), and annual and perennial medic
6
(b, d and f). Error bars represent LSD values: significant differences are indicated by *.
7
Figure 3. Rate of root growth (maximum depth of visible roots) for annual wheat,
8
perennial wheat, annual medic, and lucerne. The two wheat types were significantly
9
different to the two medic types, but there were no significant differences between annual
10
and perennial for either wheat or medic.
2
1
Figure 1.
Perennial
Wheat
0.1
0.2
0.3
Pot
depth
(m)
0.4
0.5
0.6
0.7
0.8
0.9
2
Annual
Wheat
Lucerne
Barrel
Medic
1
Figure 2.
WHEAT
0.0
0
5
MEDIC
Root Length (m/plant)
10
15
20
(a)
0
2
4
6
8
(b)
*
0.2
0.4
0.6
Perennial
Annual
LSD
0.8
1.0
Root weight (g/plant)
0.00
0.0
0.04
0.08
0.12 0.00
0.01
0.02
0.03
0.04
*
(d)
(c)
*
0.2
Soil depth (m)
1
0.4
0.6
0.8
1.0
Root diameter (mm)
0.0
0.0
0.1
(e)
0.2
0.4
0.6
0.8
1.0
0.2
0.3
0.00
0.05
(f)
0.10
0.15
0.20
2
1
Figure 3
2
Days after sowing
0
5
10
15
Depth of deepest visible root (m)
0.0
0.2
0.4
0.6
0.8
1.0
Annual wheat
Perennial wheat
Annual medic
Perennial medic
20
25
30
35