independent and combined effects of soil warming and drought stress during anthesis on seed set and...
TRANSCRIPT
DROUGHT STRESS
Independent and Combined Effects of Soil Warming andDrought Stress During Anthesis on Seed Set andGrain Yield in Two Spring Wheat VarietiesD. F. Weldearegay1,*, F. Yan2,*, D. Jiang3 & F. Liu1
1 Department of Agriculture and Ecology, Faculty of Sciences, University of Copenhagen, Taastrup, Denmark
2 Jilin University, College of Plant Science, Changchun, China
3 Key Laboratory of Crop Physiology and Ecology in Southern China, Ministry of Agriculture/Hi-Tech Key Laboratory of Information Agriculture of
Jiangsu Province, Nanjing Agricultural University, Nanjing, China
Introduction
At the end of this century, the mean global atmospheric
temperature is predicted to rise by 2.9 �C (IPCC 2007).
One of the attributes of climate warming is irregular
increase in mean and variance of temperature at the soil
surface (Jones and Mann 2004). In addition, global
warming will not only affect air and soil temperatures but
also influence the amount and distribution of precipita-
tion leading to more frequent drought spells in the sum-
mer in many European countries including Denmark
(Christensen and Christensen 2007). Therefore, it is cru-
cial to understand how crops like spring wheat respond
to climate change and to explore whether there are varie-
ties that can better adapt to soil warming and drought
stress during their critical period of development.
To date, numerous studies have been carried out evalu-
ating the impacts of increased air temperature on wheat
Keywords
abscisic acid; anthesis; drought; grain yield;
seed set; soil warming; Triticum aestivum
Correspondence
F. Liu
Department of Agriculture and Ecology
Faculty of Sciences
University of Copenhagen
Højbakkegaard Alle 13
DK-2630 Taastrup, Denmark
Tel.: +45 353 33416
Fax: +45 353 33478
Email: [email protected]
*These authors contributed equally to the
research work.
Accepted February 24, 2012
doi:10.1111/j.1439-037X.2012.00507.x
Abstract
Increase in soil temperature together with decrease in soil moisture during
anthesis of spring wheat (Triticum aestivum L) crops is predicted to occur
more frequently in a future climate in Denmark. The objective of this study
was to investigate the responses of two Danish spring wheat varieties (Trappe
and Alora) to soil warming (H), drought (D) and both (HD) during anthesis.
The plants were grown in pots in a climate-controlled glasshouse. In H, the soil
temperature was increased by 3 �C compared with the control (C). In both D
and HD treatments, the plants were drought-stressed by withholding irrigation
until all of the transpirable soil water had been depleted in the pots. Results
showed that, particularly under D treatment, Alora depleted soil water faster
than Trappe. In both varieties, flag leaf relative water content (RWC) was sig-
nificantly lowered, while spikelet abscisic acid (ABA) concentration was signifi-
cantly increased by D and HD treatments. Compared with the C plants, D and
HD treatments significantly reduced ear number, ear to tiller ratio, shoot bio-
mass, grain yield, harvest index and seed set but hardly affected tiller number
and 1000-kernel weight, whereas H treatment alone only decreased shoot bio-
mass and reduced seed set. When analysed across the varieties and the treat-
ments, it was found that the reduction in seed set was closely correlated with
the increase in spikelet ABA concentration, indicating that D and HD treat-
ments induced greater spikelet ABA concentrations might have caused seed
abortion. It was concluded that the grain yield reduction under D and HD
treatments during anthesis in spring wheat is ascribed mainly to a lowered seed
set and wheat varieties (i.e. Alora) with more dramatic increase in spikelet
ABA concentration are more susceptible to D and HD treatment.
J. Agronomy & Crop Science (2012) ISSN 0931-2250
ª 2012 Blackwell Verlag GmbH 1
crop development and yield (e.g. Leaky et al. 2009, Wang
et al. 2011), while very little is known about how soil
warming may affect wheat in a temperate climate. Gavito
et al. (2001) reported that soil temperature had significant
effects on root development, biomass and nutrient uptake
of winter wheat during vegetative growth. However,
Hartley et al. (2007) found that soil warming remarkably
increased soil respiration but had no effects on plant
development and biomass accumulation for wheat. More
recently, Patil et al. (2010a,b) reported that soil warming
by 5 �C during the whole growing season of winter wheat
speeded up crop vegetative growth. In addition, these
authors also found that soil warming did not reduce grain
yield but reduced soil N losses and increased mildew
attack on the crop. Climate change scenarios predict that
more frequent heat waves may occur from late spring to
early summer when the wheat crop enters the reproduc-
tive stages, for example, the anthesis stage. At this stage,
the plants are very sensitive to any environmental pertur-
bations and a few days of soil warming may have signifi-
cant effects on crop growth and yield. However, until
now this aspect has not been investigated.
Drought is one of the most important factors limiting
crop productivity. Crop plants are very sensitive to
drought stress particularly during the early reproductive
stage (Saini and Westgate 2000). Severe drought during
anthesis could decrease grain yield up to 50 % by reduc-
ing the number of grains (Saini and Westgate 2000). Sev-
eral physiological mechanisms may explain the increased
seed abortion caused by anthesis drought (Liu et al.
2005). It has been suggested that phytohormones are
involved in the regulation of crop reproductive develop-
ment under drought stress. For instance, abscisic acid
(ABA) concentration in crop reproductive structures
increases significantly when the plants are drought-
stressed during flowering (Morgan 1980, Morgan and
King 1984). In wheat, seed set is negatively correlated
with the endogenous ABA concentration under drought
(Westgate et al. 1996); and application of ABA to the leaf
sheath of well-watered plants inhibits floret development,
decreases the number of fertile florets and grain set (Mor-
gan 1980, Waters et al. 1984). Similarly, Liu et al. (2003)
showed that reduction of pod set in soybean is associated
with increasing ABA concentration in flowers and young
pods. Based on these findings, it is possible that crop
varieties which have lower ABA concentration in the
reproductive organs under anthesis drought may perform
better in achieving greater grain yield than those with
higher ABA concentrations. However, to date this possi-
bility has not been explored. Nevertheless, it should be
noted that increases of ABA concentration in the xylem
sap can also act as an earlier signal inducing stomatal clo-
sure and reduction of leaf expansion growth during soil
drying, which reduces transpiration water loss thereby
preventing dehydration of the shoots and enhancing the
chance for survival under prolonged drought (Liu et al.
2005). It has been reported that exogenous ABA applica-
tion increases pod set in soybean by maintaining a
favourite leaf water status and high photosynthetic rates
under anthesis drought stress (Liu et al. 2004), indicating
a protective role of the hormone in sustaining crop yield
under stressful conditions (Liu et al. 2005).
Impact of abiotic stress factors on crop plants has been
mostly studied by applying a single stress factor such as
drought or high temperature in controlled experiments
(Guilioni et al. 2003). However, field crops often encoun-
ter multiple stresses such as drought and elevated soil
temperature simultaneously during a certain stage of
development (Zhang et al. 2008). In fact, the combination
of drought and high temperature is the major stressful
factor that restricts wheat growth and production in
many regions (Khan et al. 2009). Earlier studies have
demonstrated that combined drought and air heating sig-
nificantly decreased crop growth and yield (Savin and
Nicolas 1996, Wang and Huang 2004), whereas the effects
of combined drought stress and soil warming on crop
yield remain unknown. In this study, the plant water sta-
tus, spikelet ABA concentration and yield responses of
two Danish spring wheat varieties to soil warming,
drought and both at anthesis were investigated. We
hypothesized that the two wheat varieties perform differ-
ently in response to single or combined soil warming and
drought treatments at anthesis stage; and the phytohor-
mone ABA level in the plant plays an important role in
determining seed set and grain yield formation of spring
wheat varieties grown under multiple stress.
Materials and Methods
Experimental set-up
A pot experiment in a glasshouse was conducted at the
experimental station of the Faculty of Life Sciences, Uni-
versity of Copenhagen, Taastrup, Denmark in 2010–2011.
The pots had a volume of four litters (25 cm in height
and 15.2 cm in diameter) and were filled with 2.4-kg peat
material (Sphagnum, 32 % organic matter, pH = 5.6–6.4
and EC = 0.45 ms cm)1). In the glasshouse, air tempera-
ture and relative air humidity were 16/12 �C (day/night)
and 62 %, respectively. Two spring wheat varieties, viz.
Trappe and Alora, currently being widely cultivated by
seed companies, farmers, seed multipliers and research
organizations in Denmark were used. The seeds were sown
on 5 November 2010 directly into the pots at a density of
six seeds per pot. When the plants were 15 cm in height,
thinning was carried out leaving three plants with even
Weldearegay et al.
2 ª 2012 Blackwell Verlag GmbH
size in each pot. Each variety had 25 pots but only 20 rep-
resentative pots with uniform seedlings were selected for
treatment. Each treatment had five pots (five replications).
During the vegetative growth phase (before anthesis), all
plants were drip irrigated with nutrient solution (in total
2.5 g N, 1.3 g P and 1.8 g K were applied to each pot) to
prevent any deficiency of nutrients. During treatment,
there were no nutrients given to the plants to avoid the
potential effect of nutrient availability on the treatments.
Four treatments were imposed during anthesis (anthesis
started on 31 January 2011); soil warming (H), in which
the pots were placed on a heating carpet and the soil tem-
perature was increased by 3 �C compared with the control
(C) for 13–14 days; drought (D), in which the irrigation
was withheld from the pots for 13–14 days until the tran-
spiration rate of the stressed plants was 10 % of the C
plants; and combined soil warming and drought (HD).
Measurements
Soil water status in the pots was determined by weighing
the pots daily at 9:00 am and was expressed as the frac-
tion of transpirable soil water (FTSW) (Liu and Stutzel
2002, Jøgensen et al. 2010). The total transpirable soil
water (TTSW) in the pots was the difference between the
pot weight at 100 % holding capacity (i.e. pot weight of
4.7 kg) and pot weight when the transpiration rates of
the D and HD plants decreased to 10 % of the C plants.
Then, the daily value of FTSW was estimated by the ratio
between the amount of transpirable soil water remaining
in the pot and TTSW:
FTSW ¼ ðWTn �WTf Þ=TTSW ð1Þ
where WTn is actual pot weight on a given date and WTf
is pot weight at the time when the transpiration rate of
the D and HD plants were 10 % of the control plants.
By the end of the treatment, relative water content
(RWC) of flag leaf (one leaf per pot) was determined
according to the protocol by Jensen et al. (2000). The
RWC was calculated as follows:
RWC ¼ ðFW� DWÞ=ðTW� DWÞ � 100% ð2Þ
where FW is the leaf fresh weight, DW is the leaf dry
weight and TW is the leaf turgid weight. On the same
day, spikelet samples were collected (one spike per pot)
and were frozen immediately in liquid N for the analysis
of ABA concentration. The samples were finely ground
under liquid nitrogen, extracted in distilled water using
about 1 ml per 30 mg fresh weight. The extraction was
carried out at 4 �C on a shaker overnight. The extracts
were centrifuged at 12 000 g for 10 min at 4 �C and the
supernatants were assayed for ABA by enzyme-linked
immunosorbent assay (ELISA) using monoclonal
antibody (AFRC MAC 252) according to Asch (2000). No
cross-reactions of the antibody with other compounds in
the crude spikelet extracts were detected when tested
according to Quarrie et al. (1988).
At maturity, tiller number, ear number, shoot biomass,
grain yield, harvest index and 1000-kernel weight were
recorded. Seed set was scored by observing the presence
or absence of seed in all florets of five spikes from each
pot and calculating the ratio of fertile florets to total flo-
rets (expressed as a percentage).
Statistical analysis
The experiment was arranged in a completely randomized
design with five replications (five pots for each treat-
ment). The pots were rearranged every 3 days in the
glasshouse to avoid any placement effects on the treat-
ments. Data were subjected to three-way analysis of vari-
ance (Three-way anova) procedures (SAS 9.2, Cary, NC,
USA). Standard errors of the means (S.E.) were calcu-
lated.
Results
Soil water status
Alora depleted the plant available soil water much faster
than did Trappe under D treatment, while the two varie-
ties had similar rate of soil water depletion under HD
treatment (Fig. 1). During the treatment period, FTSW in
the C and H pots was similar and was kept at 0.77–0.93
for both varieties (data not shown).
Flag leaf RWC and spikelet ABA concentration
The RWC of flag leaf was similar for the two varieties
(Fig. 2a; Table 1); when analysed across the two varieties,
it was found that RWC was significantly affected by H
and D treatments, and there were significant interactive
effects between H and D on RWC (Fig. 2a; Table 1). Both
variety and treatment had significant effects on spikelet
ABA concentration (Fig. 2b; Table 1). Between the two
varieties, Alora had significantly higher spikelet ABA con-
centration than Trappe (Fig. 2b; Table 1). For both varie-
ties, D and HD treatments significantly increased spikelet
ABA concentration, while the increases were much evi-
dent in Alora than in Trappe (Fig. 2b; Table 1).
Tiller and ear number
At maturity, tiller number per pot was significantly
greater for Alora than for Trappe, and the H, D and HD
Soil Warming and Drought Stress Effects on Wheat
ª 2012 Blackwell Verlag GmbH 3
treatments did not affect tiller number for both varieties
(Fig. 3a; Table 1). Ear number per pot was affected by
both the variety and D and HD treatment (Fig. 3b;
Table 1) and was significantly greater in Alora than in
Trappe. For both varieties, D and HD treatments signifi-
cantly decreased ear number per pot as compared with
the C and H treatments (Fig. 3b; Table 1).
Ear to tiller ratio was significantly affected by both the
variety and the D and HD treatment (Fig. 3c; Table 1).
Across the treatment, Alora had significantly higher ear to
tiller ratio than Trappe. For both varieties, D and HD
treatments decreased the ear to tiller ratio but the reduc-
tion was more pronounced for Trappe than for Alora
(Fig. 3c; Table 1).
Shoot biomass, grain yield and yield components
Shoot biomass was affected significantly by both the vari-
ety and the treatment. Between the two varieties, shoot
0
500
1000
1500
Trappe AloraVariety
Spi
kele
t AB
A c
once
ntra
tion
(pm
ol g
–1 F
W)
(b)0
20
40
60
80
100
120
140
Flag
leaf
RW
C (%
)
C H D HD (a)
Fig. 2 Flag leaf relative water content (RWC) and spikelet ABA con-
centration of two spring wheat varieties grown under soil warming
(H), control (C), combined soil warming and drought (HD) and
drought (D) treatments at anthesis. Error bars indicate S.E. (n = 5).
0.0
0.2
0.4
0.6
0.8
0 2 4 6 8 10 12 14 16DAT
HD
0.0
0.2
0.4
0.6
0.8
1.0FT
SW
TrappeAlora
D
Fig. 1 Changes of the fraction of transpirable soil water (FTSW) for
two spring wheat varieties grown under drought (D) and combined
soil warming and drought (HD) treatments. DAT denotes days after
onset of treatment. Error bars indicate the standard error of the mean
(S.E.) (n = 5).
Table 1 Three-way anova for flag leaf relative water content (RWC), spikelet ABA concentration (ABA), tiller number per pot (T), ear number per
pot (E), ear to tiller ratio (E : T), shoot biomass (SB), grain yield (GY), harvest index (HI), 1000-kernel weight (TK) and seed set (SS) of the two
spring wheat varieties (V) as affected by the soil warming (H), drought (D) and combined soil warming and drought (HD) treatments (data are pre-
sented in Figs 2–5)
Factor RWC ABA T E E : T SB GY HI TK SS
V ns *** *** *** *** *** ** ns ** ***
H * ns ns ns ns ** ns ns ns *
D *** *** ns *** *** *** *** *** ns ***
V · H ns ns ss ns ns ns ns ns ns ns
V · D ns *** ns ns *** ns ns ns ns ns
H · D * ns ns ns ns ns ns ns ns ns
V · H · D ns ns ns ns ns ns * *** ns ns
*, **, and *** indicate significance level at P £ 0.05, P £ 0.01 and P £ 0.001, respectively; ns, no significance.
Weldearegay et al.
4 ª 2012 Blackwell Verlag GmbH
biomass was significantly higher for Alora than for Trappe
(Fig. 4a; Table 1). For both varieties, H, D and HD
treatments significantly decreased shoot biomass as
compared with the C treatment. There was no interactive
effect between the variety and the treatment on shoot
biomass.
Grain yield was affected by the variety and the D and
HD treatments. Alora had significantly higher grain yield
compared with Trappe (Fig. 4b; Table 1). For both varie-
ties, D and HD treatments significantly decreased grain
yield, while H treatment alone did not significantly affect
grain yield. There was significant interactive effect
between the variety and the treatment on grain yield
(Table 1). Harvest index was similar for the two varieties
(Fig. 4c; Table 1). There was a significant interactive effect
of the variety and the treatment on harvest index. Com-
pared with the C plants, harvest index was significantly
decreased by D and HD treatments in Alora, while it was
decreased only by HD treatment in Trappe.
Trappe had significantly higher 1000-kernel weight
than Alora; however, across the treatment, there was no
difference in 1000-kernel weight (Fig. 5a; Table 1). Seed
set was significantly affected by both the variety and the
treatment (Fig. 5b; Table 1). Across the treatments, seed
set was significantly higher for Trappe than for Alora;
when analysed across the varieties, it was found that C
plants had the highest seed set, H plants had slightly
lower seed set than C plants and D and HD plants had
the lowest seed set.
Relationships between spikelet ABA concentration, seed
set and grain yield
To analyse the sensitivity of the measured bio-physiological
parameters and yield traits to H, D, and HD treatments,
the spikelet ABA concentration, seed set and grain yield
of the H, D and HD plants were expressed relatively to
those of the C plants. It was found that, across the
varieties and the treatments, there was a clear negative
linear relationship between the relative seed set and the
0
10
20
30
40
Tille
r num
ber p
er p
ot
C H D HD (a)
0
10
20
30
Ear
num
ber p
er p
ot
(b)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Trappe AloraVariety
Ear
to ti
ller r
atio
(c)
Fig. 3 Tiller per pot (a), ear per pot (b) and ear to tiller ratio (c) of
two spring wheat varieties grown under control (C), soil warming (H),
drought (D) and combined soil warming and drought (HD) treatments
at anthesis. Error bars indicate S.E. (n = 5).
0
15
30
45
60
75
1000
-ker
nal w
eigh
t (g)
C H D HD (a)
50
60
70
80
90
100
Trappe AloraVariety
See
d se
t (%
)
(b)
Fig. 4 Shoot biomass (a), grain yield (b) and harvest index (c) of two
spring wheat varieties grown under control (C), soil warming (H),
drought (D) and combined soil warming and drought (HD) treatments
at anthesis. Error bars indicate S.E. (n = 5).
Soil Warming and Drought Stress Effects on Wheat
ª 2012 Blackwell Verlag GmbH 5
relative spikelet ABA concentration (Fig. 6); and the rela-
tive grain yield was positively correlated with the relative
seed set (Fig. 7).
Discussion
It is well known that wheat plants are very sensitive to
drought and high-temperature stresses particularly when
those occur during anthesis (Balouchi 2010, Wang et al.
2011). It is also widely recognized that interspecies and
intraspecies differences in drought or high-temperature
tolerance exist which provide the opportunity for select-
ing varieties that are suitable for a future drier and war-
mer climate. In the present study, we imposed soil
warming (H), drought (D) and the combined soil warm-
ing and drought (HD) treatments to two Danish spring
wheat varieties during anthesis. The results showed that
although soil warming treatment alone hardly affected
most of the bio-physiological and yield traits investigated,
it significantly reduced shoot biomass (Fig. 4a) and seed
set (Fig. 5b; Table 1). D and HD treatments negatively
influenced the flag leaf RWC and grain yield while signifi-
cantly increased spikelet ABA concentration in both varie-
ties; however, the magnitude of changes differed between
the two varieties. For instance, under D treatment, Alora
depleted the plant available soil water much faster than
did Trappe (Fig. 1a) and which had resulted in slightly
0
20
40
60
80S
hoot
bio
mas
s (g
pot
–1) C H D HD (a)
0
15
30
45
Gra
in y
ield
(g p
ot–1
)
(b)
0.0
0.2
0.4
0.6
Trappe AloraVariety
Har
vest
inde
x
(c)
Fig. 5 1000-kernel weight (a) and seed set (b) of two spring wheat
varieties under control (C), soil warming (H), drought (D) and com-
bined soil warming and drought (HD) treatments at anthesis. Error
bars indicate S.E. (n = 5).
r2 = 0.76 (P = 0.024)
0.7
0.8
0.9
1.0
1.1
0 1 2 3 4 5 6Rel. Spikelet ABA concentration
Rel
. See
d se
t
TrappeAlora
Fig. 6 Relationship between relative spikelet ABA concentration and
relative seed set of two spring wheat varieties exposed to soil warm-
ing (H), drought (D) and both (HD) treatments during anthesis. Error
bars indicate S.E. (n = 5).
r2 = 0.79 (P = 0.018)0.4
0.6
0.8
1.0
1.2
0.7 0.8 0.9 1 1.1Rel. Seed set
Rel
. Gra
in y
ield
Trappe
Alora
Fig. 7 Relationship between relative seed set and relative grain yield
of two spring wheat varieties exposed to soil warming (H), drought
(D) and both (HD) treatments during anthesis. Error bars indicate S.E.
(n = 5).
Weldearegay et al.
6 ª 2012 Blackwell Verlag GmbH
lower RWC of the flag leaf. Coincided with this, the
increase in spikelet ABA concentration was also much
pronounced for Alora than for Trappe (Fig. 2b). This
result is in good agreement with that reported by Asch
et al. (2009) in maize, who found that plants grown in
fast drying soils increased xylem ABA concentration more
dramatically than those grown in slow drying soils. In
comparison with Trappe, the more dramatic increase in
spikelet ABA concentration under D and HD treatment
for Alora might have caused the greater reduction of seed
set in this variety as exemplified by the negative linear
relationship between the relative seed set and the relative
spikelet ABA concentration (Fig. 6). Such a relationship
implies that ABA level in the reproductive organs during
anthesis plays a crucial role in determining seed number
and hence final grain yield of wheat crops under D and
HD treatments, consistent with earlier findings in
drought-stressed soybean (Liu et al. 2003, 2004) and
wheat plants (Westgate et al. 1996). However, Asch et al.
(2001) concluded that ovary ABA concentration does not
induce kernel abortion in field-grown maize under anthe-
sis drought stress. In addition, Liu et al. (2003) suggested
that the ABA accumulated in the reproductive organs of
soybean might be transported from the roots via the
xylem, and greater ABA concentrations in the reproduc-
tive organs might indicate higher ABA concentrations in
the xylem sap. If this was also the case here for wheat,
one would have expected that the ABA concentration in
the xylem sap would have been higher in Alora as com-
pared with Trappe under D and HD treatments. A strong
xylem-borne ABA signal is expected to induce early sto-
matal closure hereby curtailing the transpiration rates.
This was obviously not the case here as the Alora plants
depleted the soil water much faster than did Trappe, the
latter had much lower ABA concentration in the spikelet
(Fig. 2b). Moreover, as the sensitivity of plant physiologi-
cal processes to ABA level differs at different stages, even
a slight increase in ABA concentration may cause repro-
ductive failure at its critical stage, for example, floral initi-
ation stage (Saini and Westgate 2000). Here, the ear to
tiller ratio decreased more pronounced in Trappe than in
Alora in response to D and HD stresses, indicating there
were more sterile tillers in Trappe (Fig. 3c). This could
have been due to that the floral initiation in the late til-
lers of Trappe occurred just around the end of the D and
HD treatment when the stresses were most severe; a mod-
erate increase of ABA concentration in the plants might
have caused tiller sterile and hence a reduced ear to tiller
ratio. However, for Alora, the floral initiation in the later
tillers might have been started after the termination of
the D and HD treatments and therefore might have not
severely been affected by the treatments. However, these
speculations need to be verified in future studies.
In the present study, it was seemingly that the soil
warming treatment alone had minor effect on most of the
traits investigated. Nevertheless, soil warming significantly
decreased shoot biomass and seed set in both wheat varie-
ties (Figs 4a and 5b). These results indicate that a 3 �C
increase in soil temperature in this experiment caused a
reduction in biomass accumulation and probably a
decrease in carbohydrate availability for reproductive
development. These effects could be due to the fact that a
higher temperature around the roots has caused greater
respiration rates and larger carbohydrate losses in the
wheat plants (Hill et al. 2007). On the contrary, Gavito
et al. (2001) observed that an increase in soil temperature
from 10 to 15 �C significantly increased leaf and stem
biomass of winter wheat during vegetative growth. The
reasons for this discrepancy are unknown, most probably
due to the differences in the growing conditions (e.g. the
optimal growing temperature could be differing between
wheat varieties investigated) and the plant developmental
stages between the two experiments.
Combined H and D treatment, viz. HD treatment, is
expected to have more pronounced effects on the wheat
plants than those caused by a single H or D treatment.
This was seemingly true in the present study even though
the differences between D and HD plants were mostly not
significant. Nonetheless, the flag leaf RWC was signifi-
cantly lower in HD than in D plants (Fig. 2a) indicating
that the combination of H and D treatment had resulted
in more severe plant water deficit than D treatment alone.
This might have caused the relatively higher spikelet ABA
concentration in the HD than in D plants (P = 0.283).
Furthermore, as has been discussed previously, greater
ABA concentrations in the spike might have led to an
increased seed abortion and thus to a reduced seed set in
the two wheat varieties under HD treatment (Fig. 6).
Compared with seed set, 1000-kernel weight was less
sensitive to either D or HD treatment for both wheat
varieties (Fig. 4a). Thus, the grain yield reduction under
D and HD treatment was attributed mainly to decrease in
seed number. This conclusion is supported by the posi-
tively linear relationship between the relative grain yield
and the relative seed set (Fig. 7). However, it should be
noted that grain yield is the product of individual grain
weight and grain number; and the two yield components
often compensate each other to achieve a maximal grain
yield under suboptimal growth conditions (Liu et al.
2005). It has been observed that a lowered seed set caused
by anthesis drought stress could lead to bigger seeds at
maturity in maize (Andersen et al. 2002). However, such
effect was not evident in the present study.
Collectively, our results showed that soil warming alone
had very little effect on leaf water status, spikelet ABA
concentration and grain yield of spring wheat varieties,
Soil Warming and Drought Stress Effects on Wheat
ª 2012 Blackwell Verlag GmbH 7
but significantly affected shoot biomass and tended to
reduce seed set. Compared with the H treatment, D and
particularly the HD treatment had more pronounced
effects on seed set and grain yield of spring wheat plants.
Between the two varieties, Alora was more susceptible to
HD and D treatments in terms of reductions in seed set
and grain yield. It was concluded that the grain yield
reduction under anthesis D and HD treatments in spring
wheat is ascribed mainly to a lowered seed set, and wheat
variety (i.e. Alora) with more pronounced increase in
spikelet ABA concentration is more susceptible to D and
HD treatment.
Acknowledgement
Financial support from ViVa – Water Research Initiative
at Faculty of Life Sciences, University of Copenhagen and
the National Natural Science Foundation of China
(31028017) is gratefully acknowledged.
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