Introduction
Water deficit is a
primary constraint for terrestrial plants (Chimungu et al. 2014) and drought stress greatly limits crop production and
food stability in arid and semiarid regions (Delpérée et al. 2003; Farooq et al.
2009). Plants have evolved various strategies to deal with drought stress,
which can be divided into drought avoidance, and the development of adaptation
mechanisms that contribute to drought
tolerance (Claeys and Inze 2013; Hussain
et al. 2018). Adaptive strategies that are induced during the drought
process help plants survive (Nguyen et
al. 2016). Plants such as maize (Zea
mays L.) reduce tissue dehydration, either via tolerance to lower tissue
water potential or by maintaining the water potential (Wu and Cosgrove 2000; Farooq
et al. 2017, 2018). To successfully
minimize water loss, plants limit transpiration by decreasing
leaf area, closing their stomata and accelerating leaf senescence, resulting in
improved water-use efficiency (WUE) (Franks
et al. 2015; Farooq et al. 2019)
or reduced photosynthesis (Shi et al.
2016). To protect against drought-induced oxidative damage to the cells,
osmotically active metabolites rapidly increase
(Vasquez-Robinet et al. 2008; Claeys
and Inze 2013). Plants also respond to water deficit by inducing sets of both
regulatory and functional genes (Yoshida
et al. 2014; Simmons and Bergmann 2016). Recent studies in Arabidopsis (Arabidopsis thaliana) have shown that phytohormones such as cytokinin or abscisic
acid induce signaling pathways that activate or negatively regulate
the
genes necessary for
drought-acclimation responses (Yoshida et
al. 2014; Nguyen et al. 2016).
Transcriptional regulation is a vital
regulatory mechanism mediating drought tolerance (Yoo et al. 2010; Simmons and Bergmann 2016).
Tartary buckwheat (Fagopyrum tataricum) is an important
traditional edible and medicinal crop with excellent nutritional and
pharmacological properties (Zhu 2016). Tartary buckwheat is usually cultivated
in the mountainous areas of western China, northern India, Bhutan, and Nepal at
high altitude. Therefore, it is extremely well-adapted to harsh climatic
conditions (Zhou et al. 2015; Zhang et al. 2017) and it regularly suffers
from drought stress in its main growing regions (Xiang et al. 2020). Thus,
Tartary buckwheat is an ideal plant to study adaptation mechanisms under
drought conditions. Therefore, to better understand the specific
drought-adaptive responses with respect to
morphological plasticity, biochemical indexes, photosynthetic and chlorophyll
fluorescence parameters, WUE and seed yield, two contrasting Tartary buckwheat
genotypes [drought-susceptible Chuanqiao No. 1 (CQ) and drought-tolerant
Jinqiao No. 2 (JQ)] were sown under progressive drought and recovery at
flowering stage.
Materials and Methods
Growth
conditions and materials
The experiment was
conducted in a greenhouse at Chengdu University (30°39′N, 104°11′E,
altitude 490 m asl). Two Tartary buckwheat genotypes, previously evaluated for
their yield responses to water stress, were used in this experiment: the drought-tolerant
cv. JingQiao No. 2 (JQ) and drought-susceptible cv. ChuanQiao No. 1 (CQ),
selected for by the Shanxi Academy of Agricultural Sciences and the
Agricultural Science Institute of Zhaojue County, Liangshan Yi Autonomous
Prefecture, respectively. The experiment was initiated on 4 Mar 2018. Seeds
were sterilized with 0.1% potassium permanganate solution for 30 min and then
rinsed three to four times with distilled water. They were then sown in plastic
pots (20 cm high, 25 cm diameter) in a waterproof shelter. One plant was
allowed to grow per plastic pot to the three-and-a-half leaf stage. The plastic
pots were allowed to drain freely from the bottom and contained 15 kg soil
(sandy loam, with 1.87 g kg-1 total nitrogen, 17.3 g kg-1
organic matter, 1.49 g kg-1 P2O5, and 15.8 g
kg-1 total potassium). At the experimental site of Chengdu
University, the annual mean air temperature was 16°C, total precipitation was
900–1300 mm, and average sunshine duration was 1042–1412 h overall year.
Drought
treatment and recovery
The pots were weighed
daily. A fully watered pot was set at 100% water-holding capacity. For the well-watered (WW)
treatment, plants were watered daily to maintain 90% water-holding capacity. For the water deficit and recovery (WD-R) treatment,
plants from bud-appearing stage were exposed to gradual soil water
depletion (50% water-holding capacity) for 14 days, then all plants were re-watered daily to the initial pot
water-holding capacity for 9 days
of recovery. Pots were weighed daily during both water-deficit and recovery
periods. There were 60 pots for each treatment. Each treatment had four independent
replicates, arranged in a completely randomized designed. The imposition of drought simulated possible conditions in the
field.
Microenvironment
for plants
During the experiment, soil
water content (SWC) and soil electrical conductivity (SEC) were measured in the
top 20 cm of the soil every 2 days using a WET-2-K1 time-domain reflectometry probe (Delta-T Devices Ltd., Cambridge, U.K.).
Evaluation of leaf
wilting (LW) and leaf relative water content (RWC)
To analyze LW, 10 plants
from 10 pots of each treatment were monitored daily at 08:00 h. LW was graded
on a five-point scale: level 1, normal; level 2, curling slightly; level 3,
curling slightly with middle and lower leaves drooping; level 4, curling
heavily with all leaves drooping; level 5, rolling into a cylindrical shape
with all leaves drooping and the tip growth point wilting.
To measure leaf RWC, leaf samples were weighed for fresh
weight (FW) and drenched with deionized
water at 4°C for saturated weight (SW), then oven-dried at 80°C for 72 h for
dry weight (DW). Leaf RWC (%) was calculated as:
%20483-491_files/image002.png){kind=small}
(Barrs
and Weatherley 1962)
Gas exchange traits
The net photosynthesis
rate (Pn), intercellular CO2
concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr) of
the last fully developed and expanded leaf was examined on days 5, 9, 14, 19 and
23 of the experimental period with a portable
photosynthesis system (LI-6400, LI-COR
Biosciences, Lincoln, NE, USA). These parameters were automatically recorded by
the machine at 10:00–12:00 h under atmospheric CO2 and full
sunlight. WUE was calculated as:
%20483-491_files/image004.png){kind=small}
Chlorophyll
(Chl) a fluorescence
Chl a fluorescence of the same leaves used for the gas-exchange
measurements was determined in leaf discs by pulse-amplitude modulated
fluorescence spectrometry (Mini-PAM, Heinz Walz, Pfullingen, Germany). The
initial Chl a fluorescence (F0)
was recorded after 30 min dark adaptation using a beam of 0.2 μmol m-1 s-1.
The maximum Chl a fluorescence (Fm)
was recorded at 8,000 μmol m-1
s-1 with a 0.8-s saturating pulse. The Overall Chl a fluorescence (Fv) was calculated as Fm – F0. Fv/Fm, the maximum efficiency of
photosystem II (PSII) photochemistry in the dark, was calculated as:
%20483-491_files/image006.png){kind=small}
(Dias and Bruggemann 2010)
Malondialdehyde
(MDA) and soluble protein (SP) contents, and activities of superoxide
dismutase (SOD) and peroxidase (POD)
Fresh leaf samples (last
fully developed leaves) were taken on the same days as the photosynthesis
measurements to determine MDA and SP contents, and the activities of SOD and
POD. Leaf samples were immediately frozen in liquid nitrogen after collection
and then stored at -80°C for further use. Contents of MDA and SP were measured
with suitable modifications of Dhindsa’s method (Dhindsa et al. 1981) and Bradford’s method (Bradford 1976), respectively.
The activities of SOD and POD were determined by Giannopolitis and Ries’ method
(Giannopolitis and Ries 1977) and Nakano and Asoda’s method (Nakono and Asada
1980), respectively.
Morphological
measurements and grain yield
In each treatment, 20 plants
from 20 pots of each treatment were harvested at physiological maturity to
determine plant height, branch number, stem diameter, and number of nodes on
the main stem. Seed numbers for each plant, 1000-seed weight and grain yield
were recorded.
Statistical
analysis
Statistical analyses were
carried out using Excel 2010, S.P.S.S.
13.0 (Chicago, I.L., U.S.A.) and SigmaPlot 10.0 (Aspire Software International,
Ashburn, V.A., U.S.A.). Two-way ANOVA was used
to determine the significance level, and means were compared by Duncan’s multiple range
tests at a significance level of P < 0.05.
Results
Leaf RWC and
LW
During the
experiment, no significant fluctuations in leaf RWC (Fig. 1A) or LW (Fig. 1B)
were observed in either CQ or JQ plants under the WW treatment. A gradual
decrease in RWC for both Tartary buckwheat cultivars were observed during the
drought part of the WD-R treatment, but the JQ plants maintained a higher RWC
level than the CQ plants. On day 14 of the WD-R treatment, the RWC of CQ leaves
was 48.2% and that of JQ was 72.5%. Upon re-watering, RWC increased in both
cultivars. When subjected to water deficit, LW markedly increased for the two
genotypes, but was significantly lower for JQ plants vs. CQ plants. On day 14
of the WD-R treatment, CQ leaves showed a high level of drooping and tip growth
point wilting, whereas JQ leaves were slightly curled with middle and lower
leaf drooping. After re-watering, the LW in both genotypes gradually returned
to the WW levels.
Soil water
content (SWC) and soil electrical conductivity (SEC)
As shown
in Fig. 2A and B, neither SEC nor SWC changed significantly in the CQ or JQ
pots under the WW treatment. SEC in the WD-R pots of both genotypes increased
significantly during the drought part of the treatment compared to the WW
treatment. On day 14 of the WD-R treatment, SEC in the CQ pots was
significantly higher than that in the JQ pots. After re-watering, SEC in the
WD-R pots of both genotypes gradually decreased to normal WW values. Opposite
trends were observed for SWC: SWC in CQ and JQ pots decreased by 44.6–51.1%
during the dehydration part of the WD-R treatment compared to the WW treatment.
On day 14 of the WD-R treatment, SWC in CQ pots had decreased more than in the
JQ pots. During the process of recovery in the WD-R treatment, the SWC in CQ
and JQ pots increased back to WW values.
Temporal dynamics of photosynthetic characteristics
and WUE
All measured
photosynthetic parameters and WUE of CQ and JQ plants in the WW treatment
remained stable during the experiment (Fig. 3A–D). Pn,
Ci, Gs and Tr of both genotypes declined consistently with decreasing soil
water availability during drought phase of the WD-R treatment. Pn,
Gs, Ci and Tr progressively increased with water application during the
recovery phase of the WD-R treatment. There were significant differences in Pn and
Gs between the two genotypes: JQ plants showed higher Pn
than CQ plants during the experiment. Pn, Gs, and Tr of the CQ and
JQ plants differed significantly (P < 0.05) between WW and WD-R
treatments on days 5, 9, 14, 19 and 23, while Ci differed on days 5, 9, 14 and
19. On day 14, Pn and Ci in CQ plants under the WD-R treatment were lowest
among all treatments and genotypes, reduced by 81.08 and 56.44%, respectively,
compared to JQ plants in the WW treatment (P < 0.01). Gs and Tr in JQ
plants under WD-R treatment were significantly lower than in the other
treatments. There was no difference in WUE between the two genotypes in the WW
treatment (Fig. 4). WUE in the JQ plants under the WD-R treatment was
significantly highest for all treatments and genotypes on days 5, 9, 14 and 19.
Especially on day 14, WUE in JQ plants under WD-R treatment was at its highest
point in the whole experimental phase.
Dynamic changes in Chl a fluorescence characteristic
F0
and Fv/Fm of CQ and JQ plants in the WW treatment were relatively constant
throughout the experiment (Fig. 5A and B). However, F0 was significantly
increased by water deficit in the WD-R treatment in both CQ and JQ plants on
days 9 and 14, while Fv/Fm was significantly decreased on day 14. Re-watering
induced a decline in F0 and an increase in Fv/Fm. On days 9 and 14, F0
of CQ under the WD-R treatment was highest among all
treatments and genotypes, while Fv/Fm of CQ under WD-R treatment was
lowest on day 14.
%20483-491_files/image008.jpg)
Fig. 1: Effect of drought stress and recovery phase on leaf relative water content (A) and leaf wilting rate (B) of two Tartary buckwheat genotypes
WW= Well-watered; WD-R= Water
deficit to recovery; CQ= Chanqiao No.1; JQ= JingQiao No.2; DAS= Days after
imposition of drought stress
%20483-491_files/image010.jpg)
Fig. 2: Effect of drought stress and recovery phase on soil
electrical conductivity (A) and soil
water content (B) of two Tartary
buckwheat genotypes
WW= Well-watered; WD-R= Water deficit to recovery; CQ=
Chanqiao No.1; JQ= JingQiao No.2; DAS= Days after imposition of drought stress
%20483-491_files/image012.jpg)
Fig. 3: Effect
of drought stress and recovery phase on Pn (A), Gs (B), Ci (C) and Tr (D) of two Tartary buckwheat genotypes. Different lowercase
letters in each figure indicate significant differences (P < 0.05) between treatments on each sampling days
WW= Well-watered; WD-R= Water deficit to recovery; CQ=
Chanqiao No.1; JQ= JingQiao No.2; DAS= Days after imposition of drought stress;
Pn=net photosynthetic rate; Gs= stomatal conductance; Ci= intercellular CO2
concentration; Tr= transpiration rate
%20483-491_files/image014.jpg)
Fig. 4: Effect of drought stress and recovery phase on WUE of
two Tartary buckwheat genotypes
WW= Well-watered; WD-R= Water deficit to recovery; CQ=
Chanqiao No.1; JQ= JingQiao No.2; DAS= Days after imposition of drought stress;
WUE=water-use efficiency; Asterisks * represent significant differences between
treatments on corresponding days (P
< 0.05)
Enzymatic antioxidants
Across the experiment, MDA and SP contents, and SOD and POD activities in both genotypes under WW
treatment were relatively constant (Fig. 6A–D). During the drought phase of the
WD-R treatment, MDA content, and SOD and POD activities were significantly enhanced
in both genotypes, and then dropped back to normal levels during the recovery
phase. SP showed the opposite trend. MDA content of CQ plants under WD-R
treatment was highest out of all treatments on days 9 and 14, leading to a 58.6% increment compared
to CQ plants under the WW treatment on day 14 of the experiment. POD activity
in JQ plants under WD-R treatment was highest among all treatments on days 5,
9, 14 and 19. During the drought phase of the WD-R treatment, SOD activity in
JQ plants increased steadily, whereas that in CQ plants increased sharply from
day 9 to day 14, then decreased back to normal WW levels on day 23. SOD
activity in JQ plants under WD-R treatment was highest—significantly higher
than that in JQ plants and CQ plants under the WW treatment—on days 5, 9, 14
and 19. The SP content of CQ and JQ plants differed significantly (P
< 0.05) between WW and WD-R treatments on days 5, 9, 14 and 19. Among all
treatments, SP content was significantly lowest in CQ plants under the WD-R
treatment.
%20483-491_files/image016.jpg)
Fig. 5: Effect of drought stress and recovery phase on F0 (A) and Fv/Fm ratio (B) of two Tartary buckwheat genotypes
WW= Well-watered; WD-R= Water
deficit to recovery; CQ= Chanqiao No.1; JQ= JingQiao No.2; DAS= Days after
imposition of drought stress; F0= initial fluorescence; Fv/Fm ratio= the
maximum efficiency of photosystem II (PSII) photochemistry;
Asterisks * represent significant differences between treatments on
corresponding days (P < 0.05)
%20483-491_files/image018.jpg)
Fig. 6: Effect of drought stress and
recovery phase on MDA content (A),
POD activity (B), SOD activity (C) and SP content (D) of two Tartary buckwheat genotypes
WW= Well-watered; WD-R= Water deficit to recovery; CQ=
Chanqiao No.1; JQ= JingQiao No.2; DAS= Days after imposition of drought stress;
MDA= Malondialdehyde; POD= peroxidase; SOD= superoxide dismutase; SP= soluble
protein; Asterisks * represent significant differences between between
treatments on corresponding days (P
< 0.05)
Growth, biomass and root-to-shoot ratio
Plant
height, branch number, stem diameter and stem node number for JQ were higher
than for CQ under both WW and WD-R conditions (Table 1). The WD-R treatment
caused a significant reduction in plant height, branch number, stem diameter
and stem node number compared to the WW treatment due to soil dehydration.
Among all treatments and genotypes, plant height, stem diameter, branch number
and stem node number were significantly lowest for CQ under the WD-R treatment.
The same trend was observed for biomass. There were significant differences in
biomass (root, stem, leaf and total) between the two genotypes. Root biomass,
stem biomass, leaf biomass and total biomass were significant lower under the
WD-R treatment than under the WW treatment. Biomass (root, stem, leaf and
total) of CQ in the WD-R treatment was the
lowest for all water conditions and genotypes. JQ
plants had a higher root-to-shoot ratio than CQ plants. Root-to-shoot ratios of
CQ and JQ were significantly increased, by 16.7 and 41.7%, respectively, under
the WD-R treatment compared to the corresponding WW treatment. Among all
treatments, JQ plants under the WD-R treatment had the significantly highest
root-to-shoot ratio.
Table 1: Effect of drought stress and recovery phase on growth,
biomass, root to shoot ratio, yield component and yield of two Tartary
buckwheat genotypes
|
Varieties |
Treatments |
Plant height (cm) |
Stem
diameter (mm) |
Branch
Number |
Stem
node number |
Biomass
(g/plant) |
Root
to shoot ratio |
Seeds number per plant |
1000-grain weight (g) |
Yield per plant (g) |
|||
|
Root |
Stem |
Leaf |
Total |
||||||||||
|
CQ |
WW |
63.8b |
3.71b |
9.9b |
10.0bc |
0.64c |
1.83b |
2.67b |
5.14b |
0.12c |
158.2b |
22.97a |
3.63a |
|
WD-R |
51.3c |
3.37c |
7.0d |
9.7c |
0.56d |
1.23c |
2.25c |
4.04c |
0.14b |
112.7c |
22.13a |
2.49b |
|
|
JQ |
WW |
76.4a |
4.28a |
10.7a |
11.1a |
0.72b |
2.04a |
3.12a |
5.88a |
0.12c |
179.5a |
22.85a |
4.10a |
|
WD-R |
65.1b |
3.90b |
9.1c |
10.3b |
0.92a |
1.75b |
2.74b |
5.41b |
0.17a |
166.5ab |
22.62a |
3.76a |
|
Within each column, different
small letters denote significant differences among treatments at P < 0.05
WW= Well-watered; WD-R= Water deficit to recovery; CQ=
Chanqiao No.1; JQ= JingQiao No.2
%20483-491_files/image020.jpg)
Fig. 7: Relationship between leaf RWC and Gs in CQ (A) and JQ (B) plants during the water stress and recovery phase
CQ= Chanqiao No.1; JQ= JingQiao
No.2; Gs= stomatal conductance; RWC=relative water content
%20483-491_files/image022.jpg)
Fig. 8: Relationship between Gs and SWC in CQ (A) and JQ (B) plants during the water stress and recovery phase
CQ= Chanqiao No.1; JQ= JingQiao No.2; Gs= stomatal
conductance; SWC= soil water content
Yield components and total yield
Under WW
conditions, there were no significant differences in seed yield between the two
genotypes. However, the seed yield of CQ decreased more than that of JQ under
the WD-R treatment (Table 1). The yield of CQ reached under the WD-R treatment
was 31.4% of that under to the WW treatment, but there was no significant yield
loss for JQ grown under the WW vs. WD-R treatments. Seed number per JQ plant
was significantly higher than that per CQ plant under both WW and WD-R
conditions. Seed number per CQ and JQ plant decreased under the drought part of
the WD-R treatment. The number of seeds per CQ and JQ plant decreased
significantly, by 28.8 and 12.8%, respectively, under the WD-R treatment
compared to the corresponding WW treatment. There were no significant
differences in grain weight among genotypes or treatments.
Correlation analysis
Relationships
between Gs, leaf RWC and SWC were studied to determine those that might
indicate responses to drought stress and recovery in CQ and JQ plants. The Gs
was positively correlated with leaf RWC in CQ and JQ plants (P <
0.01; Fig. 7). Positive correlations were also found in CQ (R2 =
0.8626, P < 0.001; Fig. 8A) and JQ (R2 = 0.8474, P
< 0.001; Fig. 8B) plants between SWC and Gs.
Discussion
Water loss, photosynthetic inhibition, growth
restriction, and cell membrane damage are the most common symptoms in plants
under drought stress (Farooq et al.
2009; Bodner et al. 2015; Hussain et al. 2017). Some of these effects can
be restrictive or even devastating. In this study, the CQ and JQ plants
survived 14 days of progressive water deficit. A significant difference in LW
was found between WW and WD-R treatments. Compared to JQ plants, CQ plants were
more sensitive to dehydration, as wilting appeared earlier and were more
severe. The leaf is an important organ for photosynthesis. Under normal
conditions, with sufficient water for photosynthesis, plants utilize a large fragment
of light constant. When subjected to drought, the equilibrium between light
capture and water utilization can be disrupted (Chaves et al. 2003). At first, CQ and JQ plants showed higher WUE to cope
with decreasing water availability. To minimize water loss and maintain
adequate leaf water status during dehydration in the WD-R treatment, CQ and JQ
plants initially reduced Gs to adapt to the drought, along with a significant
reduction in Tr (Chaves et al. 2002;
Pompelli et al. 2010; Tardieu 2012).
Furthermore, the dramatic reduction in Pn, Gs and Ci suggested that
photosynthesis is mainly restricted by stomatal limitations in the two Tartary
buckwheat genotypes (Flexas and Medrano 2002; Hu et al. 2018). A decline in leaf RWC is another way for plants to
increase water availability, contributing to increased carbon delivery to the
roots (Rosales-Serna et al. 2004). As
expected, a large decrease in leaf RWC was found for both genotypes under the
WD-R treatment. In addition, correlation analysis suggested that the two plants
lost part of their leaf RWC at low stomatal aperture. At the same time, they
reduced Gs to cope with the decreasing SWC. A similar finding was reported in Jatropha curcas (Jatropha curcas L.) (Sapeta et al. 2013).
Compared to the CQ plants, JQ plants exhibited higher WUE by lowering
transpiration through a decrease in Gs, supporting JQ as a drought-tolerant
genotype (Blum 2009; Rahbarian et al.
2011). Upon re-watering, the stomata reopened and leaf RWC recovered rapidly,
with increasing Pn. Comparatively higher Pn of WD-R-treated JQ plants was also
detected after re-watering, implying
better photosynthetic recovery from water deficit in the JQ plants.
On the other hand, drought
usually causes photoinhibition when light exceeds the capacity for
photosynthesis (Shi et al. 2016). The
two Tartary buckwheat plants had to subtract excrescent light by preventing (Havaux
and Tardy 1999) or dissipating (Chaves et
al. 2003) absorbance. For the WD-R-treated plants, increased LW on days 5,
9 and 14 was an effective way to minimize light absorption. At the same time, a
decrease in Fv/Fm was observed in the two genotypes during the drought phase of
the WD-R treatment. Fv/Fm is an important indicator of PSII photochemical
efficiency. Thus, plants exhibited a reduction in PSII photochemical efficiency
(Rahbarian et al. 2011), together
with yield losses, under drought conditions (Pathan et al. 2014). In the current study, decreased Fv/Fm occurred after
net assimilation decreased, reinforcing the notion that stomatal limitations
primarily limit Pn, rather than photochemistry (Sapeta et al. 2013). PSII efficiency was limited due to reduced CO2
supply, which has a negative effect on the Calvin cycle (Sapeta et al. 2013). JQ plants maintained a
higher Fv/Fm ratio compared to CQ plants during the drought, suggesting rapid
adjustments in the former to avoid the decline in PSII photochemical
efficiency. Upon re-watering, Fv/Fm quickly recovered to pre-drought levels,
suggesting that the damage had been successfully repaired.
Reactive oxygen species (ROS)
are frequently generated under drought stress, causing serious oxidative damage
to plants (Oliver et al. 2010).
Antioxidant enzymes function in quenching the ROS (Luna et al. 2005). Of these, SOD, the first key enzyme in the active
oxygen-scavenging system, plays a crucial role in catalyzing superoxide free
radical dismutation into H2O2 and O2 (Blokhina et al. 2003). SOD activity increased in
the leaves of the two Tartary buckwheat genotypes under water stress. The same
trend was found for leaf POD activity at the end of the drought stress.
Increased activities of SOD, POD, and catalase in response to water deficit
have been reported in potato (Solanum
tuberosum L.) (Boguszewska et al. 2010). Drought-tolerant white
clover (Trifolium repens L.) retained
significantly higher POD and SOD activity in roots and leaves during drought
treatments (Li et al. 2012). We also
found that JQ leaves maintained higher SOD and POD activity on days 5 and 9 of
the drought phase compared to CQ leaves. The steadily decreasing trend of SOD
and POD activity during the recovery phase revealed the successful
reinstatement of osmosis and antioxidation. Typically, ROS-induced oxidative
damage under water stress can be adjusted by osmoregulatory molecules, such as
proline, soluble sugars, SP, flavonoids, and late-embryogenesis abundant
proteins (Reddy et al. 2004; Claeys
and Inze 2013). These substances help the
membranes defend against drought stress and stimulate plants to absorb more
water to retain normal metabolic functions (Claeys and Inze 2013). The MDA, a
marker of oxidative stress stemming from lipid peroxidation, is generally employed to assay oxidative damage (Sarker
and Oba 2018). In the present study, MDA content in the leaves of both Tartary
buckwheat genotypes increased during the drought phase, but a slower increase
in MDA was found in the JQ vs. CQ plants, which could be attributed to the
former's better tolerance to drought stress.
Along with the physiological
responses to water stress, morphological adaptions were observed. The most
prominent of these in the two Tartary buckwheat genotypes were the reductions
in plant height, stem node number and branch number in the water-stressed
plants. Genotypes with maximum root length and minimum shoot length under water
deficit are regarded as drought-tolerant (Chaves et al. 2003; Oliver et al.
2010). A significant increase in root-to-shoot ratio was found in WD-R plants
compared to WW plants, greater in JQ vs.
CQ plants. Such responses might be attributed to root system development: root
growth in JQ was relatively more enhanced than in CQ during the drought phase.
Drought-induced yield loss is a
major problem in many plants (Bodner et
al. 2015). The primary yield-determining components—grain number and grain
weight—are influenced by water stress, depending on the duration and magnitude
of the drought, and the phenological stage at which it occurs (Farooq et al. 2014; Bodner et al. 2015). In the
present study, water deficit at the flowering stage markedly reduced seed
number in CQ plants. Yield loss due to lack of water in CQ plants reached
31.4%, mainly due to the reduced number of seeds, whereas neither yield nor
seed number were significantly affected by the water deficit in JQ plants.
Thus, the higher yield and higher number of seeds in the JQ plants might have
been a result of the higher photosynthetic activity with more functional leaves
and lower transpiration, due to increased WUE, in those plants under water
stress.
Conclusion
Findings of this study imply that physiological
acclimation of Tartary buckwheat variety JQ to progressive drought includes
elevated water-use efficiency through stomatal closure, and decreased
photosynthesis rate, transpiration rate and PSII photochemical efficiency. At
the same time, JQ has a more active ROS-scavenging system with higher
activities of superoxide dismutase and peroxidase, which may be one of the
crucial routes to avoiding oxidative membrane damage and lipid peroxidation.
Furthermore, accumulation of malondialdehyde helped maintain cell turgor and
metabolic functions. Upon re-watering, these parameters eventually returned to,
or close to, normal levels. These combined strategies enable JQ plants to
survive without significant yield loss under drought conditions with unreliable
precipitation.
Acknowledgements
All authors acknowledge the financial grant from the Key Research and
Development Program of the Ministry of Science and Technology of China
(2019YFD1001302/ 2019YFD1001300), National Natural Science Foundation of China
(31601260), the Applied Basic Research Programs of the Project Science and Technology
Commission of Sichuan Province (2016JY0209) and Supported by the earmarked fund
for China Agriculture Research System (CARS-08-02A).
Author Contributions
Y.W and D.X analyzed the data, wrote the manuscript and
revised the manuscript; J.O, X.G and L.L conducted the experiment and collected
data; X.W, Q.W, L.Z and G.Z reviewed and supervised the manuscript; all authors
mentioned approved the final manuscript.
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