Introduction
Drought stress has a negative influence
on to the growth and development, resulting in crop yield reduction (Farooq
et al. 2009). Crops show various morphological and
physiological responses to drought stress (Qi et al. 2010) causing water deficits, leaf gas
exchange decrease, and metabolic changes in plants (Anjum
et al. 2011), which limits crop productivity (Farooq
et al. 2009, 2014), reducing average yields by 50% or
more (Wang et al. 2003; Farooq et al. 2014). Photosynthesis
is an important biosynthetic reaction and the foundation of crop yield. The
contribution of gas exchange, and especially the rate
of photosynthesis, to crop productivity under sub-optimum conditions has
received much attention worldwide (Samarah et
al. 2010). Photosynthetic rate and chlorophyll (Chl) content are important indicators to
evaluate plant health and environmental situation (Amane
2011). Senthil-Kumar et al. (2007) found that drought stress affected leaf gas exchange and enzymatic
antioxidants activity, resulting in imbalance of the production of enzymatic
system and the electron-transfer chain. In this case,
the excess electrons can cause the production of reactive oxygen species (ROS) (Abidet al. 2016b). In this way, drought stress harms structure and function of cell, which leads to cell death (Sergi
and Josep 2003).
Consequently, plants have evolved an antioxidant system to protect them from ROS (Blokhina
et al. 2003; Carvalho 2008).
Unluckily, drought stress can affect the function of the antioxidant enzymes,
thus induce lipid peroxidation damage of membrane to plant (Dat
et al. 1998).
Plants’ responses to drought are extremely
complicated and vary among different crops and growth stages (Aslam et al. 2015). The growth and development of crops are significantly
affected by water limitation. Sarker et
al. (1999) found that drought stress decreases the water potential (Ψw)
and relative water content (RWC), which lead to the changes of water status in
wheat. Some crops have adaptive strategies to withstand adverse conditions.
Morphological plasticity, improved water use efficiency, and gene regulation
are possible mechanisms of plants which respond to drought during the
vegetative stage (Lotscher and Hay 1997). Optimal cultivation conditions are critical for
plants to withstand subsequent drought, such as optimal rates of fertilizer
application, water, and light.
Nitrogen (N) is required by crops for the synthesis
of chlorophyll, proteins, and enzymes. Certainly, N is very important to
increase stromal and thylakoid proteins to affect photosynthetic capacity (Ahmad et al. 2014). In agricultural production, it is one of important strategy for crop
productivity increase by applying N (Ataulkarim
et al. 2016). Brennan
(1992) found the N availability has a great impact to the functional
activity of photosynthetic apparatus. A previous study reported that plant growth and development are limited by water
restrictions, especially under the condition of low N availability. Water
deficit and limited N have been shown to affect plant-water relations and
photosynthetic ability, which lead to premature senility and low productivity
of crops (Madani et al. 2010).
Furthermore, appropriate N application has been shown to alleviate drought
stress damage by allowing plants to maintain metabolic activity (Wu et al. 2018).
Tartary
buckwheat (Fagopyrum tataricum (L.) Gaertn.) is an excellent plant resource grown worldwide and processed
into foods and drinks (Bonafaccia et al. 2003; Fabjanet al.
2003; Xiang et al. 2016). It has the concomitant function of
both medicine and foodstuff because of various pharmaceutical ingredients, such
as rutin, quercetin and isoquercetin in the different organs of plant (Zhao et al. 2012). Due to its abundant
nutrition ingredients and health
value, Tartary
buckwheat is becoming highly attractive (Fabjan et al. 2003; Kreft 2016). However, Tartary buckwheat is mainly cultivated in marginal
land of Southwest China. Owing to infrequent rain in
these areas, drought stress has become a major hindrance for production of Tartary buckwheat (Ohnishi
and Tomiyoshi 2005;
Xiang et al. 2013). A previous study found that Tartary buckwheat could not tolerate
drought stress during its initial growth stages (Zhao
and Shang 2009).
Therefore, it is important to find ways to ameliorate the adverse effects of
drought on Tartary buckwheat at the seedling stage to improve its growth and
yield. Previous studies found that N application can reduce the negative
influence of drought on yield in other crops (Saneoka
et al. 2004; Dinh et al. 2017). However, the reported results differ
among studies because of differences in crops or species, environmental
conditions, N application rates, drought stress levels, and growth stages of
crops (Ping
et al. 2011; Shi et al. 2014; Wu et al. 2018). A comprehensive understanding of
compensation effect of N nutrition under water stress is scarce in Tartary buckwheat.
The specific objective of this study was
to assess the effects of N application under different water regimes on the
growth and development, physiological activities, and yield of Tartary
buckwheat. The results will provide information about the physiological
mechanism by which N promotes the growth of Tartary buckwheat, and highlight
the potential of N to improve yield in arid or semiarid regions.
Materials and Methods
Experimental materials
and site description
A trial was conducted during two years (2017 and
2018) at experimental farm of Chengdu University (30°39′ N, 104°11′ E, 490 m altitude),
Sichuan Province, China. In each
growing season, the Tartary buckwheat cultivar (XiQiao-1), was obtained from Chengdu University
and is the most widely grown cultivar in southwest China. Before sowing, the
10% (v/v) hydrogen peroxide was used for seed sterilization, rinsed four times
with deionized H2O, then collected and stored for further use. Ten
seeds were sown in each plastic pot (30 cm height × 25 diameter). The pots were
filled with 9 kg air-dried soil with 13% soil moisture. The soil was alkaline
(pH 7.6) containing 48.2 mg kg-1 available N,
20.3 mg kg-1 Olsen-P, 52.5 mg kg-1 available K, 1.8 g kg-1
organic matter, 0.62 g kg-1 total N, 0.41 g kg-1 total P, and
14.6 g kg-1 total K, 0.43 dS m-1 electrical conductivity (EC), 1.31 g
cm-3 bulk
density, and 37.9% field capacity (FC) by volume, respectively. When soil was
added to the pots, the base fertilizer (0.6 g P2O5 and
1.2 g K2O) was added to each pot. The seedlings were thinned to three plants per pot.
Thirty pots were used for each treatment and five plants were maintained after
thinning at 7 days after germination. Each pot was maintained with soil
moisture at 80% of FC until drought stress was imposed. The pots were placed
randomly and moved to a different place every week to ensure that all plants
had equal growth conditions.
Experimental design and
management
The experiment was arranged in
completely randomized design. There were three soil water levels (well-watered (WW); moderate drought stress (MD); severe
drought stress (SD)) and two N rates (0.05 and 0.2 g N kg-1
soil, represented as low N (N1) and high N (N2), respectively). The N
fertilizer was applied at sowing (50%) and the five-leaf stage (50%),
respectively. Drought was imposed on Tartary buckwheat seedlings at the
three-leaf and five-leaf stages. At each stage, three drought levels (35–40%,
55–60% and 80% FC, respectively) were maintained by water application to the
desired FC (Zlobin et al. 2018). At
each stage, drought stress was maintained for 10 days. After the drought
stress, the soil of each pot was re-watered to 80% of FC and the plants were
grown until maturity. The experimental design and management were consistent
during two growing seasons.
Data collection
Fully expanded leaves were randomly
selected at 1 day before drought stress (0 D), days 5 and 10 of drought stress
(5 D, 10 D), and 1 and 3 days after irrigating (1 DR, 3 DR). These samples were
used to analyze the RWC, Ψw, Chl and soluble protein (SP) content, and gas exchange parameters.
Leaf water status
To determine the RWC, leaf fresh weight (FW) was measured
(Sartorius CPA225D balance, Sartorius Co., Beijing, China) immediately after
leaves were cut from the Tartary buckwheat plants. Later, the leaves were floated on deionized water
for 18 h and weighed to determine turgid weight
(TW). These leaves were dried in a drying oven for 72 h
at 75°C to measure dry weight (DW). The RWC was
calculated as: RWC (%) = [(FW−DW)/(TW −
DW)] × 100. The Ψw was measured as described by Canny (1997) using portable pressure chamber
3115 (Soil moisture Equipment Cor., California, U.S.A.).
Chlorophyll and soluble
protein content
The Chl
content was measured following the method of Xiong (2009). Leaf samples was
ground and placed in centrifuge tube with 80% acetone and then covered with
black cloth and kept at
darkroom until the leaf changed to white. The Chl content was measured using a spectrophotometer at the
wavelength of 645 and 663 nm. A leaf sample of 0.5 g was used to determine SP content using the
method (Coomassie brilliant blue G-250 staining) described by Xiong (2009).
Leaf gas exchange
The gas exchange of fully expanded
leaves was measured using portable photosynthesis system
(GFS-3000, WALZ Inc., Effeltrich, Germany) between
09:00 and 11:30 h. During the measurement, a photosynthetic active radiation of
1200 µmol m−2 s−1 was provided by an
automatic light source. The net photosynthetic rate (Pn)
and stomatal conductance (gs) of
Tartary buckwheat leaf were recorded by this photosynthesis system.
Malondialdehyde (MDA)
content and superoxide dismutase (SOD) activity
The SOD activity and MDA
content were measured following the method of Beauchamp and Fridovich
(1971) and Wang et al. (2018), respectively. 0.5 g of
frozen leaf sample was homogenized in a mortar and pestle, and the homogenate
was centrifuged (4°C) at 10,000 × g
for 30 min. Later, the supernatant was used to analyze the
SOD activity and MDA content.
Dry matter, relative
growth rate, drought index, yield, and yield components
Whole plants were cut at the
three-leaf, five-leaf, anthesis, and maturity stages to determine FW. These
samples were dried in a drying oven for 72 h at 75°C to measure DM. The RGR,
RGR = (1/DM) × (△DM/△d), DI, DI = YD/YW were calculated according to Zhang et al. (2007). △DM and △d were assessed by the change in DM and
days between two adjacent samplings stages, respectively. The YD and YW
represented the yield of Tartary buckwheat under the conditions of drought
stress and WW, respectively. Certainly, the 1000-grain weight and grain number
per plant of each treatment were also measured at the maturity stage.
Statistical analysis
Two yeas data were analyzed by SPSS
Statistics 17.0 (IBM, Chicago, I.L., U.S.A.). There
were consistent physiological characteristics of Tartary buckwheat in 2017 and
2018, and no significant differences were found in
across years and in interaction effects (Year × Nitrogen; Year × Drought and
Year × Nitrogen × Drought). Hence, data was analyzed from the mean of two
years, and the differences among treatments were assessed by Duncan’s multiple
range test (P < 0.05).
Results
Leaf RWC and Ψw
The leaf RWC and Ψw of
Tartary buckwheat decreased under drought stress (Fig. 1). The difference in RWC and Ψw between the treatments of two N rates under drought stress conditions were
significantly, but not under WW conditions. The RWC and Ψw were
lower at low N than at high N treatments, and the decreases in RWC and Ψw
under drought stress were greater at the three-leaf stage than at the
five-leaf stage. After re-watering, the RWC and Ψw recovered to
different extents among the different treatments. The RWC and Ψw
of drought-stressed plants showed greater recovery at high N than at low N.
Leaf Chl and SP contents
Drought and N application significantly affected the Chl and soluble protein (SP) contents of
Tatary buckwheat leaves (P < 0.05)
(Fig. 2). Under drought stress, the Chl
and soluble protein contents decreased significantly under both N application rates and decreased to lower
levels at low N treatments than at high N treatments under drought stress and
WW conditions. After re-watering, the Chl
and SP contents recovered slowly in plants subjected to drought at the
three-leaf and five-leaf stages, but the recovery was stronger in the high N treatments than in the low
N treatments.
Leaf gas exchange
%201167-1177_files/image002.png)
Fig. 1: Effect of drought stress on leaf relative water content
(RWC) and water potential (Ψw) of
Tartary buckwheat under two nitrogen (N) application rates. Panels A–B and C–D show results obtained when drought was applied at three-leaf
and five-leaf stage, respectively. S, M, severe and moderate
drought stress, respectively; W, well-watered conditions. 0 D, 1 day
before drought stress; 5 D, 10 D, day 5 and 10 of drought stress, respectively;
1 DR, 3 DR, 1 and 3 days after re-watering, respectively. N1 and N2 represent
the low and high N levels, respectively. Data are means ± SD of two years (2017
and 2018). Different letters denote significant differences between treatments
at the same time. NS: non-significant difference
The leaf Pn was significantly influenced by N application,
which was lower in the low N than in the high N treatments. However, N application
had no remarkable effects on gs (Fig. 3). The Pn
and gs of Tartary buckwheat leaf responded to the different
drought levels significantly. The Pn and gs declined with
increasing intensity of drought, and the lowest values of Pn and gs were
in the low N treatments. Seedlings subjected to drought at the three-leaf and
five-leaf stage showed the same trends, but the decreases in Pn and
gs were greater when drought was applied at the three-leaf stage.
After re-watering, the Pn
and gs of Tartary buckwheat
showed better recovery in the high N treatments than in the low N treatments.
SOD activity and MDA
content
The SOD activity and MDA content were
higher in drought-stressed plants than in plants in
the WW treatment (Fig. 4). There were no remarkable differences in MDA contents
and SOD activity between the two N levels under WW conditions. Under drought
stress, the MDA content tended to decline and SOD activity tended to increase
with increasing N application rate, and the same trend was observed in drought-stressed
plants at the three-leaf stage and at the five-leaf stage. After re-watering,
SOD activity and MDA contents decreased in the drought-stressed plants, but
decreased more in the treatments of high N than low N.
Dry matter and relative
growth rate
In both growing seasons, drought stress
at the three-leaf and five-leaf stages significantly reduced the DM
accumulation and RGR (except FLA, five-leaf to anthesis stage) of Tartary
buckwheat plants (P < 0.01; Table
1). The reduction in RGR was lower from the three-leaf to five-leaf stage (TFL)
and from anthesis to the maturity stage (AM), and higher from the five-leaf to
anthesis stage (FLA) under drought stress than in WW conditions. The DM accumulation showed similar decreasing trends
in the two drought treatments, and decreased more in the severe drought
treatments
(by 13.1% in ST and 4.9% in SF) than in the moderate drought treatments.
Consistently, drought
stress had less impact on RGR and DM accumulation in the high N treatments than
in the low N treatments at the four growth stages, and the patterns were
similar in 2017 and 2018. Additionally, no significantly
difference were observed in
across years and in interaction effects (Y × N; Y
× D and Y × N × D).
%201167-1177_files/image004.png)
Fig. 2: Effect of drought stress on the chlorophyll (Chl)
and soluble protein (SP) content of Tartary buckwheat under two nitrogen (N)
application rates. Panels A–B and C–D show results obtained when drought
was applied at three-leaf and five-leaf stage, respectively. S,
M, severe and moderate drought stress, respectively; W, well-watered
conditions. 0 D, 1 day before drought stress; 5 D, 10 D, day 5 and 10 of
drought stress, respectively; 1 DR, 3 DR, 1 and 3 days after re-watering,
respectively. N1 and N2 represent the low and high N levels, respectively. Data
are means ± SD of two years (2017 and 2018). Different letters denote
significant differences between treatments at the same time. NS: non-significant
difference
DI, yield, and yield
components
The drought and N application
significantly affected the grain number per plant, 1000-grain weight, grain yield, and DI, and the magnitude of the effects depended
on the severity of drought, growth stage, and N levels, and the patterns were
similar during two years (Table 2). The loss of yield was greater when drought
was applied at the three-leaf stage than at the five-leaf stage. The grain
yield decreased with increasing drought stress intensity, but increased with
increasing N level. The yield in ST, MT, SF, and MF was reduced by 34.2, 21.9,
23.9 and 10.3%, respectively, in low N, and by 31.5, 18.2, 15.2 and 4.2%,
respectively, in high N, compared with grain yields in WW conditions. The DI of
moderate drought stress was greater than severe drought stress, and this trend
was observed in both the low N and high N treatments of two years. However, the
DI was higher in high N treatments than in low N treatments under the same
degree of drought stress. The DI in ST, MT, SF, and MF was 4.3, 4.9, 10.6 and 6.3%
lower, respectively, in low N than in high N.
The number of grains per plant
significantly increased with increasing N application rate in all treatments.
Compared with the WW treatments, the ST, MT, SF, and MF treatments showed 32.9,
25.4, 19.8 and 12.3% decreases, respectively, in grain number per plant in low
N, and 34.6, 22.5, 13.9 and 5.1% decreases, respectively, in grain number per
plant in high N. The 1000-grain weight was impacted by drought and N
application. Compared with WW treatment, all stress treatments (except MF in
N2) indicted significantly reduced 1000-grain weight. The weight of
1000-grain in ST, MT, SF and MF was 5.9, 2.4, 7.3 and 2.3% higher,
respectively, in high N than in low N.
Discussion
The present study assessed the
influences of N nutrition on the growth and development, physiological
performance and yield of Tartary buckwheat under drought conditions. The
Tartary buckwheat plants at the three-leaf and five-leaf stages showed
different responses to severity of and N levels. The responses to drought
included a decrease in Ψw, which resulted in stomatal closure
and reduced photosynthesis (Fig. 1 and 3). Flexas and Medrano (2002) suggested
that Ψw is very important for normal crop growth, and a
decrease in Ψw adversely affects CO2 assimilation
and water use efficiency (WUE) due to metabolic impairment of photosynthesis.
Thus, under a higher N application rate, the maintenance of Ψw
allowed Tartary buckwheat plants to sustain leaf processes under conditions of drought stress, and then to recover faster after re-watering than under
a lower N application level. In this case, the stomatal activity was higher
under a higher N rate than under a lower N rate. These results may be
attributed to the decreased Ψw under lower N application, which
could lead to the limited growth and development of cell. The decline in Ψw
may decrease mesophyll conductance (Warren et al. 2004). Lower WUE under low Ψw
has been shown to decrease DM accumulation and yield (Grassi
and Magnani 2005).
%201167-1177_files/image006.png)
Fig. 3: Effect of drought stress on the net photosynthetic rate (Pn)
and stomatal conductance (gs) of Tartary
buckwheat under two nitrogen (N) application rates. Panels A–B and C–D show results
obtained when drought was applied at three-leaf and five-leaf stage,
respectively. S, M, severe and moderate drought stress,
respectively; W, well-watered conditions. 0 D, 1 day before drought
stress; 5 D, 10 D, day 5 and 10 of drought stress, respectively; 1 DR, 3 DR, 1
and 3 days after re-watering, respectively. N1 and N2
represent the low and high N levels, respectively. Data are means ± SD of two years (2017 and 2018).
Different letters denote significant differences between treatments at the same
time. NS: non-significant difference
Chlorophyll plays a key role in
determining the intensity of photosynthesis, which is strongly affected by
adverse conditions. Mafakheri et al.
(2010) found that drought stress significantly decreased the Chl content also evident from present study (Fig. 2). The decrease in Chl content under drought stress would
reduce the photochemical activity of chloroplasts, leading to decreased
photosynthesis. In this study, the Chl
content in Tartary buckwheat leaves increased by higher N application rates
under drought and well-watered conditions (Fig. 2). Because N is an important
component of Chl and proteins, it
strongly affects plant metabolism during drought stress (Amane
2011). Sufficient
N can enhance the recovery of photosynthesis, and so N-deficient crops show
limited recovery after severe drought stress conditions (Grassi
and Magnani 2005).
Therefore, enough N may increase the
photosynthetic capacity and stomatal control under drought conditions, owing to
more than half of the N nutrient in plants’ green tissues take part in
collecting solar energy to drive photosynthesis (Sinclair
and Jamieson 2006).
Certainly, photosynthesis can be improved by increasing the total Chl content via appropriate N
fertilization. Further research is required to explore the detailed mechanism by which N
enhances photosynthesis in Tartary buckwheat under drought stress.
Drought-stressed plants produce excess H2O2,
which can cause oxidative damage through the formation of ROS which damage
proteins (Mohammadi et al.
2018). Sofo et al. (2010) suggested that the MDA
content was a vital indicator of oxidative damage in plants, and closely
related to the serious degree of drought and N available. In this study,
Tartary buckwheat plants under the condition of severe drought with Low-N
application caused excess MDA accumulation. Hence, a higher content of MDA
under conditions of Low-N application during drought stress may reduce the
ability of antioxidation in cell, resulting in greater ROS accumulation (Jiang et al. 2007). Lipid peroxidation can lead to further damage such as
the loss of Chl, improved the
permeability of cell membrane, breakdown of macromolecules, reduction of
nutrient availability, and early senescence, which eventually lessen the growth
period of grain (Calatayud et al.
2001). It was observed that lower
MDA contents and higher SOD activity in the high N treatments than in the low N
treatments, indicating that greater N availability increased the ROS scavenging
capability of drought-stressed Tartary buckwheat. Cheng (2013) also suggested
that greater SOD activity and lower MDA content in plants in a High-N treatment
was indicative of improved redox defenses to scavenge ROS. In this study, the
drought-stressed Tartary buckwheat plant under higher N rates had the stronger
activity of the ROS-detoxifying antioxidant system, thus may have protect the
photosynthetic process, consistent with Zandalinas et al. (2017). Hence, present study
results indicate that appropriate N fertilization can improve the production
and drought tolerance of Tartary buckwheat by enhancing antioxidant enzyme
activities and reducing lipid peroxidation.
%201167-1177_files/image008.png)
Fig. 4: Effect of drought stress on the MDA content and SOD activity of
Tartary buckwheat under two nitrogen (N) application rates. Panels A–B and C–D show results obtained when drought was applied at three-leaf
and five-leaf stage, respectively. S, M, severe and moderate
drought stress, respectively; W, well-watered conditions. 0 D, 1 day
before drought stress; 5 D, 10 D, day 5 and 10 of drought stress, respectively;
1 DR, 3 DR, 1 and 3 days after re-watering, respectively. Data are means ± SD
of two years (2017 and 2018). N1 and N2 represent the low
and high N levels, respectively. Different letters denote significant differences between treatments at
the same time. NS: non-significant difference
It was also found that severe drought
stress during seedling stage affected normal physiological processes (Fig. 3
and 4), leading to the inhibition of growth, development (Table 1) and yield
formation in Tartary buckwheat (Table 2). The decrease in the RGR of Tartary
buckwheat under drought stress was greater under a lower N application rate
than under a higher N, and the recovery of photosynthesis after re-watering was
also slower at the lower N rate. Abid et
al. (2016a) suggested that the decline in Pn under stress
conditions leads
to the imbalance of photosynthesis and respiration, resulting in a decreased
crop growth rate. In the present study, the RGR recovered after re-watering
(Table 1), which was indicative of the reversibility of some physiological
damage caused by drought. However, the DM accumulation in Tartary buckwheat at
maturity was lower in the drought treatments than in the WW treatments under
both N levels (Table 1). In this sense, drought stress had some irreversible
effects on the growth and development of Tartary buckwheat, consistent with the
report of Xu et al. (2010).
Certainly, other factors may have affected the plants pre-drought, but further
studies are required to explore this.
Drought stress has been shown to limit
the growth, development and yield formation of crops under Low-N supply (Tuong
et al. 2002; Bernier et al. 2007), and its effects were studied in
detail by analyzing individual yield components (Hattori et al.
2010). In this
study, Tartary buckwheat under higher N application rates had higher grain
number per plant, higher 1000-grain weight, and produced higher yield than
under lower N application rates (Table 2). The drought-stressed plants under
lower N level showed significantly weaker performance in terms of yield and
yield components. The results showed that there were significant differences in
the growth, yield and its component of Tartary buckwheat plants among different
drought and N conditions (Table 1 and 2).
Table 1: Effect of drought stress and nitrogen application rate
on dry matter accumulation and relative growth rate of Tartary buckwheat during
several growth stages
|
Year |
N Levels |
Drought Treatment |
Relative growth rate
(mg/g/d) |
Dry matter
accumulation (g) |
|||||
|
TFL |
FLA |
AM |
Three-leaf |
Five-leaf |
Anthesis |
Maturity |
|||
|
2017 |
Low N |
ST |
73.1 |
164.1 |
29.4 |
1.40 |
2.74 |
9.47 |
18.66 |
|
MT |
76.8 |
146.9 |
36.0 |
1.58 |
3.16 |
10.11 |
22.11 |
||
|
SF |
78.4 |
127.4 |
32.1 |
1.77 |
3.58 |
10.42 |
21.44 |
||
|
MF |
81.2 |
126.0 |
34.1 |
1.84 |
3.78 |
10.92 |
23.21 |
||
|
WW |
81.9 |
136.1 |
38.5 |
1.85 |
3.82 |
11.62 |
26.35 |
||
|
High N |
ST |
89.3 |
190.0 |
31.1 |
1.61 |
3.48 |
13.4 |
27.13 |
|
|
MT |
79.8 |
182.0 |
38.2 |
1.93 |
3.93 |
14.66 |
33.12 |
||
|
SF |
93.3 |
128.6 |
36.2 |
2.21 |
4.89 |
14.32 |
31.42 |
||
|
MF |
92.0 |
138.1 |
39.1 |
2.26 |
4.95 |
15.21 |
34.81 |
||
|
WW |
95.3 |
148.7 |
40.6 |
2.26 |
5.06 |
16.35 |
38.25 |
||
|
2018 |
Low N |
ST |
71.0 |
159.9 |
30.8 |
1.41 |
2.71 |
9.21 |
18.56 |
|
MT |
73.4 |
159.6 |
33.7 |
1.59 |
3.11 |
10.56 |
22.30 |
||
|
SF |
82.9 |
121.5 |
33.5 |
1.75 |
3.64 |
10.26 |
21.61 |
||
|
MF |
83.9 |
123.4 |
34.1 |
1.85 |
3.86 |
10.99 |
23.36 |
||
|
WW |
81.7 |
137.1 |
37.1 |
1.89 |
3.89 |
11.89 |
26.44 |
||
|
High N |
ST |
88.9 |
184.2 |
32.0 |
1.63 |
3.52 |
13.24 |
27.22 |
|
|
MT |
80.7 |
179.6 |
38.2 |
1.95 |
3.99 |
14.73 |
33.30 |
||
|
SF |
91.0 |
128.8 |
36.5 |
2.24 |
4.88 |
14.32 |
31.57 |
||
|
MF |
91.7 |
142.6 |
38.5 |
2.24 |
4.91 |
15.41 |
35.00 |
||
|
WW |
94.0 |
148.8 |
39.9 |
2.29 |
5.09 |
16.46 |
38.11 |
||
|
Mean (2017 and 2018) |
Low N |
ST |
75.1c |
166.0c |
30.6f |
1.47g |
2.84h |
9.63i |
19.06j |
|
MT |
79.8bc |
153.6d |
35.5d |
1.65f |
3.30g |
10.62h |
22.72h |
||
|
SF |
83.9b |
126.4f |
33.7e |
1.83e |
3.75e |
10.63h |
22.11i |
||
|
MF |
85.0b |
127.3f |
34.8d |
1.91d |
3.94d |
11.24g |
23.83g |
||
|
WW |
84.6b |
138.7e |
38.7b |
1.93d |
3.99d |
12.04f |
27.02f |
||
|
High N |
ST |
91.7a |
190.2a |
32.0e |
1.69f |
3.62f |
13.60e |
27.64e |
|
|
MT |
83.1b |
182.6b |
38.8b |
2.00c |
4.10c |
14.98c |
33.79c |
||
|
SF |
95.9a |
128.8f |
36.7c |
2.29b |
5.06b |
14.61d |
31.96d |
||
|
MF |
94.5a |
141.5e |
39.3a |
2.31ab |
5.08b |
15.60b |
35.43b |
||
|
WW |
96.6a |
150.7d |
40.6a |
2.34a |
5.20a |
16.69a |
38.66a |
||
|
ANOVA |
|
|
|
|
|
|
|
|
|
|
F-value |
|
Year (Y) |
0.036NS |
0.036NS |
0.035NS |
1.33NS |
2.18NS |
0.56NS |
1.77NS |
|
Nitrogen (N) |
120.85** |
156.97** |
50.02** |
1185.46** |
5334.9** |
2883.57** |
2028.01** |
||
|
Drought (D) |
16.53** |
196.07** |
38.47** |
392.89** |
1374.48** |
131.2** |
1868.1** |
||
|
Y × N |
0.23NS |
0.10NS |
0.030NS |
0.16NS |
0.12NS |
0.029NS |
0.0037NS |
||
|
Y × D |
0.31NS |
1.67NS |
1.17NS |
0.41NS |
0.36NS |
1.22NS |
0.41NS |
||
|
N × D |
3.76* |
9.98** |
3.50* |
15.49** |
51.69** |
2.64* |
70.24** |
||
|
Y × N × D |
0.89NS |
2.17NS |
0.47NS |
0.32NS |
2.10NS |
0.42NS |
0.22NS |
||
TFL: from three-leaf to five-leaf stage. FLA: from five-leaf to anthesis stage. AM: from anthesis
to maturity stage. ST and SF represent severe drought stress at three-leaf and
five-leaf stage, respectively. MT and MF represent moderate drought stress at
three-leaf and five-leaf stage, respectively. Within each column, different
small letters denote significant differences among treatments (P
< 0.05). For ANOVA, Y × N represents interaction between year
and N. Y × D represents interaction between year and drought. N × D
represents interaction between N and drought. Y × N × D represents interaction among year, N and drought. NS, not significant.
*, significant
(P < 0.05). **, significant (P < 0.01)
Interestingly, the magnitude of yield
loss differed depending on the timing of the drought treatment, with greater
reductions when drought was applied at the three-leaf stage than at the
five-leaf stage. These results indicate that Tartary buckwheat plants are more
sensitive to drought stress at an early stage than at a later stage during
vegetative growth. Zhao and Shang (2009), also found
that Tartary buckwheat could not tolerate drought stress at the early stage of
vegetative growth. Davatgar et al.
(2009) also suggested that plants’ responses to water deficit rely on the
stress condition and plant status, such as stress time, severity, duration and growth stage. For Tartary buckwheat, the root
system is smaller in seedlings than in older plants, so seedlings’ ability to
take up water from the soil is weaker than older plants. Eneji et al. (2008) reported that the N uptake
and utilization under drought stress is crucial for improving growth and
productivity of crops. The response of plants to N is strongly related with the
ability of roots to absorb nutrients and water (Ding
et al. 2015). This explains why adverse environmental conditions during this earlier growth stage
resulted in greater decreases in physiological activities (Fig. 2, 3 and 4) and
grain yields (Table 2).
Drought-stressed Tartary buckwheat had
a significantly higher grain yield under high N treatment than in the low N
treatment. Similar promoting effects of N were observed in the WW treatments,
but the effect of N to increase yield was stronger in the drought treatments
than in the WW treatments. In general, a higher N application rate resulted in
stronger growth, higher physiological activity, and improved yield performance
of drought-stressed Tartary buckwheat. These findings show that appropriate N
application can decrease drought damage during the seedling stage by enhancing
the growth potential of Tartary buckwheat plants. Optimal nitrogen nutrition is
fundamental to improve the growth and yield of Tartary buckwheat in in arid and
semi-arid zones.
Table 2: Effect of nitrogen application
rate and drought on yield and its components in Tartary buckwheat
|
Year |
N Levels |
Drought
Treatment |
Grains
Number per Plant |
1000-Grain
Weight (g) |
Yield per
Pot (g) |
DI |
|
2017 |
Low N |
ST |
109.9 |
18.5 |
10.2 |
0.66 |
|
MT |
117.9 |
20.6 |
12.0 |
0.78 |
||
|
SF |
123.5 |
19.2 |
11.8 |
0.76 |
||
|
MF |
130.2 |
21.4 |
13.8 |
0.90 |
||
|
WW |
142.5 |
21.5 |
15.4 |
0.00 |
||
|
High N |
ST |
115.1 |
19.5 |
11.2 |
0.68 |
|
|
MT |
127.0 |
21.5 |
13.5 |
0.81 |
||
|
SF |
136.8 |
20.5 |
14.1 |
0.85 |
||
|
MF |
145.8 |
22.1 |
15.6 |
0.94 |
||
|
WW |
150.3 |
21.9 |
16.6 |
0.00 |
||
|
2018 |
Low N |
ST |
110.8 |
18.7 |
10.1 |
0.65 |
|
MT |
117.9 |
21.0 |
12.2 |
0.78 |
||
|
SF |
123.5 |
19.4 |
11.8 |
0.76 |
||
|
MF |
131.7 |
21.1 |
14.0 |
0.90 |
||
|
WW |
144.0 |
21.8 |
15.6 |
0 |
||
|
High N |
ST |
116.3 |
20.0 |
11.4 |
0.70 |
|
|
MT |
128.5 |
21.1 |
13.6 |
0.83 |
||
|
SF |
135.9 |
20.8 |
13.9 |
0.85 |
||
|
MF |
144.5 |
21.6 |
15.9 |
0.98 |
||
|
WW |
150.3 |
22.3 |
16.3 |
0 |
||
|
Mean |
Low N |
ST |
110.4j |
18.6g |
10.2i |
0.66h |
|
MT |
117.9h |
20.8d |
12.1f |
0.78e |
||
|
SF |
123.5g |
19.3f |
11.8g |
0.76f |
||
|
MF |
131.0e |
21.3c |
13.9d |
0.90b |
||
|
WW |
143.3c |
21.7b |
15.5c |
0 |
||
|
High N |
ST |
115.7i |
19.7e |
11.3h |
0.69g |
|
|
MT |
127.8f |
21.3c |
13.5e |
0.82d |
||
|
SF |
136.4d |
20.7d |
14.0d |
0.85c |
||
|
MF |
145.2b |
21.8ab |
15.8b |
0.96a |
||
|
WW |
150.3a |
22.1a |
16.5a |
0 |
||
|
ANOVA |
|
|
|
|
|
|
|
F-value |
Year (Y) |
3.00NS |
2.86NS |
2.31NS |
1.87NS |
|
|
Nitrogen (N) |
1456.54** |
133.46** |
725.16** |
188.56** |
||
|
Drought (D) |
2061.37** |
213.8** |
977.18** |
669.72** |
||
|
Y × N |
1.60NS |
0.39NS |
0.38NS |
1.61NS |
||
|
Y × D |
1.13NS |
4.39NS |
0.96NS |
0.90NS |
||
|
N × D |
42.55** |
7.28** |
15.46** |
9.98** |
||
|
Y × N × D |
2.05NS |
1.59NS |
1.23NS |
0.57NS |
||
TFL: from three-leaf to five-leaf stage. FLA: from
five-leaf to anthesis stage. AM: from anthesis to maturity stage. ST and SF represent severe
drought stress at three-leaf and five-leaf stage, respectively. MT and MF
represent moderate drought stress at three-leaf and five-leaf stage,
respectively. Within each column, different small letters denote significant differences
among treatments (P < 0.05). For
ANOVA, Y × N represents interaction between year and N. Y × D represents
interaction between year and drought. N
× D represents interaction between N
and drought. Y × N × D represents interaction among year, N and drought. NS, non-significant.
**, significant (P < 0.01)
Conclusion
The results demonstrated that the
combination of drought stress and N level during the seedling stage severely
affected the growth potential and physiological performance of Tartary
buckwheat plants while N application effectively ameliorated the adverse
effects of drought stress. An adequate N fertilizer application under drought
stress could promote increased antioxidant activity, Ψw, RWC, Chl, and SP content as well as
photosynthesis ability, which ultimately results in high-yield of Tartary
buckwheat. Appropriate culture techniques may optimize these traits to enhance
drought tolerance and achieve higher yields of Tartary buckwheat in field
production. These results provide insights into the role of N nutrition to
improve the performance of drought-stressed Tartary buckwheat, which also
provide a theoretical and practical guide for cultivation of Tartary buckwheat
crops under drought stress.
Acknowledgments
We
are grateful for research support from National Natural Science Foundation of
China (31771716; 31601260) and China Agriculture Research System (CARS-07-02A).
Author Contributions
DBX, WW and LXP designed the experiments and
wrote the manuscript, WW, JYOY and LQL performed the experiments, GZ, LXP and
YW statistically analyzed the data, reviewed the manuscript and made
illustrations.
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