BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-14-3851-2017Morphological plasticity of root growth under mild water stress increases
water use efficiency without reducing yield in maizeCaiQianZhangYulongylzsau@163.comSunZhanxiangsunzhanxiang@sohu.comZhengJiamingBaiWeiZhangYueLiuYangFengLiangshanFengChenZhangZheYangNingEversJochem B.ZhangLizhenhttps://orcid.org/0000-0003-1606-6824College of Land and Environment, Shenyang Agricultural University,
Shenyang, 110161, Liaoning, ChinaTillage and Cultivation Research Institute, Liaoning Academy of
Agricultural Sciences, Shenyang, 110161, Liaoning, ChinaCollege of Resources and Environmental Sciences, China Agricultural
University, Beijing, 100193, ChinaWageningen University, Centre for Crop Systems Analysis (CSA),
Droevendaalsesteeg 1, 6708 PB Wageningen, the NetherlandsYulong Zhang (ylzsau@163.com) and Zhanxiang Sun (sunzhanxiang@sohu.com)29August201714163851385822March201731March201725June201722July2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/14/3851/2017/bg-14-3851-2017.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/14/3851/2017/bg-14-3851-2017.pdf
A large yield gap exists in rain-fed maize (Zea mays L.) production in semi-arid
regions, mainly caused by frequent droughts halfway through the crop-growing period
due to uneven distribution of rainfall. It is questionable whether irrigation
systems are economically required in such a region since the total amount of
rainfall does generally meet crop requirements. This study aimed to
quantitatively determine the effects of water stress from jointing to
grain filling on root and shoot growth and the consequences for maize grain
yield, above- and below-ground dry matter, water uptake (WU) and water use
efficiency (WUE). Pot experiments were conducted in 2014 and 2015 with a
mobile rain shelter to achieve conditions of no, mild or severe water
stress. Maize yield was not affected by mild water stress over 2 years,
while severe stress reduced yield by 56 %. Both water stress levels
decreased root biomass slightly but shoot biomass substantially. Mild water
stress decreased root length but increased root diameter, resulting in no
effect on root surface area. Due to the morphological plasticity in root
growth and the increase in root / shoot ratio, WU under water stress was
decreased, and overall WUE for both above-ground dry matter and grain yield
increased. Our results demonstrate that an irrigation system might be not
economically and ecologically necessary because the frequently occurring
mild water stress did not reduce crop yield much. The study helps us to
understand crop responses to water stress during a critical water-sensitive
period (middle of the crop-growing season) and to mitigate drought risk in dry-land
agriculture.
Introduction
Maize (Zea mays L.) is the most important crop globally, and also a major food crop
in north-eastern China with an average yield around 5.3 t ha-1 (Dong et
al., 2017). However, the yield gap to the potential of 10.9 t ha-1 is
still large (Liu et al., 2012), mainly due to frequent summer droughts. Due
to the increasing probability of extreme climate events (IPCC, 2007), water
stress for agricultural production is likely to increase in this region
(Song et al., 2014; Yu et al., 2014) which is detrimental for crop
photosynthesis and yield (Richards, 2000).
Although the averaged total rainfall during the crop-growing season can meet the
requirements of rain-fed maize in the semi-arid north-east of China, the
yearly and seasonal variations often cause droughts (mostly mild water
stress) during summer, resulting in yield loss. Since quantitative
information on the effects of water stress on maize performance is lacking,
it can be questioned whether irrigation systems using underground water are
economically and ecologically required in this situation.
Yield reduction by water stress has been attributed to decreased crop growth
(Payero et al., 2006), canopy height (Traore et al., 2000), leaf area index
(NeSmith and Ritchie, 1992) and root growth (Gavloski et al., 1992). Crop
shoot development and biomass accumulation are greatly reduced by soil water
deficit at seeding stage (Kang et al., 2000). Short-duration water deficit
during the rapid vegetative growth period causes around 30 % loss in final
dry matter (Cakir, 2004). The reduction in maize yield by water stress can
be observed in all yield components such as ear density, number of kernels
per ear and kernel weight (Ge et al., 2012), especially for stress during or
before the maize silk and pollination period (Claassen and Shaw, 1970). Biomass
and harvest index (the ratio of grain yield over total above-ground dry
matter) are decreased under water stress during anthesis (Traore et al.,
2000).
Water use efficiency (WUE, expressed in kg yield obtained per m3 of
water) is notably reduced by severe water stress. However, a moderate water
stress at V16 (with 16 fully expanded leaves) and R1 (silking) stages in
maize increased WUE (Ge et al., 2012). Intentional irrigation deficits
before the maize tasselling stage are often used for improving WUE in regions
with serious water scarcity, e.g. the North China Plain (Qiu et al., 2008; Zhang
et al., 2017). Under water stress, plant photosynthesis and transpiration
decrease due to a decrease in stomatal conductance (Killi et al., 2017)
induced by increasing concentration of abscisic acid (ABA) (Beis and
Patakas, 2015). However, limited knowledge exists on how much the growth and
biomass partitioning between shoot and root in maize is affected by water
stress during the middle and late growing stages, and whether changes in root growth and
morphology caused by water stress could affect maize yielding and water use
efficiency.
Since field experiments that aim to quantify the effects of water stress
are difficult to carry out in rain-fed agriculture, a mobile rain shelter is
often used in studies to control water stress in the field (NeSmith and
Ritchie, 1992). The objective of this study was to quantify maize shoot and
root growth, grain yield and WUE under different water stress levels during
the middle of the crop-growing season with a well-controlled mobile rain shelter to
understand the crop response to water stress.
Water treatments during crop-growing seasons from 2014 to 2015.
Actual water supply at three growing periods (mm) YearWaterInitial volumetricEarlyMiddleLateTotaltreatmentsoil moisture(16–29 DAS*)(30–102 DAS)(103–121 DAS)content (%)2014No stress24.411.947856545Mild stress24.811.929956366Severe stress24.911.9122561902015No stress25.311.951032553Mild stress25.311.933432378Severe stress24.411.915932203
* DAS refers to days after maize sowing.
Materials and methodsExperimental design
The experiments were conducted at Shenyang (41∘48′ N,
123∘23′ E), Liaoning province, north-eastern China in 2014 and 2015.
The experimental site is 45 m above sea level. On average from 1965 to 2015,
annual potential evaporation is 1445 mm, with a total precipitation 720 mm,
and mean air temperature 8 ∘C. The frost-free period is
150–170 days. Average relative humidity is 63 %. Annual mean wind speed
is 3.1 m s-1. The climate is a typical continental monsoon climate
with four distinct seasons, characterized by a hot summer and cold winter.
The annual mean air temperature was 9.5 ∘C in 2014 and
9.1 ∘C in 2015. The mean air temperature during the crop-growing season
(May to September) was 20.2 ∘C in 2014 and 19.4 ∘C in 2015
(Fig. 1).
Daily maximum and minimum air temperatures in 2014 and 2015 in
Shengyang, Liaoning, China.
Maize plants were grown in pots in three treatments: (1) no water stress,
(2) mild water stress and (3) severe water stress (Table 1). The levels of
water stress were based on historical rainfall frequency analysis. The water
supply was controlled by a mobile rain shelter with a steel frame and
transparent PVC cover. The mobile rain shelter is built on a mechanical
movement track equipped with an electricity motor to move the shelter with a
remote control. The shelter was moved away from the experimental plots on no
rain days and covered before the rain came; therefore the effect of shelter on
incoming radiation could be ignored. The mobile rain shelter is 9 m in
width, 30 m in length and 4.5 m in height. The top and both sides of the
shelter have transparent PVC boards to prevent outside rainfall from entering. There is a
water gutter outside the movement track to drain the rainwater. Therefore
the rainwater intrusion can be avoided. Water treatments began from maize
jointing (V6, with 6 fully expended leaves) to filling stages (R3, milk)
(Abendroth et al., 2011). Water treatments were conducted by supplying
irrigation once every 5 days before starting water treatments with the same amount
for all pots, and once every 3 days during the period of water treatment. The
amount of water supplied to each treatment is listed in Table 1. The
experiments entailed a completely randomized block design with three
replicates. Each treatment consisted of 12 pots (one plant per pot) and was
divided into 3 replicates (4 pots each). At each sampling (4 samplings in
total at an interval of approximately 30 days), one pot was used.
Each pot was 40 cm in diameter and 50 cm in height, filled with 40 kg
naturally dried soil with a bulk density of 1.31 g cm-3. The large size of
pots in the experiments effectively avoided the space effect for growing good
maize. The soil was sandy loam with a pH of 6.15, total N of
1.46 g kg-1, total of P 0.46 g kg-1 and total K of
12.96 g kg-1. 46.5 g compound fertilizer (N 15 %, P2O5
15 % and K2O 15 %) and 15.5 g diammonium phosphate (N 18 %
and P2O5 46 %) were applied to each pot before sowing. No other fertilizer was applied during the maize-growing season. Maize cultivar
used in both years was Liaodan 565, a local commonly used drought-resistant
cultivar. One plant was grown in each pot. Maize was sown on 13 May and
harvested on 30 September in both 2014 and 2015.
Dry matter and grain yield measurements
To determine maize dry matter, four plants were harvested on 49 (V6,
jointing), 77 (VT, tasselling), 113 (R3, milk) and 141 (R5, dent) days after
sowing (DAS) in 2014, and one sampling was done on 132 DAS in 2015. The
samples were separated into roots and shoots and oven-dried at 80 ∘C
for 48 h until they reached a constant weight. The shoot / root ratio was
calculated using measured organ-specific dry matter.
Grain yield was measured by harvesting all cobs in a pot at maize-harvesting
time. The grain was sun-dried to a water content of 15 %. Yield
components,
i.e. ear (cob) numbers per plant, kernel numbers per ear and thousand kernel
weight were measured for each plot.
Root measurements
Root growth and morphological traits (root length, diameter and surface
area) were measured four times during the crop-growing season on 49, 77, 113,
141 DAS in 2014. All of the roots were collected for each pot at the time of dry
matter measurements. Root samples were carefully washed with tap water to
remove soil. The cleaned roots were placed on the glass plate of a root system
scanner. Scanned root images were analysed by a plant root image analyser
WinRHIZO PRO 2009 (Regent Instruments Inc., Canada) to quantify total root
length (m), diameter (mm) and surface area (m2) per plant (pot).
Measuring soil moisture content, water uptake and water use
efficiency
Soil moisture contents were measured by a soil auger at sowing and harvesting
times for each plot (three replicates per treatment). Soil cores were taken from
the middle pot for each 10 cm soil layer. After measuring fresh soil weight,
soil samples were oven-dried at 105 ∘C for approximately 48 h until
a constant weight was reached. The gravimetric soil moisture contents (%,
g g-1) measured by soil auger were calculated into volumetric soil
moisture content (%, m3 m-3) by multiplying them with soil bulk
density.
Water uptake (WU) of maize was calculated using a simplified soil water
balance equation (Kang et al., 2002). Because the experiments were
sheltered, rainfall, drainage and capillary rise of water did not occur in
this situation and therefore were not taken into account in the calculation:
WU=I+ΔS,
where WU (mm) is crop water uptake (mm) during the whole of the crop-growing season, I
is the amount of water supplied to each pot (mm). ΔS is the change of
total soil water between sowing and harvesting dates.
Water use efficiency (WUE) was calculated by measuring final yield or
above-ground dry matter and total WU during the crop-growing season (Zhang et
al., 2007).
WUE=Y/WU,
where WUE (g m-2 mm-1 or kg m-3) is water use efficiency
expressed in gain yield WUEY or dry matter WUEDM. Y
(g m-2) is grain yield or dry matter.
Statistical analysis
Analysis of variance on yield, WU, WUE and dry matter for shoot and root were
performed using a general linear model of SPSS 20 (SPSS Inc., Chicago, USA).
The differences between means were evaluated through least significant difference multiple comparison tests
at a significant level of 0.05.
Yield and yield components affected by different water stress from
2014 to 2015.
The same lower-case letters indicate no significant difference between water treatments within the same year at a=0.05.
ResultsVariation and frequency distribution of rainfall
The average rainfall during the maize-growing season (May to September) at an
experimental site from 1965 to 2015 was 531 mm with a standard deviation of
134 mm (Fig. 2a). Rainfall in the experimental years was much less than in a
normal year, 296 mm in 2014 and 379 mm in 2015. The frequency of years with
rainfall above 500 mm was 68.6 % over the past 51 years. For years with
mild drought stress (350–450 mm), this was 27.5 % and with severe
drought stress (200–300 mm) it was 3.9 % (Fig. 2b), indicating that
maize growing in this region mainly suffered from mild water stress.
Anomalies and cumulative frequency of rainfall during the maize-growing
season (May to September) from 1965 to 2015 at Shengyang, Liaoning.
Yield and yield components
The maize yield under mild water stress over 2 years was not significantly
different, while in severe stress the yield was 55.6 % lower than in the no water
stress control (Table 2). The decrease of maize yield in severe water
treatment was due to the decreases in ear and kernel numbers as well as the
harvest index (HI). However, water stress did not affect kernel weight, while
other yield components were decreased. The yearly effect was only significant for
HI, which was likely caused by the variation in air temperature: the cooler
weather in 2015 during the maize-growing season decreased the HI compared with a
warmer year in 2014. There were no interactions between year and treatment.
Above- and below-ground dry matter
Mild water stress did not reduce root dry matter (Fig. 3a, b), but greatly
reduced shoot dry matter, especially at grain-filling stage (113 DAS)
(Fig. 3c, d). The severe water stress decreased both root and shoot dry
matter compared with no stress control, but the magnitude of the decrease in
shoot was much larger than in root. At maize tasselling stage (77 DAS), as
taproots reached their maximum size, root dry matter under severe water
stress was much lower than mild and no water stress treatments. However, it
was less different later in the season, which indicated a strong
complementarily growth of root system under water stress. Due to the
different responses of shoot and root to water stress, the root/shoot ratios
under water stress increased (Fig. 3e, f), especially during crop rapid
growing period (77 to 113 DAS).
Root and shoot dry matter of maize under water stress at different
growing stages in 2014–2015.
Root length, diameter and total surface area affected by water
stress
Root length per plant was much lower under severe water stress than in the
control, especially at the tasselling stage (77 DAS). The decrease of root length
under mild water stress during the middle of the maize-growing season was much smaller than
under severe stress (Fig. 4a). Root diameters under both mild and severe
water stress treatments were much higher than under the no water stress control
(Fig. 4b), especially during the late growing season. The total root surface area was
less changed (Fig. 4c), especially during the reproductive growth period
(113 DAS).
Total root length, average diameter and total surface area per plant
affected by water stress in 2014.
Water uptake and use efficiency
Total water uptake (WU) reduced by 28.9 % under mild water stress and by
54.6 % under severe stress compared with no stress control (588 mm)
(Fig. 5). Water use efficiency for maize above-ground dry matter
(WUEDM) under both water stress treatments across all years
increased by 31.2 % compared with no stress control (Fig. 5b). The
WUEDM in severe water stress was the highest (14.4 kg m-3),
which was 42.2 % higher than the control, while that in mild stress
increased by 20.2 %. However, WUE for grain yield under severe water stress
(3.51 kg m-3) was not significantly different from that in the control
(3.38 kg m-3), while WUEY in mild water stress over 2 years
increased by 17.3 % (Fig. 5c). The difference between WUEs in dry matter and
grain yield was due to the extent of decreasing HI under the levels of water
stress (Table 2).
Total water uptake (WU) during the crop-growing season and water use
efficiency for above-ground dry matter (WUEDM) and grain yield
(WUEY) under water stress in 2014–2015.
Discussion
Mild water stress from maize jointing (V6) to filling stages (R3) did not
significantly reduce maize grain yield. This is different from a previous report
which claimed that maize yield is much more affected by water stress during the flowering
stage than at other stages (Doorenbos et al., 1979). Our result differed from a
previous study, which showed that mild water stress seriously reduced crop
production (Kang et al., 2000). This is likely due to our choice of a
drought-resistant variety (Zhengdan 565) and the difference in ecological
zones. Genotype-dependent relationships between yield and crop growth rate
would be stronger under water stress than under the no stress condition (Lake
and Sadras, 2016).
Mild water stress during the middle of the crop-growing period can maintain maize yield but
substantially reduces the water consumption at the same time in our study.
Thus, the water use efficiency was increased (Liu et al., 2016). Mild water
stress reduced total water uptake, resulting in a 20.2 % higher WUE in dry
matter and 17.3 % in yield. The increase in WUE under mild water stress
benefitted from the morphological responses of shoot and root growth to
water stress with an increase in root / shoot ratio. The water stress
reduced root length; however, this reduction was compensated by an increase
in root diameter. The maintenance of crop growth under water deficit was
limited by the severity of the stress. Under severe water stress, maize
growth fails to be compensated by plant plasticity.
Severe water stress greatly reduced both shoot and root biomass. A large
decrease in shoot growth, i.e. less biomass and leaf area, reduces the light
interception and transpiration (Monteith, 1981). Under mild water stress
during vegetative and tasselling stages, the shoot growth was not
significantly reduced in this study but was in a previous report, e.g. in plant
height and leaf area (Cakir, 2004). Mild soil water deficit may also reduce
water loss of plants through physiological regulation (Davies and Zhang,
1991). Moderate soil drying at the vegetative stage encourages root growth
and distribution in deep soil (Jupp and Newman, 1987; Zhang and Davies,
1989), which is consistent with our findings. A large root system with deep
distribution is beneficial for water-limited agriculture (McIntyre et al.,
1995). These mechanisms explained why maize yield under mild water stress
did not decrease in our study.
We found an increase in root diameter under water stress. This result
indicated that there were fewer lateral roots under water stress than
under no water stress. This may limit water absorption since the lateral
roots is younger and more active in uptake function (Lynch, 1995). Average
root diameters in all treatments decreased from 77 to 113 DAS, which was
caused by highly emerged lateral roots after the taproot reached its maximum
(VT stage). The higher root diameter under water stress than in the no water
stress control at 141 DAS was probably due to a fast senescence of late-developed lateral roots.
Our results on root morphological plasticity under mild water deficit
provided more evidence for the explanation of enhancing WUE and
maintaining yielding in relation to the crop–water response. However, the
mechanism that determines the crop response to water stress may also involve
other processes, e.g. intercellular CO2, stomatal conductance,
photosynthetic rate, oxidative stress, sugar signaling, membrane stability
and root chemical signals (Xue et al., 2006; Dodd, 2009). The relationship
between carbon assimilation and water stress has been widely explored to
understand the physiological mechanism for improving WUE (Ennahli and Earl,
2005; Xue et al., 2006; Zhang et al., 2013). The abscisic acid (ABA)-based
drought stress chemical signals regulate crop vegetative and reproductive
development and contribute to crop drought adaptation (Killi et al., 2017).
Increased concentration of ABA in the root induced by soil drying may
maintain root growth and increase root hydraulic conductivity, thus
alleviating the water deficit in the shoot (Liu et al., 2005). The increase of ABA
can also induce stomatal closure and reduce crop transpiration (Haworth et
al., 2016), net photosynthesis and crop growth (Killi et al., 2017).
The maize yield in 2015 was much lower than in 2014 independent of water
stress. That might be caused by a higher maximum air temperature in 2015
(32.0 ∘C) than in 2014 (29.1 ∘C) during the flowering period.
High air temperature reduces maize pollination (Muller and Rieu, 2016) and
directly affects yield formation and HI.
Conclusions
This study clearly demonstrates that the maize yield under mild water stress
during summer does not decrease but the water use efficiency increases due to
changes in root and shoot growth. A higher root / shoot ratio under mild
water stress allows plants to efficiently use limited soil water. In the studied
region (Liaoning province), maize mainly grows in rain-fed conditions (2.4 million ha), covering 73 % of the total area for grain crops. To reduce the
possible effect of drought on maize production, a well system that pipes
ground-water to irrigate crops has recently been planned. The wells need to be 60 to
70 m deep and have an average cost of 12 000 Yuan each. Each well can only
irrigate 9 to 10 ha of maize. According to our results, only severe water
stress significantly reduces maize yield by 55.6 % across two
experimental years (Table 2), which occurs only 3.9 % during 1965 to
2015. Mild water stress occurs much frequently (27.5 % of years);
however, it does not significantly affect maize yield. Our study suggested
that the well system in this region might not be economically and
ecologically necessary. Other agronomy practices such as intercropping maize
with crops requiring less water (e.g. peanut), cultivar selection, adjusting
sowing windows (Liu et al., 2013; Lu et al., 2017) and ridge-furrow with
covering plastic film (Dong et al., 2017) are likely more applicable in
optimizing crop yield and regional sustainability. Our study provides more
evidence to understand crop responses to water stress, especially in
relation to root morphological plasticity in a drought environment. The
results can be further applied by combining them with a crop model (Mao et al., 2015)
to mitigate climate risk in dry-land agriculture.
The data are available at
http://pan.baidu.com/s/1skGRASd and in the
Supplement.
The Supplement related to this article is available online at https://doi.org/10.5194/bg-14-3851-2017-supplement.
ZS, YZ, JZ and QC conceived and designed the
experiments. QC, WB, YZ, YL, LF, CF, ZZ and NY performed the experiments. LZ,
QC and JBE analysed the data and wrote the paper.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Ecosystem processes
and functioning across current and future dryness gradients in arid and
semi-arid lands”. It is not associated with a conference.
Acknowledgements
This research was supported by the National key research and development
programme of China (2016YFD0300204), the International Cooperation and
Exchange (31461143025) and the Youth Fund (31501269) of the National Science
Foundation of China, Liaoning BaiQianWan Talent Program (201746), Outstanding
Young Scholars of National High-level Talent Special Support Program of
China.Edited by: Zisheng Xing
Reviewed by: two anonymous referees
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