YIN Wen, FAN Zhi-long, HU Fa-long, FAN Hong, HE Wei, SUN Ya-li, WANG Feng, ZHAO Cai,YU Ai-zhong, CHAI Qiang#

1 State Key Laboratory of Aridland Crop Science, Lanzhou 730070, P.R.China

2 College of Agronomy, Gansu Agricultural University, Lanzhou 730070, P.R.China

Abstract Straw returning to the field is a technical measure of crop production widely adopted in arid areas.It is unknown whether crop yield can be further increased by improving the eco-physiological characteristics when straw returning is applied in the crop production system.So, a three-year field experiment was conducted with various straw returning treatments for wheat production: (i) no-tillage with straw mulching (NTSM), (ii) no-tillage with straw standing (NTSS),(iii) conventional tillage with straw incorporation (CTS), and (iv) conventional tillage with no straw returning (CT, control).The eco-physiological and yield formation indicators were investigated to provide the basis for selecting the appropriate straw returning method to increase wheat yield and clarifying its regulation mechanism on eco-physiology.The results showed that NTSM and NTSS treatments had better regulation of eco-physiological characteristics and had a higher yield increase than CTS and CT.Meanwhile, NTSM had a relatively higher yield than NTSS through better regulation of eco-physiological characteristics.Compared to CT, the leaf area index of NTSM was decreased by 6.1–7.6% before the Feekes 10.0 stage of wheat, but that of NTSM was increased by 38.9–45.1% after the Feekes 10.0 stage.NTSM effectively regulated the dynamics of the photosynthetic source of green leaves during the wheat growth period.NTSM improved net photosynthetic rate by 10.2–21.4% and 11.0–21.6%, raised transpiration rate by 4.4–10.0% and 5.3–6.1%,increased leaf water use efficiency by 5.6–10.4% and 5.4–14.6%, at Feekes 11.0 and 11.2 stages of wheat, compared to CT, respectively.NTSM had higher leaf water potential (LWP) by 7.5–12.0% and soil water potential (SWP) by 8.9–24.0%from Feekes 10.3 to 11.2 stages of wheat than CT.Meanwhile, the absolute value of difference on LWP and SWP with NTSM was less than that with CT, indicating that NTSM was conducive to holding the stability of water demand for wheat plants and water supply of soil at arid conditions.Thus, NTSM had a greater grain yield of wheat by 18.6–27.3% than CT, and the high yield was attributed to the synchronous increase and cooperative development of ear number, grain number per ear, and 1 000-grain weight.NTSM had a positive effect on regulating the eco-physiological characteristics and can be recommended to enhance wheat grain yield in arid conditions.

Keywords: Triticum aestivum L., straw management, grain yield, eco-physiological traits, arid ecological environment

1.Introduction

The scarcity of water resources has become the core problem affecting global agricultural production (Caoet al.2017; D’Odoricoet al.2020).Globally, arid and semiarid areas with water shortages account for more than 40% of the total arable area and feed more than 30%of citizens around the world (Reynolds 2007; Schimel 2010).Agricultural production in dry regions where water resources are scarce plays a significant role in grain supply around the world.As is known to all, the inland irrigated areas in Northwest China are the main areas of grain production, and extreme climate and water shortage are the major constraints on agricultural production in this region (Shareefet al.2018; Wang Let al.2019).

Photosynthesis in plants is a very complicated process involving biochemical, electrochemical, and physicochemical reactions (Nobel 2005; Baker and Habershon 2017).Soil hydro-thermal characteristics are the key ecological factors that regulate photosynthesis and affect crop production (Fernandez-Marinet al.2020; Rathodet al.2022).Numerous publications have demonstrated that water scarcity and high soil temperature decreased leaf water potential (LWP) and chlorophyll relative content (SPAD) for green leaves by inhibiting the root action of crops, thus reducing the net photosynthetic rate (Pn) and transpiration rate(Tr) and affecting the grain filling (Hassan 2006).Soil drought inhibited the photochemical activity of PSII by reducing the water content and leaf area index (LAI) of crop green leaves, which resulted in the decrease ofPnandTrand affected the yield formation (Gentyet al.1989; Hassan 2006).Previous studies have shown that photosynthesis of crops from flowering (Feekes 10.5.2)to maturating (Feekes 11.4) accumulates more than 65%of the carbohydrate, which was used for grain formation(Bidingeret al.1977; Baker and Habershon 2017), it is very important to keep relatively high photosynthetic capacity in the late growing period for crop yield increase and stability.Hence, it is of great significance to improve grain yield by optimizing cultivation measures to create a suitable soil ecological environment and maintain the higher photosynthetic capacity of crops in the late growth period.

It is well known that the photosynthetic capacity of plants is determined by their own genetic characteristics and surrounding environmental conditions (Nobel 2005;Silva-Pérezet al.2019).The influence of environmental conditions on plant photosynthetic efficiency was further enhanced in arid areas with a water shortage and serious desertification (Shareefet al.2018; Guoet al.2021).The selection of tillage methods and mulch materials is the key strategy to affect the above-ground photosynthetic performance of crops by adjusting the soil ecological environment (Jiaet al.2020; Chenet al.2023).Straw returning has been widely used in semi-arid and arid areas where there is a scarcity of water resources due to its advantages of harvesting and maintaining soil moisture (Huet al.2016; Wanget al.2018; Wuet al.2021).Straw returning can achieve high and stable crop yield and efficient water utilization by reducing ineffective evaporation of soil water, enhancing soil water retention and increasing soil water potential (SWP), and reducing soil heat loss to air (Chenet al.2007; Sarkaret al.2007; Liet al.2022).Straw returning is beneficial to uniform and rapid seed germination, promotes cooperative growth of above- and below-ground crops,and enhances crop productivity by improving the hydrothermal microenvironment of topsoil (Lópezet al.2018;Yanet al.2018).However, straw returning sometimes affects normal growth and development by reducing the photosynthetic performance, which is because the lower soil temperature inhibits the root activity of crops(Khanet al.2019; Meyeret al.2021).Hence, it is very urgent to study an effective straw returning method to boost crop production by optimizing the eco-physiological characteristics in areas of water scarcity.

The practice of crop production has proved that the cultivation of cool-loving wheat often relies on straw returning technology in arid areas in Northwest China where water resources are extremely scarce (Chaiet al.2014; Yinet al.2018).Most of the previous publications were single studies on optimizing soil hydrothermal effect and physical and chemical properties through straw returning in wheat production (Chenet al.2007; Wanget al.2019).Few studies have focused on the effects of different straw returning ways on ecophysiological characteristics in crop production systems.The central hypothesis of this common and promising cultivation practice is that it can improve the physiological characteristics for boosting wheat yield in arid oasis regions.We further point out that wheat’s physiological ability improves since straw returning effectively regulates the difference between LWP and SWP to maintain the stability of water demand for wheat plants and soil water supply at arid conditions.However, the mechanism of straw returning regulating the eco-physiological traits of crops and thus affecting wheat yield has not been well explained.Thus, the present study aimed: 1) to investigate the effects of straw management practices on LAI, SPAD,Pn, andTrof green leaves in wheat; 2) to explore the effects of straw management practices on LWP of green leaves in wheat plants and SWP of wheat field, and quantify the difference on LWP and SWP can be used to evaluate the capacity of water absorption in green leaves and the stability of soil water supply for wheat plants; 3) to detect the effects of straw management practices on grain yield of wheat; and 4) to select a way of straw returning that is useful and suitable for wheat production in arid, semiarid, and other areas with similar climatic and ecological conditions.

2.Materials and methods

2.1.Study site

A three-year field experiment was carried out in 2014–2016 at the Huangyang Town in Wuwei City of Northwest China.The soil classification in the experimental area was Aridisol.The annual mean rainfall in the area over the past 20 years was less than 180 mm, which obviously could not meet the water demand of crop production, so crop production in this region is completely dependent on irrigation.In the last 20 years, the annual average temperature was 7.2°C and the accumulated temperature higher 10°C is about 2 900°C, which is in line with the thermal requirements for spring wheat (the required accumulated temperature above 10°C is 1 700–1 900°C)of the main local cultivated crop.Total rainfall was 100.9,108.7, and 106.9 mm, and daily average air temperature was 17.6, 18.0, and 18.3°C, across the three-growth seasons of wheat in 2014–2016, respectively.

2.2.Experimental design

In this study, four different straw management treatments(Table 1) were tested using a completely randomized block design.There were 12 plots in total due to each treatment was repeated three times in this study, and each plot had an area of 48 m2.The experiment was initiated in 2013 to set up various ways of returning straws to the field, and the amount of straw returning to the field was 4 200 kg ha–1; the experimental data from 2014 to 2016 are presented in this paper.This study consisted of four ways to return wheat straws in the field: (i) no-tillage with straw mulching (NTSM,25–30 cm length of wheat straws were cut 3-5 cm andevenly distributed on the soil surface), (ii) no-tillage with straw standing (NTSS, the wheat straws were maintained at the length of 25-30 cm stand the soil surface), (iii)conventional tillage with straw incorporation (CTS, the length of 25-30 cm wheat straws were cut up and plowed with depths of 25-30 cm), and (iv) conventional tillage with no straw returning (CT, wheat straws were removed and plowed the depths of 25-30 cm).The field operations of different treatments are shown in Fig.1.Plots were fertilized, rotary tilled, and compacted, and wheat seeds were sown by a special planter for all the treatments.A small harvester was used for wheat harvesting in each plot.

The irrigation amount was 2 400 m3ha–1during the entire growth season of wheat, and 750, 900, and 750 m3ha–1were irrigated at Feekes 3.0, 10.0, and 11.0 stages of wheat, respectively.The nitrogen (pure N) and phosphorus (pure P2O5) amounts for the base fertilizer were 225 and 150 kg ha–1for wheat production,respectively.Wheat variety Ningchun 4 was selected in the experiment, and the planting density was 675 plants m–2.Wheat was planted on 21, 29, and 30 March and harvested on 24, 28, and 21 July in 2014, 2015, and 2016, respectively, and the planting and harvesting dates of wheat in all plots were consistent.

2.3.Data collection

Leaf area indexAfter the emergence of wheat, 20 wheat plants were selected randomly by S-shape to measure the leaf area at an interval of 15–20 d for each treatment,repeated three times at various growth stages (Table 2).The leaf length (ai) and the greatest leaf width (bi) were measured using a ruler, and the LAI was determined using the following formula:

wherePwas the seedling number of wheat, 0.83 was the compensation coefficient of wheat.

Table 2 The growth stage of wheat at sampling times in 2014–2016

Chlorophyll relative contentThe SPAD with flagleaf for three wheat plants in the center of each plot was measured by a portable chlorophyll meter (SPAD 502Plus, Konica Minolta, Japan) at Feekes 10.3, 11.0 and 11.2 stages of wheat.

Photosynthetic indicesIn this study,PnandTrwith flagleaf for three wheat plants in the center of each plot were measured by a Portable Photosynthesis System (LI-Cor 6400XT, Licoln, Nebraska, USA) at Feekes 10.3, 11.0 and 11.2 stages of wheat.PnandTrwere measured in the sunlight of the morning at 9:00–11:00 a.m.to avoid the stomatal closure at noon.At the same time, leaf water use efficiency (WUEL) was determined byPndivided byTr.

Soil water potential and leaf water potentialSWP(MPa) was measured with soil tension meters (TEN-45,Tuopu, Zhejiang, China) in 0–120 cm (at 30 cm intervals)soil depths in the central rows in each treatment with three plots.The measurement date was consistent with physiological indexes.The SWP in each plot was the average of the four measuring values in various soil depths.

Leaf water potential (MPa) in the upper three green leaves for three wheat plants in the center of each plot for all treatments was measured by a dew point potentiometer (WP4C, Decagon Inc., Washington, USA)at Feekes 10.3, 11.0, and 11.2 stages of wheat.

Additionally, the difference in LWP and SWP was calculated at each measuring time to judge the stability of soil water supply and crop water demand for wheat.

Grain yield and its componentsWhen the wheat crop was mature, all the experimental plots were manually harvested, and the grain yield (GY, kg ha–1) in each treatment with three plots was quantified according to the air-drying weight.The sampling 3.6 m2(3 m in length with 10 rows) was selected to investigate the ear number(EN) for wheat in each treatment with three plots.Also,30 wheat plants were randomly sampled to determine the grain number per ear (GNE).The grain yield per unit area and 1 000-grain weight (TGW) are converted to the standard grain water content of 13%.

2.4.Statistical analysis

All of the experimental data were analyzed with the Statistical Analysis Software (SPSS 18.0, Chicago, IL,USA) for ANOVA analysis after the homogeneity test of variance was performed.The significances of separate means of all observed indicators in this manuscript among treatments were presented atP<0.05 using Duncan’s test.

3.Results

3.1.Straw management regulated LAI of wheat

Straw returning significantly increased the average LAI during the whole growth season of wheat (Fig.2).The average LAI with NTSM, NTSS, and CTS was 14.6–17.2%, 10.4–11.9%, and 7.3–9.4% higher than that with CT, and that with NTSM was 6.6–7.1% higher than that with CTS.LAI of wheat increased first and then decreased across the three experimental years, and LAI reached the peak at the Feekes 10.0 stage.LAI of NTSM and NTSS was 6.1–7.6% and 4.6–9.8% lower than that of CT, respectively, before the Feekes 10.0 stage of wheat.However, after the Feekes 10.0 stage, NTSM, NTSS,and CTS increased LAI by 38.9–45.1%, 30.7–32.6%, and 16.4–17.5% compared to CT, and NTSM increased LAI by 6.2–9.4% and 19.3–24.5% compared to NTSS and CTS,respectively.These results indicated that no-tillage with straw mulching could effectively regulate the dynamics of the photosynthetic source of green leaves for the wheat growth period, guaranteeing a higher LAI in the later growth period.It is beneficial to maintain higher photophysiological characteristics and promote grain filling to achieve high yields.

3.2.Straw management optimized SPAD and photosynthetic traits for wheat

Fig.2 Leaf area index (LAI) dynamic and mean LAI for the entire growth period of wheat as affected by straw management in 2014–2016 at arid irrigated regions.NTSM, no-tillage with straw mulching; NTSS, no-tillage with straw standing; CTS, conventional tillage with straw incorporation; CT, conventional tillage with no straw returning.Bars are standard errors (n=3).Different letters indicate significant differences (P<0.05) among treatments.

Chlorophyll relative content of green leaves for wheatStraw management significantly affected the SPAD of green leaves for wheat (Table 3).The SPAD value of green leaves for wheat with NTSM was decreased at the Feekes 10.3 stage but was increased at Feekes 11.0 and 11.2 stages.At the Feekes 10.3 stage, NTSM and NTSS had less SPAD value of green leaves for wheat by 7.9–13.2% and 5.9–10.1% than CT and 5.3–8.2% and 4.5–4.9% than CTS, respectively.However, at the Feekes 11.0 stage, the SPAD value of green leaves for wheat in NTSM,NTSS, and CTS was 9.9–15.7%, 9.8–15.9%, and 4.9–6.4%higher than that in CT, and that in NTSM was 4.7–8.8%and 4.8–8.9% (except 2016) higher than that in CTS, but no significant difference was observed between NTSM and NTSS.Similar to the Feekes 11.0 stage of wheat, NTSM,NTSS, and CTS had greater SPAD values of green leaves for wheat by 20.8–29.4%, 15.9–19.8%, and 6.8–9.8% than CT, and NTSM and NTSS had greater by 13.1–17.8% and 8.0–9.1% than CTS, respectively, at the Feekes 11.2 stage.Although NTSM reduced the SPAD value of green leaves for wheat at the Feekes 11.0 stage, it maintained a higherSPAD value of green leaves for wheat at Feekes 11.0 and 11.2 stages, indicating that NTSM could maintain a higher physiological activity in the later growth stage of wheat.

Table 3 Chlorophyll relative content (SPAD), net photosynthetic rate (Pn, μmol m–2 s–1), and transpiration rate (Tr, mmol m–2 s–1) of wheat with different growth stages as affected by straw management in 2014–2016 at arid irrigated regions

Net photosynthetic rate of green leaves for wheat Notillage with straw returning decreasedPnof green leaves for wheat at the Feekes 10.3 stage, but it was increased at Feekes 11.0 and 11.2 stages (Table 3).At the Feekes 10.3 stage, NTSM and NTSS reducedPnof green leaves for wheat by 6.6–7.6% in 2014–2016 and 3.8–9.5% in 2014–2015, respectively, over CT, and NTSM decreasedPnof green leaves for wheat by 4.4–6.0% compared to CTS in 2014–2016.However, at the Feekes 11.0 stage,Pnof green leaves for wheat was 10.2–21.4% and 7.6–14.4%greater with NTSM and NTSS than that with CT, and was 6.0–11.8% greater with NTSM than that with CTS.Similarly, at the Feekes 11.2 stage,Pnof green leaves for wheat was 11.0–21.6% and 8.2–18.1% greater with NTSM and NTSS than that with CT, and was 7.4–11.9% and 4.8–8.6% greater with NTSM and NTSS than that with CTS.Although thePnof green leaves for wheat was reduced with NTSM and NTSS at the Feekes 10.3 stage, a higherPnvalue of green leaves for wheat with NTSM and NTSS was kept at Feekes 11.0 and 11.2 stages, this effectively made up for the earlier growth reduction and had a compensatory effect on the high grain yield for wheat.

Transpiration rate of green leaves for wheat At the Feekes 10.3 stage, theTrvalue of green leaves for wheat was decreased by 10.9–11.3% and 7.9–12.9% with NTSM and NTSS in comparison to CT (Table 3) and was decreased by 8.2–9.8% and 5.0–10.3%, respectively, in comparison to CTS.At Feekes 11.0 and 11.2 stages,only NTSM had 4.4–10.0% and 5.3–6.1% greaterTrof green leaves for wheat than CT.It indicated that NTSM maintained higherTrof green leaves in the middle and late stages of grain-filling for wheat.This is beneficial to enhance the effective use of water in wheat.

Leaf water use efficiency of wheat No-tillage with straw returning significantly improvedWUELof wheat at three growth stages across the three studied years(Fig.3).NTSM and NTSS treatments had 3.7–4.9% and 4.0–5.2% greaterWUELof wheat than CT at the Feekes 10.3 stage.Also, theWUELvalue with NTSM and NTSS was 5.6–10.4% and 4.4–6.0% at the Feekes 11.0 stage and 5.4–14.6% and 4.7–11.6% at the Feekes 11.2 stage than that with CT.NTSM had greaterWUELby 3.7–6.9%and 4.0–8.9% than CTS at Feekes 11.0 and 11.2 stages,respectively.Both NTSM and NTSS treatments had the potential to enhance efficient water use, and the potential of NTSM to improve the water use efficiency of wheat is greater than that of NTSS.

3.3.Straw management affected the leaf and SWP of wheat

Fig.3 Leaf water use efficiency (WUEL) of wheat as affected by straw management across the various wheat growth stages, in 2014–2016 at arid irrigated regions.NTSM, no-tillage with straw mulching; NTSS, no-tillage with straw standing; CTS, conventional tillage with straw incorporation; CT, conventional tillage with no straw returning.Bars are standard errors (n=3).Different letters indicate significant differences (P<0.05) among treatments.

Leaf water potential of green leaves for wheat At three measuring stages, no-tillage with straw returning had greater LWP of green leaves for wheat than conventional tillage (Table 4).At the Feekes 10.3 stage,NTSM increased the LWP of green leaves for wheat by 7.5–12.1% compared to CT in 2014–2015 and 7.3–9.5%over CTS.At Feekes 11.0 stage, the LWP value of green leaves for wheat was 8.4–10.9% with NTSM and NTSS in 2014–2016 and 5.3–7.6% in 2014–2015 than that of CT, and was greater 7.8–12.8% and 4.4–6.1% with NTSM and NTSS in 2014–2016 than that of CTS, and NTSM had greater LWP of green leaves for wheat by 7.1% than NTSS in 2016.NTSM and NTSS had 8.7–12.0% and 4.0–8.3% greater LWP of green leaves for wheat than CT,respectively, and NTSM had 5.2–8.3% greater than CTS at the Feekes 11.2 stage across the three studied years.These results indicated that no-tillage with straw mulching could maintain a high moisture content for wheat plants.It was beneficial to further enhance the drought resistance of wheat and reduce the restriction degree of soil moisture change on the physiological activity of wheat.

Soil water potential of wheat field Across three measuring stages, no-tillage with straw returning significantly increased the SWP of wheat fields over conventional tillage (Table 4).In 2014–2016, at the Feekes 10.3 stage, SWP of wheat field with NTSM and NTSS was increased by 10.9–24.0% and 4.7–11.0%compared to CT, and that with NTSM was increased by6.2–12.7% over CTS.At the Feekes 11.0 stage, the SWP of wheat fields was 8.9–15.1% in 2014–2016 and 5.2–8.3% in 2014 and 2016, with NTSM and NTSS greater than that with CT, and 6.2–11.3% with NTSM greater than that with CTS in 2014–2016.Similarly, NTSM and NTSS had greater SWP of wheat fields by 12.6–15.7% and 6.8–8.7% than CT, and greater by 5.1–11.3% and 4.0–5.2%than CTS, at the Feekes 11.2 stage, respectively, during the three research years.NTSM maintained a high SWP of wheat fields, thus creating a good soil moisture environment for the healthy growth of wheat plants.

Table 4 Leaf water potential (LWP, MPa), soil water potential (SWP, MPa), and the difference value (DV, MPa) between leaf and soil water potential of wheat with different growth stages as affected by straw management in 2014–2016 at arid irrigated regions

Differences in LWP and SWP The difference in LWP and SWP can be used to judge the influence of straw returning on the driving force of soil water flow to leaves.At three measuring stages, the absolute value of the difference in LWP and SWP with NTSM was less than that with CTS and CT (Table 4).In 2014–2016, NTSM decreased the absolute value of difference on LWP and SWP by 3.5–11.9%, 8.3–10.9%, and 8.6–12.0% than CT, and 7.2–9.6%, 7.8–13.0%, and 5.2–8.3% than CTS,at Feekes 10.3, 11.0, and 11.2 stages, respectively.It indicated that NTSM reduced the driving force of water flow from soil to wheat leaves due to high LWP, which was conducive to maintaining the stability of soil water supply and water demand for wheat plants at water scarcity regions.So, NTSM treatment could stabilize soil water supply for wheat demand and relieve the excessive influence of water deficit changes on wheat growth, which laid the foundation for wheat to achieve high yield.

3.4.No-tillage with straw returning enhanced grain yield of wheat and coordinated yield components

Grain yield Straw returning had a significant effect on increasing wheat grain yield across the three experimental years.Compared to CT, NTSM, NTSS,and CTS increased grain yield of wheat by 18.6–27.3%,16.6–24.9%, and 10.2–18.7%, and NTSM and NTSS had 7.2–9.5% and 5.2–5.9% higher grain yield of wheat than CTS, respectively (Table 5).The grain yield of NTSM was the highest among the four treatments, reaching 7 203–8 035 kg ha–1.These results indicated that no-tillage with mulching of 25–30 cm long wheat straws had the potential to increase wheat grain yield.

Yield components The ear number (EN) per unit area,grain number per ear (GNE), and 1 000-grain weight(TGW) of wheat were significantly increased by straw returning (Table 5).NTSM, NTSS, and CTS increased EN value by 10.3–11.2%, 7.4–8.3%, and 5.3–6.7%compared to CT, and NTSM increased EN value by 4.1–4.8% compared to CTS.The GNE value with NTSM, NTSS, and CTS was 18.4–22.5%, 15.0–18.2%,and 5.7–9.0% higher than CT, and that with NTSM and NTSS was 12.0–12.7% and 5.8–9.7% higher than CTS,respectively.Additionally, NTSM and NTSS had greater TGW values by 13.7–14.2% and 7.2–8.7% than CT, and NTSM had greater TGW value by 9.5–10.9% than CTS.In conclusion, no-tillage with mulching of 25–30 cm long wheat straws (NTSM) treatment was the highest yield component among the three straw returning treatments,which was the basis for wheat to achieve high yield.

3.5.Relationship among grain yield and eco-physiological parameters and yield components of wheat

Correlation analysis Principal component analysis(PCA) was conducted to explain the relationship among wheat grain yield (GY) and eco-physiological parameters and yield components (Fig.4).The first principal component was composed of GY, TGW, GNE,EN, LWP, DV (the absolute value of difference on LWP and SWP), SPAD, andWUEL, and its contribution rate reached 62.8%.The second principal component was composed ofTr,Pn, and SWP, and its contribution rate reached 27.8%.GY was significantly positively correlated with GNE, TGW, EN, and LWP, and GY was significantly correlated with SPAD,WUEL,Pn,Tr, and SWP.However,GY was significantly negatively correlated with DV.At the same time, yield components were more closely related to grain yield than eco-physiological parameters.The above analysis indicates that the wheat grain yield improvement of no-tillage with straw mulching was because of the coordination of yield components through the optimization of eco-physiological characteristics.

Incidence matrix analysis In order to clarify the influence degree of various factors on the grain yield of wheat, the incidence matrix analysis of grain yield and its influencing factors showed that the dominant factor affecting the grain yield is yield components, followed by eco-physiological parameters (Table 6).In terms of yield compositions, the order of influencing degrees of grain yield for wheat was GNE, TGW, and EN.Among the ecophysiological parameters, the order of influencing grain yield of wheat was SPAD,Pn,WUEL,Tr, LWP, DV, and SWP.The above results showed that no-tillage with straw mulching could promote the coordinated development of three yield components of wheat by optimizing the soil moisture environment, increasing moisture content in wheat plants, and improving photo-physiological characteristics of green leaves.It is a feasible way to enhance the grain yield of wheat by using appropriate agronomic measures to coordinate and synchronously improve yield components in arid conditions.

4.Discussion

4.1.Regulation effect of straw management on the photosynthetic source of wheat

Leaves are the most important component of photosynthetic sources in crops.Increasing the green leaf area and prolonging the green holding time play a key role in the utilization of light energy, the absorption of soil water and nutrients, and the formation of yield (Peltonen-Sainioet al.1997; Hafeezet al.2019).The increase in grain yield is not only the result of a prolonged reproductive growth period but also the increase in total leaf area and delayed leaf senescence, both of which lead to prolonged photosynthetic time (Ahmadet al.2018; Hafeezet al.2019).Since nutrient uptake by plant roots is dependent on the continuous supply of carbohydrates by plants, a longer photosynthetic time is beneficial to nutrient uptake by plants during the reproductive growth period (Ibrahimet al.2020; Liet al.2020).In turn, the duration of canopy photosynthesis can be prolonged by maintaining a large LAI during the reproductive growth period, and finally, the grain yield can be increased (Ciampitti and Vyn 2011).In this study, straw returning (NTSM, NTSS, and CTS)effectively regulated the dynamics of the photosynthetic source of green leaves during the wheat growth period,guaranteeing a higher LAI in the later growth period.Meanwhile, the regulated effect on LAI of NTSM wasbetter than NTSS and CTS.Because the regulation effect of NTSS and CTS on soil hydro-thermal traits was weaker than that of NTSM (Yinet al.2020a).So, NTSM can maintain higher photo-physiological characteristics and promote grain filling for achieving high yield.The possible reasons for this result are mainly shown in four aspects:1) no-tillage with straw mulching reduced soil temperature at the early growth stage of low air temperature season(Yinet al.2020a), delayed the emergence date of wheat(Yinet al.2020b), resulting in lower photosynthetic area,thus decreasing LAI before wheat Feekes 10.0 stage;2) no-tillage with straw mulching was beneficial to the root growth of wheat at the low air temperature season,but delayed shoot growth of wheat (Fenget al.2010),resulting in low LAI before wheat Feekes 10.0 stage;3) no-tillage with straw mulching had less moisture and nutrient consumption because of the comparatively slower growth at low air temperature season than conventional tillage with no straw returning, the remaining moisture and nutrient meet the demand for wheat at the reproductive growth stage to delay root and leaf senescence (Fenget al.2010; Yinet al.2018, 2020a), thus maintaining the higher LAI with NTSM than that with CT after the Feekes 10.0 stage of wheat; and 4) straw returning had the effects of enhancing soil enzyme and microorganisms activity and increasing the number and of soil microorganisms (Soareset al.2021; Liuet al.2022), reducing the temperature difference between day and night, which could create a suitable soil microecological environment for wheat growth (Yinet al.2020a).Therefore, no-tillage with straw mulching is an appropriate measure to create a good photosynthetic source basis for increasing and stabilizing wheat yieldsviaregulating leaf area dynamics in arid environments.

Fig.4 Principal component analysis (PCA) was conducted based on grain yield (GY) and eco-physiological parameters and yield components at arid irrigated regions.SPAD, chlorophyll relative content; Pn, net photosynthesis rate; Tr, transpiration rate; WUEL, leaf water use efficiency; LWP, leaf water potential;SWP, soil water potential; DV, the difference value between leaf and soil water potential; EN, ear number, GNE, grain number per ear; TGW, 1 000-grain weight.

4.2.The eco-physiological characteristics of wheat with straw management

Water scarcity is the main factor that regulates the ecophysiological characteristics of crops and affects their growth and development under drought conditions(Hassan 2006; Machianiet al.2021).Increased and stabled crop yields are partly attributed to optimized soil physicochemical properties and improved photophysiological characteristics (Agegnehuet al.2016; Wuet al.2022).Previous studies have shown that optimal tillage and mulching practices can strengthen crop productivity by improving soil physicochemical properties and leaf photosynthetic characteristics (Ramtekeet al.2022; Wanget al.2022).It is urgent to develop suitable agronomic practices to enhance crop yield by improving eco-physiological characteristics.Relatively moist soil water environment and suitable water conditions of plant leaves can improve SPAD,Pn, andTrof green leaves,especially in the late growing period of crops (Zhanget al.1995; Donget al.2019).The absorption, distribution,and transmission of light energy by plants are directly affected by SPAD in green leaves, which indirectly affects the photosynthetic efficiency (Slatteryet al.2017; Calvo and Lagorio 2019).Published studies have shown that no-tillage with straw returning inhibited soil evaporation,increased water infiltration, and maintained relatively good soil moisture conditions (Chenet al.2007; Grumet al.2017), thus enhancing the photosynthetic capacity of green leaves.Therefore, no-tillage with straw returningis an effective practice to promote chlorophyll synthesis and photosynthesis of leaves by conserving soil moisture(Lampteyet al.2020; Yinet al.2021).In this study, NTSM and NTSS decreased SPAD,Pn, andTrof green leaves at the Feekes 10.3 stage of wheat, which was because notillage with straw returning decreased the growth rate and consumed less moisture and nutrients of wheat at low air temperature season (Yinet al.2020a, b), thus reducing the photosynthetic capacity of green leaves of wheat.On the contrary, as soil temperature reached the suitable heat demand of wheat with the rise of air temperature (Yinet al.2020a), the remaining soil moisture and nutrients before the Feekes 10.3 stage boosted the vigorous growth of wheat after the Feekes 10.3 stage.So, the values on SPAD,Pn, andTrof green leaves in wheat with NTSM and NTSS were greater than CT at Feekes 11.0 and 11.2 stages, and NTSM had a higher increasing effect than NTSS.The main reasons were that NTSM was most effective in conserving soil moisture during the entire wheat growth period.Its soil moisture loss was slow, and the available moisture was kept for a long time, compared to NTSS (Yinet al.2018, 2020a).Meanwhile, NTSM effectively regulated the dynamics of photosynthetic sources of green leaves during the wheat growth period,guaranteeing a large LAI at the later growth period of wheat, so the values of SPAD andPnwere higher than other treatments.

The water status of green leaves in plants is related not only to soil moisture availability but also to moisture demand for atmospheric evaporation, resistance to moisture flow of organs, and root distribution within plants(Maeset al.2009; Hartzellet al.2017).Thereby, LWP is often used as an indicator to characterize the water condition of the green leaves in plants.Also, SWP is closely related to soil water content and is often used as an indicator to characterize soil water status (Bianchiet al.2017; Ketet al.2018).In this study, straw returning (NTSM,NTSS, and CTS) significantly increased the SWP of wheat fields over CT, and the increasing effect on SWP of NTSM was better than NTSS and CTS.This was because NTSM kept more soil moisture than NTSS and CTS, which was reflected in this study.Similarly, NTSM, NTSS, and CTS had greater LWP of green leaves for wheat than CT, and NTSM had greater LWP than NTSS and CTS, which was due to higher soil moisture content (Al-Darbyet al.1987;Yinet al.2021).This study further found that NTSM was conducive to maintaining the stability of soil water supply and water demand for wheat plants according to the absolute value of the difference in LWP and SWP.In short, no-tillage with straw mulching can stabilize soil water supply and relieve the excessive influence of water deficit changes on wheat growth and development.

4.3.Effects of straw management on grain yield of wheat

The practice of crop production has proved that the cultivation of cool-loving wheat often relies on straw returning technology in arid areas in Northwest China,where water resources are extremely scarce (Chaiet al.2014; Yinet al.2018; Liet al.2022).Straw returning(NTSM, NTSS, and CTS) boosted wheat grain yield compared to CT.The main reason for the grain yield increase was the synchronous increase and cooperative development of EN, GNE, and TGW.The grain yield increase of NTSM was greater than that of NTSS and CTS.The high yield of NTSM comes from five aspectsviathe analysis of photosynthetic source size, photosynthetic physiology, and SWP and LWP.First, NTSM effectively regulated the dynamics of the photosynthetic source of green leaves during the wheat growth period in this study,guaranteeing a higher LAI in the later growth period,which is beneficial to maintain higher photo-physiological characteristics and promote grain filling for achieving highyield.Second, the values of SPAD,Pn, andTrof green leaves in wheat at Feekes 11.0 and 11.2 stages were higher under NTSM than other treatments.It showed that NTSM could maintain the higher physiological activity of green leaves in wheat across the late growth period.Third, NTSM reduced the driving force of soil water flow to wheat leaves due to high leaf water potential.This was conducive to maintaining the stability of water demand for wheat plants and soil water supply in arid conditions.Fourth, NTSM stabilized soil water supply for wheat demand and decreased the excessive influence of water deficit changes on wheat growth, which laid the foundation for wheat to achieve high yields.Fifth, the growth period and functional period of green leaves of wheat were prolonged by NTSM (Yinet al.2020b), thus maintaining a higher photosynthetic capacity of green leaves in wheat.Additionally, many publications have indicated that notillage with straw returning enhanced the distribution and translocation of photoassimilates from vegetative organs to reproductive organs compared to conventional tillage with no straw returning (Figueiredoet al.2017; Yinet al.2017).A similar finding is reflected in this study: NTSM promoted grain filling and increased TGW.Therefore,no-tillage with straw mulching can not only meet the increasing population’s demand for food but also alleviate the shortage of water resources in arid regions.

5.Conclusion

No-tillage with straw mulching (NTSM) effectively regulated the dynamics of photosynthetic sources of green leaves for the wheat growth period, guaranteeing an extensive LAI at the later growth period of wheat.Compared to CT control, NTSM increased SPAD,Pn,Tr,andWUELof green leaves in wheat plants at Feekes 11.0 and 11.2 stages, which can keep a higher physiological activity at the late growth period of wheat.NTSM kept relatively high LWP for wheat green leaves and SWP for wheat fields and favorably formed an optimal soil moisture environment to strengthen the drought resistance of wheat crops.Meanwhile, the absolute value of difference on LWP and SWP with NTSM was lower than that with CT at three growth stages of wheat.It indicated that NTSM was conducive to maintaining the stability of water demand for wheat plants and soil water supply in arid conditions.Thereby, NTSM boosted the wheat grain yield by 18.6–27.3% compared to CT.This study concluded that no-tillage with straw mulching can be a promising technique to coordinate the conflict between high yield and water scarcity in arid conditions.

Acknowledgements

We are very grateful for financial support of the National Natural Science Foundation of China (32101857,32372238, and U21A20218), the Fuxi Young Talents Fund of Gansu Agricultural University, China (Gaufx-03Y10),the Science and Technology Program of Gansu Province,China (23JRRA1407), and the ‘Double First-Class’ Key Scientific Research Project of Education Department in Gansu Province, China (GSSYLXM-02).

Declaration of competing interest

The authors declare that they have no conflict of interest.