Effect of chemical regulators on the recovery of leaf physiology,dry matter accumulation and translocation,and yield-related characteristics in winter wheat following dry-hot wind

Yanan Xu ,Yue Wu ,Yan Han ,Jiqing Song ,Wenying Zhang ,Wei Han ,Binhui Liu#,Wenbo Bai#

1 Institute of Environment and Sustainable Development in Agriculture,Chinese Academy of Agricultural Sciences,Beijing 100081,China

2 Institute of Dryland Farming Research,Hebei Academy of Agriculture and Forestry Sciences,Hengshui 053000,China

3 Shandong General Station of Agricultural Technology Extension,Jinan 250100,China

Abstract Dry-hot wind stress causes losses in wheat productivity in major growing regions worldwide,especially winter wheat in the Huang-Huai-Hai Plain of China,and both the occurrence and severity of such events are likely to increase with global climate change.To investigate the recovery of physiological functions and yield formation using a new noncommercial chemical regulator (NCR) following dry-hot wind stress,we conducted a three-year field experiment(2018-2021) with sprayed treatments of tap water (control),monopotassium phosphate (CKP),NCR at both the jointing and flowering stages (CFS),and NCR only at the jointing stage (FSJ) or flowering stage (FSF).The leaf physiology,biomass accumulation and translocation,grain-filling process,and yield components in winter wheat were assessed.Among the single spraying treatments,the FSJ treatment was beneficial for the accumulation of dry matter before anthesis,as well as larger increases in the maximum grain-filling rate and mean grain-filling rate.The FSF treatment performed better in maintaining a high relative chlorophyll content as indicated by the SPAD value,and a low rate of excised leaf water loss in flag leaves,promoting dry matter accumulation and the contribution to grain after anthesis,prolonging the duration of grain filling,and causing the period until the maximum grain-filling rate reached earlier.The CFS treatment was better than any other treatments in relieving the effects of dry-hot wind.The exogenous NCR treatments significantly increased grain yields by 12.45-18.20% in 2018-2019,8.89-13.82% in 2019-2020,and 8.10-9.00% in 2020-2021.The conventional measure of the CKP treatment only increased grain yield by 6.69% in 2020-2021.The CFS treatment had the greatest mitigating effect on yield loss under dry-hot wind stress,followed by the FSF and FSJ treatments,and the CKP treatment only had a minimal effect.In summary,the CFS treatment could be used as the main chemical control measure for wheat stress resistance and yield stability in areas with a high incidence of dry-hot wind.This treatment can effectively regulate green retention and the water status of leaves,promote dry matter accumulation and efficient translocation,improve the grain-filling process,and ultimately reduce yield losses.

Keywords: preparation,stress,foliar spraying,grain-filling,remobilization

1.lntroduction

The Huang-Huai-Hai Plain is one of the most important agricultural regions in China,and it produces more than 60% of China’s winter wheat (NBSC 2013).Since winter wheat is the main staple food crop in China,its production plays an essential role in ensuring China’s food security(Xiaoetal.2018).During the grain-filling stage of winter wheat,abiotic stresses such as high temperature and drought can cause irreversible damage to production(Mirosavljevicetal.2021;Correiaetal.2022).Meanwhile,the rapidly changing climate has affected global and regional food security due to increasing temperatures,changing precipitation patterns,and more frequent extreme events (Assengetal.2019).

Many studies reporting on abiotic stresses have focused on individual factors.For example,the responses to either drought or heat stress have been studied extensively in wheat,and serious physiological damages,such as the reductions in the photosynthetic rate,the carbohydrate metabolism rate and grain weight,have all been observed (Mirosavljevicetal.2021;Ullahetal.2022).The combination of both of these environmental stresses has only recently become a hot topic in crop science research.The combined stress of both high temperature and drought has a negative,additive impact on plant phenology and physiology,i.e.,growth,chlorophyll content,leaf photosynthesis,grain number,and grain filling duration (Correiaetal.2022).The combined influences of a heat wave and drought are negatively correlated with crop yield (Heinickeetal.2022).

Dry-hot wind is an agrometeorological hazard with a high air temperature,low air humidity,and a certain wind speed threshold.According to the Chinese Meteorological Industry Standard (QX/T 82-2019 2019),if the maximum temperature exceeds 31°C,the relative air humidity is less than 30%,the wind speed exceeds 3 m s-1,the relative soil water content at a depth of 20 cm is lower than 60%,and these four conditions all occur at 2 p.m.,then winter wheat is at risk of exposure to dry-hot wind events.This type of hazard usually occurs during the reproductive and grain-filling periods of winter wheat and can have severe impacts both on crop yield and quality (Chenetal.2018).Recent global warming has increased the frequency and intensity of dry-hot wind events,thus creating the urgent need for effective prevention and defensive measures(Moore and Lobell 2014).

To improve wheat quality and quantity,some methods have been used for protection in dry-hot windy climates,which include disaster prediction that aims to adjust the crop planting structure in advance in order to alleviate the impact of meteorological disasters on wheat yield (Wangetal.2021).The adoption of biological,physical,and chemical technologies such as breeding and selection of varieties tolerant to drought or high temperature (Jingetal.2020),regulation of field microclimates using sprinkler misting (Liu and Kang 2006),and the application of exogenous chemical regulatory preparations have been used to improve the stress resistance of wheat (Zhangetal.2019;Luetal.2022;Maetal.2022).However,breeding targets are highly complex for explaining and achieving the specific tolerance characteristics (Driedonksetal.2016).In comparison,exogenous plant growth regulators are relatively simple and effective,with a low cost and a quick effect,and they have gradually become the inevitable choices.Plants can be prepared to better tolerate abiotic stress conditions through the exogenous application of chemical compounds as well as transgenic approaches.Relevant studies have shown that applications of exogenous polyamines,6-benzylamino adenine,and gibberellins reduce the sensitivity of plants to abiotic stresses (Zhangetal.2019;Jingetal.2020;Luetal.2022).Until now,existing chemical compounds have often suffered from technical problems,such as singular function,unstable effects,or complicated application techniques.The research and development of environmentally friendly multifunctional plant growth regulators have become increasingly important for the development of high-yield and high-efficiency agriculture.Moreover,the application methods of plant growth regulators,such as seed dressing and foliar spraying,can regulate leaf physiology,improve the antioxidant system,and promote the stress resistance of plants (Perveenetal.2013;Jingetal.2020).Foliar applications might have the benefit of allowing the substances to enter the plant and potentially react more rapidly with the biological processes than when the materials are applied to the soil.Furthermore,simple and efficient operations have led to the widespread use of foliar application for agricultural chemical regulators.

Under adverse environmental stresses,the green retention (Nawazetal.2013) and water status (Geravandietal.2011) of plant leaves are sensitive to external damage.The photosynthate that is mainly produced by leaves is closely related to dry matter (DM) accumulation and transport,which can further affect the grain-filling process (Kumaretal.2006).A few studies have primarily focused on these issues in controlled field experiments and remote-sensing simulations (Liu and Kang 2006;Wangetal.2021),and both field experiments and remotesensing studies are indispensable for understanding the physiological mechanisms responsible for the damage caused by dry-hot wind.However,they are not fully capable of representing crop responses to dry-hot wind in a natural environment with different degrees of dryhot wind stress.In addition,little is known about the relationships among new plant growth regulators,leaf physiological characteristics,and DM accumulation and translocation in the control of wheat grain filling under dryhot wind stress conditions.

The main objective of this study was to investigate the effect of dry-hot wind stress on the grain yield (GY) of winter wheat,and also to determine whether the exogenous application of a new chemical regulator (NCR) at different growth stages could regulate GY by improving the leaf physiology and DM partitioning characteristics,while also regulating the grain-filling process under natural dry-hot wind stress.An additional objective was to provide a theoretical basis for further elucidating the mechanism of dry-hot wind resistance,and developing a suitable protection technology for wheat production under dry-hot wind conditions.

2.Materials and methods

2.1.Experimental site

The field experiments were conducted during the 2018-2021 growing seasons in the experimental station of the Institute of Dryland Farming Research,Hebei Academy of Agriculture and Forestry Sciences,Hengshui City,Hebei Province,China (37°54´N,115°42´E).The experimental area is a typical dry-hot wind climate area in the Huang-Huai-Hai Plain of China,located in a semi-humid climatic region where the mean annual air temperature is 13.3°C,the annual sunshine duration is 2,509 h,and the potential evapotranspiration is 1,785.4 mm.The mean annual precipitation is 497.1 mm,about 70% of which occurs during July-September.Winter wheat-summer maize rotation is adopted in this region.The field had a deep layer of silty loam with the composition of 22.5% clay,64.1% silt,and 11.4% sand.The pH of the 0-30 cm soil layer was approximately 7.6.The chemical characteristics of the soil are shown in Table 1.

2.2.Field microclimate observations

The diurnal variations of field microclimatic factors after anthesis during the 2018-2021 growing seasons are shown in Fig.1.Temperature and relative humidity were monitored at 30-min intervals by a recorder (LR5001 Humidity Logger,Deruikong Electronic Co.,Ltd.,Suzhou,China) hanging in the experiment plot.A small weather station (Tianqi Weather Station,Oriental Zhigan Science and Technology Co.,Ltd.,Zhejiang,China) was used to measure wind speed,which was monitored at 1 h intervals during the growing seasons.Relative soil water content at a depth of 20 cm was calculated using field capacity and soil moisture content.According to the Chinese Meteorological Industry Standard (QX/T 82-2019 2019),dry-hot winds occurred at 17,21,and 26 days after anthesis (DAA) in 2019,with severe dry-hot wind at 17 and 26 DAA and mild dry-hot wind at 21 DAA.According to the classification of the disaster grades of dry-hot wind for wheat,2018-2019 was classified as a severe grade year.Both 2019-2020 and 2020-2021 were moderate grade years,with the occurrences of mild dry-hot wind at 28 DAA in 2020,and at 24 and 26 DAA in 2021.In addition,the dry-hot winds occurred mainly in the middle and late grain filling periods during the three years of the experiment.

Fig.1 Diurnal variations of microclimatic factors in the winter wheat field after anthesis during the 2018-2021 growing seasons.Daily max-temperature,relative air humidity at 2.00 p.m.,relative soil water content in the soil layer of 20 cm at 2.00 p.m.,and wind speed at 2.00 p.m.are shown for 2018-2019 (A),2019-2020 (B) and 2020-2021 (C).Of these parameters,the relative soil water contents were missing for 2018-2019,since that index was not included in the QX/T 82-2007 (2007),and just amended in the QX/T 82-2019 (2019).

2.3.Experimental design

Winter wheat variety Jimai 22,which is widely planted in the Huang-Huai-Hai Plain,was grown during the three growing seasons.The plant densities were about 2,200,000 plants ha-1.Before sowing,405 kg ha-1of organic fertilizer was applied as base fertilizer,which represented 225 kg ha-1pure nitrogen,90 kg ha-1P2O5and 90 kg ha-1K2O.There were five treatments,with three replicates,organized in a randomized complete block design (Table 1).The planting area of each treatment was 9 m2(6.0 m×1.5 m).Tap water and monopotassium phosphate (KH2PO4),labeled here as CK and CKP,respectively,were compared with the NCR,which is a non-commercial chemical developed by the Innovative Team of Water-Saving MaterialsApplications and Agricultural Film Pollution Prevention and Control in the Institute of Environment and Sustainable Development in Agriculture,Chinese Academy of Agricultural Sciences.The exogenous chemical regulator was diluted 100-fold before foliar spraying,and the spray amount of each treatment was approximately 300 kg ha-1applied at the jointing or flowering stages.The sowing,harvest,and regulator application dates during the three growing seasons are shown in Table 2.

Table 1 Chemical characteristics of the soil in 2018-2021

2.4.Relative chlorophyll content (SPAD)

The SPAD values of the flag leaves were determined using a chlorophyll meter (Konica Minolta Co.,Japan).Measurements were taken during the afternoon and recorded as the mean of 10 measurements taken along the leaf blade (five on each side of the leaf rib).Five flag leaves for each treatment were measured with a 5-day interval during 5-30 DAA.

Table 2 The field experimental design during three growing seasons of winter wheat (2018-2019,2019-2020,and 2020-2021)

2.5.Rate of excised leaf water loss (RWL)

The five youngest fully-expanded leaves were collected for each of three replications at the anthesis stage during 5-30 DAA.The leaf samples were transported to the laboratory and weighed (FW1) at t1,wilted for 2 h at 25°C,reweighed (FW2h) at t2,and then oven-dried for 24 h at 50°C to obtain the dry weight (DW).The RWL was calculated using the following formula (Gunesetal.2008):

where (t2-t1) is the time interval between the two subsequent measurements (2 h).

2.6.DM

The DM was measured in different organs at anthesis and maturity.Thirty spikes from each treatment were randomly sampled and divided into leaf,stem (including sheath),glume (spike axis and kernel husks),and grain(only at harvest stage),which were weighed after drying to a constant weight at 80°C.The following parameters related to DM accumulation and remobilization within the wheat plant were calculated following Mengetal.(2017):

2.7.Grain-filling process

During the flowering period,the plants that flowered on the same day were selected and marked.The first sampling was conducted at 5 DAA,and then continued sampling from anthesis to maturity with a 5-day interval until the seeds were ripened on 35 DAA.The grains on the spike were packed by high temperature desiccation at 105°C for 30 min,then dried to a constant weight at 80°C and the grains were weighed.The grain-filling process was fitted by the Richards equation:

whereWis grain weight (mg),Ais maximum grain weight (mg),tis DAA (d),andB,k,andNare coefficients determined by regression.That equation was used to calculate various grain-filling parameters: the period until the maximum grain-filling rateTmax·G(d),maximum grainfilling rateGmax(mg/grain d-1),mean grain-filling rateGmean(mg/grain d-1),and active grain-filling periodD(d) (Luetal.2022).

2.8.GY and yield components

At the maturity stage,the yield trait of spike number(SN) was randomly taken by counting an area of 0.18 m2(1 m×0.18 m) in each plot.Ten spikes were randomly selected to record grain numbers per spike (GNP).In each plot,1,000 randomly selected grains were used to determine thousand-grain weight (TGW).Excluding the outer rows,the central samples of each plot were handharvested to calculate the GY.All these measurements were repeated three times.

2.9.Statistical analysis

Data are presented as means of all replicates.Duncan’s test was used in the analysis of variance to detect significant differences among the mean values for different treatments atP＜0.05.All statistical analyses were performed using Microsoft Excel 2013,SAS version 9.4 and SPSS version 19.0.

3.Results

3.1.SPAD values and RWL in flag leaves

In both growing seasons,the SPAD values in flag leaves showed an increasing-decreasing trend with DAA(Fig.2).A sharp decline was observed at 25-30 DAA(the late grain-filling stage),and all treatments of winter wheat were affected to different degrees by the dry-hot wind.Compared with CK,the influences of the other treatments on SPAD values did not show a consistent regularity during 5-20 DAA.At 30 DAA,the SPAD values were significantly increased by 7.68-9.67% in all NCR treatments,including FSJ,FSF,and CFS,and by 4.13-14.52% in the CKP treatment (except in 2018-2019),compared with the CK.The degree of increase followed the order of CFS＞FSF＞FSJ＞CKP.Compared with CKP,the CFS treatment was sharply enhanced by 3.46% in 2019 and 3.22% in 2020 (Fig.2).

Fig.2 Changes of the SPAD values and rates of excised leaf water loss (RWL) in the flag leaves under different treatments with the days after flowering in 2018-2019 (A and B),2019-2020 (C and D) and 2020-2021 (E and F).CKP and CK denote foliar spraying with tap water and KH2PO4 at the jointing and flowering stages,respectively;FSJ,FSF and CFS indicate foliar spraying with chemical regulator at the jointing stage,at the flowering stage,and successive foliar spraying at the jointing and flowering stages,respectively.Bars indicate SD (n=3).The same letters within each bar imply no significant difference according to the Duncan’s multiple range test (P＜0.05).

All the NCR treatments,except FSJ in 2019,promoted a significant reduction of RWL by 9.87% compared with CK starting from 25 DAA (Fig.2).For the CKP treatment,the RWL significantly reduced only at 30 DAA,by 17.60%in 2018-2019 and 14.23% in 2020-2021,compared with CK.Moreover,the RWL values of the FSF and CFS treatments were significantly lower than those of CKP at 30 DAA,and they were reduced by 2.38-20.98% and 13.32-25.69%,respectively.

These results indicated that the CKP treatment helped to alleviate the sharp reduction of chlorophyll and water loss under dry-hot wind stress in the middle and late grain-filling stages.However,spraying NCR had significantly greater beneficial effects on the chlorophyll content and water retention capacity of flag leaves,and spraying the NCR at the flowering stage (FSF) was better than spraying at the jointing stage (FSJ).The greatest regulatory effects on SPAD and RWL occurred with foliar spraying of the NCR at both the jointing and flowering stages (i.e.,the CFS treatment).

3.2.DM accumulation and translocation

Compared with CK,the CPDMAG in the CKP treatment in 2020 significantly increased by 4.25%,but the indexes of DM accumulation and translocation in other years did not significantly differ from CK (Table 3).Compared to the CK,all the treatments including FSJ sharply increased the DMVA by 7.11% in 2018-2019 and 15.28% in 2020-2021,while for the CFS treatment,DMVA increased by 8.57%in 2018-2019 and 12.52% in 2020-2021.This indicated that regulator spraying at the jointing stage enhanced the formation of vegetative organs before flowering.Furthermore,the FSJ treatment significantly increased the DMR by 52.81% in 2020-2021.There were no significant differences in the DMRE or the CDMRG between the CK and FSJ treatments.

For the FSF and CFS treatments,the PDMA was 19.58-65.92% greater than the CK in 2018-2019 and 2019-2020 (Table 3).Correspondingly,CPDMAG also increased by 10.10-36.63%.The higher PDMA and CPDMAG values in the CFS treatment were mainly attributed to the additive effect of the single application of FSF.This indicated that foliar spraying at the flowering stage was conducive to DM accumulation after anthesis,and so it might be better to spray the regulator at anthesis in order to affect the source of grain filling that is mainly associated with assimilate accumulation after anthesis.

3.3.Grain-filling process

The grain-filling process was affected considerably after application of the different chemical regulators.The exogenous regulators affected the grain weight remarkably in the three seasons,with varying responses(Fig.3).During the 2018-2021 growing seasons,compared with CK,the final grain weights at 35 DAA of the different NCR treatments significantly increased by 2.84-3.86%,3.68-5.41%,and 5.94-7.32% underthe FSJ,FSF,and CFS treatments,respectively;while they were enhanced by 1.20-3.51% compared with the CKP treatment.Exogenous NCR significantly alleviated the influence of dry-hot wind on wheat grain,and foliar spraying was superior at the flowering stage compared to the jointing stage.The greatest increase was obtained with the combined spraying treatment of CFS.Note that the traditional dry-hot wind prevention measure could also alleviate the inhibitory effects on wheat grain.According to the dynamic changes of grain weight with DAA,the exogenous NCR measures had more beneficial effects on grain weights,not only after the occurrences of different stresses,but also under normal conditions at the early and middle grain-filling stages.

Fig.3 Changes in grain weight and grain-filling rate of winter wheat under different treatments with the days after flowering in 2018-2019 (A and B),2019-2020 (C and D) and 2020-2021 (E and F).CKP and CK denote foliar spraying with tap water and KH2PO4 at the jointing and flowering stages,respectively;FSJ,FSF and CFS indicate foliar spraying with chemical regulator at the jointing stage,at the flowering stage,and successive foliar spraying at the jointing and flowering stages,respectively.Bars indicate SD (n=3).The same letters within each bar imply no significant difference according to the Duncan’s multiple range test (P＜0.05).

Table 3 Dry matter accumulation and translocation under the different treatments during the 2018-2021 growing seasons1)

The change in grain-filling rate followed a single peak increasing-decreasing trend with DAA,in which peaks appeared at 20 DAA and then the rates sharply decreased(Fig.3).The grain-filling rate was also affected after spraying different chemical regulators,but there were no regular changes among the various treatments.The grain-filling rates of the CKP,FSF,and CFS treatments in 2020-2021 were significantly increased only at 35 DAA,when they were increased by 2.91,3.25,and 3.52%compared to the CK,respectively.

3.4.Grain-filling parameters

The grain-filling process could be simulated very well by the Richards model,with all coefficients of determination exceeding 0.99 (Table 4).TheGmax,Gmean,andDall increased with the CKP and NCR treatments in 2018-2019,but theTmax·Goccurred earlier.The amplitudes were larger in the NCR than the CKP treatment,and the largest changes were for the CFS treatment.TheGmax,Gmean,andDof the CFS treatment were enhanced by 0.04 mg/grain d-1,0.04 mg/grain d-1and 0.33 d compared to the CKP,respectively,and itsTmax·Gdecreased by about 0.52 d.In 2019-2020,similar trends of change occurred only in the CFS treatment.Compared with CK and CKP treatment,theGmax,Gmean,andDof the CFS treatment were enhanced by 0.02 and 0.05 mg/grain d-1,0.02 and 0.04 mg/grain d-1,and 1.78 and 1.13 d,respectively.TheTmax·Gof the CFS treatment was about 0.11 and 0.01 d ahead of CK and CKP,respectively.After the application of the different regulators,theDof the CKP,FSJ,FSF,and CFS treatments were 4.54,0.35,0.90,and 4.02 d longer than that of the CK treatment in 2020-2021.TheTmax·Gof the FSF and CFS treatments were also about 0.71 and 1.37 d ahead of the CK treatment,respectively.These results showed that the effects of the FSJ treatmenton grains were mainly attributed to the largerGmax,Gmean,andDvalues during the three growing seasons.The FSF and CFS treatments could change the grain weight by modulatingTmax·Gand prolongingD,and a similar effect was observed in the CKP treatment,suggesting that the exogenous regulators ameliorated the negative effect of dry-hot wind on grain filling.

Table 4 The grain-filling characteristic parameters,yield and yield components of winter wheat under different treatments during the 2018-2021 growing seasons

3.5.Yield and yield components

The effects of exogenous chemical regulators on yield traits differed among the treatments (Table 4).There were no significant differences in SN between the NCR treatments and CK,except for the CFS treatment in 2018-2019.Compared with CK,the NCR treatments increased the GNP of wheat by 5.31-10.34% in 2018-2019 and 7.89-10.13% in 2020-2021;and these increases were in the order of CFS＞FSJ＞FSF.There was no significant difference between CKP and CK for the three seasons.The TGW of wheat was also significantly increased by 3.06-9.71% under the exogenous NCR treatments compared to CK,except for the FSJ treatment in 2019-2020;and the order of increase was CFS＞FSF＞FSJ.For the different spray treatments,spraying at the jointing stage had a greater effect on the increase of GNP,while spraying at the flowering stage was more conducive to the increase of TGW.Only in 2018-2019,with the severe dry-hot wind,was the TGW of the CKP treatment notably increased by 3.72% compared to that of the CK treatment.

The GY in the CKP treatment was not significantly higher than in the CK treatment in 2018-2020,but it was sharply increased by 6.69% in 2020-2021.Compared with the CK,the GY in the FSJ,FSF,and CFS treatments were significantly higher by 8.10-13.61%,8.54-12.45%,and 9.00-18.20%,respectively.The increasing amplitude due to the different NCR treatments was consistent in the three seasons,and the order of increase was 2019＞2020＞2021.These results indicated that exogenous NCR increased the wheat yield through GNP and TGW.

Severe dry-hot wind occurred in 2018-2019,in which the exogenous NCR treatments had more obvious effects on the increases in wheat yield,suggesting that the stress degree of dry-hot wind obviously amplified the differences between the CK and the treatments,and the regulatory effect of the growth regulators was more prominent.In addition,the regulatory effect on the yield might also be related to the growth period of winter wheat when the dryhot wind occurs.

3.6.Correlation analysis of the investigated grain parameters of wheat plants

Correlations among the parameters related to GY were evaluated.This analysis showed that GY was negatively correlated with RWL at all grain-filling stages;positively correlated with SPAD values in leaves at the late grainfilling stage (SPADl) and with PDMA,GW,GNP and TGW;but not significantly correlated with the SPAD values in leaves at the early and middle grain-filling stages (SPADeandSPADm),DMVA,or with the characteristic parametersof grain filling,i.e.,Tmax·G,Gmax,Gmean,andD(Table 5).The SPAD values were negatively correlated with RWL,but positively correlated with PDMA,GW and TGW;whereas RWL was negatively correlated with PDMA,GW,and TGW.The DMVA was positively correlated with GNP,and the same relationships were found between PDMA and GW,and between PDMA and TGW.The GW was closely correlated with GNP and TGW.Among the grainfilling characteristic parameters,Tmax·Gwas negatively correlated withD,andGmaxwas positively correlated withGmean.

Table 5 Correlation analysis of yield formation factors in different regulator treatments under dry-hot wind conditions1)

4.Discussion

4.1.Effects of NCR on yield,DM accumulation and translocation

Dry-hot wind is a frequent threat to Chinese mainland food production,and always occurs during the grainfilling period of wheat.Managing the impact of dryhot wind stress on yield production and improving the resistance of wheat have become significant agricultural research topics.Previous studies have shown that the use of exogenous substances to induce heat or drought resistance in wheat may represent an important breakthrough and countermeasure for enhancing crop resistance (Jingetal.2020;Fanetal.2022).The influence of the NCR on dry-hot wind stress resistance was confirmed in our study.Foliar spraying of NCR at different stages significantly increased the wheat yield in 2018-2021 under dry-hot wind stress (Table 4).Compared with CK,pre-spraying of NCR before the disaster significantly increased wheat yields under all dry-hot wind conditions by 8.10-13.61%,8.54-12.45%,and 9.00-18.20% in the FSJ,FSF,and CFS treatments,respectively.NCR application at both the flowering and jointing stages was the best at abating the reduction in wheat yield caused by dry-hot wind stress at the grain-filling stage,and the FSF treatment was better than the FSJ treatment.This is probably because the effect of spraying at the flowering stage on promoting the contribution of post-anthesis assimilate to grain formation was greater than that of the pre-anthesis DM accumulation caused by spraying at the jointing stage.As demonstrated in this study,the accumulation of assimilate after anthesis contributed more than 50% to the grain,and the FSF treatment had a slightly greater contribution(Table 3).

The formation of yield was positively related to the accumulation of photosynthetic product after anthesis and normal transport from the leaves to the spike.The efficient transfer and distribution of assimilates stored in vegetative organs to grain are very important for achieving a higher grain weight (Ehdaieetal.2006).With plant growth and development,the change of the growth center would transfer the photosynthate to the organs,which need more nutrients to maintain development.Nitrogen,phosphorus,and trace elements can significantly enhance the transfer of DM from plant vegetative organs to grain(Dordas 2009).The main effective constituents of our self-developed NCR include potassium,phosphorus,fulvic acids,and some trace elements (e.g.,zinc,molybdenum,copper,and manganese).Therefore,the large accumulation of DM before anthesis in this experiment might be effectively transferred and applied to the establishment of post-anthesis grain (Table 3).Moreover,spraying NCR at the jointing stage promoted greater pre-anthesis DM accumulation than spraying at the flowering stage (Table 3).This was consistent with the results of Brazienetal.(2021),in which the use of fulvic acid in spring wheat during the vegetative stage increased the number of productive stems by 30%,reliably increased GY,and improved grain quality.Maetal.(2014) reported that yield losses were partly compensated by DM remobilization under different water stress conditions.Compared with DM accumulation,photosynthate mobilization and allocation are more important for grain growth.In this study,the amounts of PDMA and CPDMAG were about 19.58 and 10.10%greater,respectively,than those of CK in 2018-2021(Table 3),and this enhancement was more obvious in the combined treatment of CFS.As also shown by Luoetal.(2022) and Fanetal.(2022),spraying at the jointing stage contributed to DM accumulation before anthesis,while leaf physiology was more regulated by spraying at the flowering stage.The exogenous NCR was sprayed at both the jointing and flowering stages,which might have a synergistic effect in promoting DM remobilization and alleviating the effect of hot-dry wind on yield reduction.

Narimanietal.(2010) reported that applying microelements as foliar fertilizers could promote wheat grain quality and weight,and we found a similar result.The exogenous NCR contained several mineral elements,which induced significant increases in TGW at the maturity stage.Foliar spraying of KH2PO4on wheat leaves is a conventional dry-hot wind control measure in the Huang-Huai-Hai area (Huoetal.2019).In this study,the GY of the CKP treatment was slightly higher than that of the CK treatment in the three growing seasons,but there were no significant differences between them under the various dry-hot wind stresses (Table 5).Liuetal.(2019) confirmed that the improvement effect with a compound preparation of KH2PO4,zinc sulfate,and urea on yield was greater than that of a single ingredient preparation,indicating that combinations of regulators could enhance the stress mitigation effect.The development and application of low-cost multifunctional compound regulators might be the future direction of sustainable agricultural development.

4.2.Effects of NCR on leaf physiology and grain filling characteristics

The level of photosynthetic efficiency is an important indicator of crop yield.Chlorophyll is one of the major pigments involved in plant photosynthesis,and it plays a critical role in the absorption,transmission,and transformation of light energy.Dry-hot wind leads directly to intensive evapotranspiration,water imbalance in plants,and irreversible damage to leaf physiology (Wangetal.2016;Caietal.2022).A low rate of excised leaf water loss (i.e.,RWL) is also related to drought resistance(Gunesetal.2008).Maintaining high SPAD and low RWL values have been proposed as important indicators for evaluating the ability of leaf green retention and water status under abiotic stress (Gunesetal.2008;Fanetal.2022).In this study,dry-hot wind stresses in the three seasons caused different levels of damage to the green retention abilities and water status in wheat leaves,and such unfavorable effects were weakened by use of our self-developed NCR (Fig.2).Fulvic acid was the main effective element of the exogenous NCR.Studies have shown that fulvic acid can improve leaf photosynthetic activity (Lotfietal.2015;Gengetal.2020) and increase the air-dry weight of shoots and roots (Brazienetal.2021).Our study showed that foliar spraying of NCR could contribute to increasing trends of the SPAD values in flag leaves,and to reductions in RWL values,especially in the late growth stage (Fig.2).The alleviating effect on water stress caused by hot-dry wind also seemed greater,which might be closely related to the effects of fulvic acid on increasing the number of roots and the ability of the crop to absorb water and nutrients,and the reduction of stomatal openings and water transpiration (Yaoetal.2019).

Grain weight is determined by the grain-filling rate and duration.High temperature during the grain-filling period can significantly accelerate crop senescence and inhibit yield (Assengetal.2015).In this study,the final grain weights at 35 DAA (Fig.3) andGmaxandGmeanin 2018-2020 were increased the most by the NCR treatments,suggesting a positive effect onGmaxandGmean;and these results were similar to a previous study in which exogenous gibberellic acid was significantly correlated with the maximum grain weight,Gmax,andGmean(Cuietal.2020).However,another study found that an exogenous regulator was not significantly correlated with the maximum grain weight,GmaxorGmeanin wheat (Liuetal.2013).We observed thatTmax·Gwas reached earlier after the application of NCR,except for the FSJ treatment in 2020-2021,and the values ofDwere all obviously increased (Table 4).Together,these results indicated that exogenous NCR increased the grain weight by increasing the grain-filling rate,improving the grain-filling process,and prolonging the duration of grain filling.Moreover,we compared the effects of exogenous NCR among the spray treatments,and found that the FSJ treatment induced larger increases inGmaxandGmeanthan the FSF treatment,except for 2018-2019.The FSF treatment had more positive impacts on prolonging the duration of grain filling and causing theTmax·Gto be reached earlier,indicating that the external factors affecting the grain-filling process were not limited to regulator type,and the spraying date could also be a major factor affecting the regulatory process in the plants (Lietal.2020).

The results of this study showed that the effects of different dry-hot wind stresses on winter wheat were alleviated by exogenous NCR,by maintaining green retention and the water-holding status of flag leaves,which contribute to delaying leaf premature senescence,promoting the amount and contribution rate of postanthesis dry matter accumulation during grain filling,increasing TGW and achieving the goal of increasing GY.The significant and well-defined relationships among GY,SPADl,RWL,PDMA,GNP,and TWG indicate that these related parameters and mechanisms should be further investigated.

5.Conclusion

The results of this study showed that the NCR was strongly involved in the response of wheat to dry-hot wind stress.Exogenously applied NCR played multiple roles in improving stress tolerance in wheat.Both methods of application significantly increased GY by more than 8.10%,although their responses to the influence of dry-hot wind differed.Compared with CK,exogenous NCR improved the physiological characteristics of photosynthesis and water-holding status by alleviating the reduction in the SPAD value and maintaining a lower RWL value in flag leaves.It also promoted the accumulation of DM and the effective transport of DM after anthesis,increased the CPDMAG,alleviated the damage of dry-hot wind to the grain-filling process,and promoted the increases in GNP and TGW to different degrees.A comparison of different exogenous spraying treatments showed that spraying at the jointing stage was conducive to PDMA and DMRE.Spraying at the flowering stage was better for maintaining higher SPAD and lower RWL values,promoting assimilate accumulation after anthesis,and prolonging grain-filling time.In comparison,the combined treatment of CFS had a synergistic promotion effect,and its mitigation effect on the adverse effects of dry-hot wind was significantly greater than those of either the conventional CKP treatment or any other single spray treatment.Therefore,it is suggested that the NCR can be an effective chemical control agent for improving winter wheat resistance under dry-hot wind stress in the Huang-Huai-Hai winter wheat planting region.We hypothesize that the exogenous application of NCR by foliar spraying contributes to the dry-hot wind tolerance of wheat,while the involvement of the various components in the mixture needs to be clarified.Further work is necessary to compare the different effective components,and to test the NCR using genotypes that differ in their stress tolerance to further understand the mechanism of NCR action through exogenous application.

Acknowledgements

This study was supported by the National Key Research and Development Program of China (2019YFE0197100),the earmarked fund for China Agriculture Research System (CARS-03-01A),and the Agricultural Science and Technology Innovation Project of the Chinese Academy of Agricultural Sciences.

Declaration of competing interest

The authors declare that they have no conflict of interest.