Responses of photosynthetic characteristics and leaf senescence in summer maize to simultaneous stresses of waterlogging and shading

Baizhao Ren,Weizhen Yu,Peng Liu,Bin Zhao,Jiwang Zhang

State Key Laboratory of Crop Biology and College of Agronomy,Shandong Agricultural University,Tai’an 271018,Shandong,China

Keywords:Zea mays L.Photosynthetic Antioxidant enzymes Waterlogging and shading treatment

ABSTRACT A field experiment was performed to investigate the physiological mechanism of the simultaneous stresses of waterlogging and shading on leaf photosynthetic and senescence during three growth stages of summer maize.The responses of leaf gas exchange parameters and antioxidant enzyme activities of the summer maize hybrids Denghai 605(DH605)to waterlogging(W),shading(S),and their combination(W+S)for 6 days at the third leaf stage(V3),the sixth leaf stage(V6),and the tasseling stage(VT)were recorded.Shading,waterlogging,and their combination disturbed the activities of protective enzymes and increased the contents of H2O2 and O2-,accelerating leaf senescence and disordering photosynthetic characteristics.Under waterlogging,shading and their combination,leaf Pn,the photo-assimilates and grain yield was decreased.The greatest reduction for waterlogging and the combined stresses occurred at V3 and that for shading stress occurred at VT.The individual and combined stresses reduced the activities of protective enzymes and inhibited photosynthesis,reducing the accumulation of photosynthetic compounds and thereby yield.Waterlogging and the combined stresses at the V3 stage showed the greatest effect on leaf photosynthetic and senescence,followed by the V6 and VT stages.The greatest effect for shading stress occurred at VT,followed by the V6 and V3 stages,and the combined influence of shading and waterlogging was greater than that of either single stress.

1.Introduction

Maize has become the largest crop of China in terms of planting area,owing to its rapid development in the past 10 years[1].The Huang-Huai-Hai region is the dominant maize-producing area in China,with a maize planting area accounting for more than one third of the total maize area of China.Light and water resources are abundant in this area,but their spatial and temporal distribution is uneven,and there are many disastrous weather events[2].Summer maize is vulnerable to drought,waterlogging,low light,and high temperature in the growing season,especially in the flowering stage,leading to low and unstable yields.From 10%to 16% of cultivated land worldwide is subject to waterlogging every year,and the frequency and intensity of waterlogging disasters will gradually increase in the future[3–5].With climate change,the frequency of overcast and rain events in the Yangtze River Basin and Huang-Huai-Hai region of China is also increasing,and the simultaneous stress of waterlogging and shading has become one of the main non-biological factors impairing stable maize yield in this region.

Waterlogging sharply lowers the content of soil oxygen,rapidly reducing root dry weight,length,surface area,and volume[6].The root system mainly forms adventitious root and aerenchyma to adapt to waterlogging stress[7–9].Waterlogging affects not only root morphology,but physiological function.Waterlogging leads to a decrease in root activity,accelerating root aging and death.It impedes the production and transportation of carbohydrate,reducing dry matter accumulation of grain,grain number,ear length,and ear diameter,and eventually yield[6,10,11].Shading of crops reduces the size and function of leaves.Shading stress reduces the area of green leaves,chlorophyll content and photosynthetic rate,increases the anthesis–silking interval,and reduces the vitality of pollen and filaments,leading to an increase of grain abortion rate and a decrease in yield[12,13].Under shading stress,the contents of reactive oxygen species and malondialdehyde(MDA)in leaves are increased and leaf senescence is accelerated[14].Shading stress reduces not only leaf growth and physiological function,but root traits including dry weight,length,density,diameter[15],activity,total absorption area,active absorption area and other parameters,leading to the weakening of maize root absorption function[16].

Most previous studies have focused on the influences of waterlogging or shading alone on maize growth and physiological function,even though waterlogging is often accompanied with shading in actual agricultural production.There are few studies of the combined influence of shading and waterlogging on photosynthesis,oxidation resistance,and yield formation.The objective of our study was to investigate the independent and combined effects of waterlogging and shading on photosynthetic characteristics and leaf senescence of summer maize at various growth stages.The results were expected to suggest the physiological mechanism underlying the response of summer maize to persistent rainfall conditions and to provide a theoretical basis for the effective cultivation of summer maize under the simultaneous stresses of waterlogging and shading.

2.Materials and methods

2.1.Plant materials and experimental design

The field experiment was conducted at the State Key Laboratory of Crop Biology and the experimental farm of Shandong Agricultural University,China(36°10′N,117°04′E)in 2019 and 2020.The region has a temperate continental monsoon climate.The summer maize hybrid Denghai 605(DH605),a popular cultivar,was used as experimental material.Maize was sown on June 10 in both years at a plant density of 67,500 plants ha-1.The preceding crop was wheat.Disease,weeds,and pests were well controlled in each treatment.N,P,and K fertilizers were applied as base fertilizer:210 kg ha-1N(urea,46% N),52.5 kg ha-1P2O5(calcium superphosphate,17% P),and 67.5 kg ha-1K2O(muriate of potash,50%K).Experimental treatments were applied at three stress stages:the third leaf stage(V3),the sixth leaf stage(V6),and the tasseling stage(VT)and stress treatments(waterlogging(W),shading(S),andthesimultaneous stressof waterlogging and shading(W+S)).The duration of each stress treatment was six days.Non-stressed plants served as control(CK,the ambient sunlight treatment,with soil moisture content maintained at 75%of the field capacity).The microclimate in the experimental field was measured dailyand hourly from5:00 AMto 6:00 PM during treatment(Fig.S1).Irradiance was measured with a CI-110 plant canopy digital image analyzer(CID Co.,Camas,WA,USA)placed 30 cm above the canopy.Canopy temperature and relative humidity of each plot were measured with a Gsp-6 temperature and humidity recorder(Elitech Instrument Co.,Suzhou,Jiangsu,China).The S treatment was imposed with shade cloth(Hongda Shade Cloth Company,Shouguang,Shandong,China)allowing almost 40%of ambient light to pass through.The shade cloth was supported with scaffolding about 2 m above the crop to keep the microclimate under the cloth consistent with that of the control.For the W treatment,water applied through pipes was maintained 2–3 cm deep on the soil surface.The W+S treatment was imposed by combining the waterlogging and shading treatments.At the end of the treatments,the shade cloth was removed and all remaining water was drained from the soil surface.The soil water was reduced to the same level as the control about five days after the water was released from the waterlogging treatment.The plot size of each treatment was 2.5 m×2.5 m(Fig.S2).Each treatment was repeated three times,and the experiment was laid out in a randomized complete block design.

2.2.Leaf area index(LAI)

Fifteen representative plant samples were marked in each plot at the next day after each treatment to measure leaf length and maximum leaf width.Leaf area and leaf area index(LAI)were calculated as follows[17].

2.3.Leaf SPAD value

The SPAD value of a functional leaf(the latest fully expanded leaf for treatment at V3 and V6,and the ear leaf for treatment at VT)was measured as leaf chlorophyll content on the day after each treatment using a chlorophyll meter(SPAD-502,Soil-plant Analysis Development Section,Minolta Camera Co.,Osaka,Japan),and measurements were replicated at least 15 times.

2.4.Leaf gas exchange parameters

Leaf gas exchange parameters of ear leaves were measured using a portable infrared gas analyzer(CIRASII,PP System,Hertfordshire,UK)at the milk stage(R3).

Five plants were randomly chosen to be measured from 10:00 AM to 12:00 AM on sunny days.Measurement conditions were kept consistent:a LED light source and a photosynthetically active radiation(PAR)of 1600 μmol m-2.CO2concentration was maintained at a constant level of 360 μmol mol-1using a CO2injector with a high-pressure liquid CO2cartridge source.

2.5.Activities of antioxidant enzymes and lipid peroxidation

On the day after each treatment,the functional leaves(the latest fully expanded leaf for treatment at V3 and V6,and the ear leaf for treatment at VT)were removed from three plants sampled from the center of each plot.Washed fresh leaves(～0.50 g)were homogenized with a mortar and pestle at 0–4 °C in 5 mL of 50 mmol L-1phosphate buffer(pH 7.8).The homogenate was filtered through muslin cloth and centrifuged at 15,000×g for 20 min at 4°C,and the supernatant was used to assay the enzyme activities of superoxide dismutase(SOD),peroxidase(POD),and catalase(CAT),and MDA content using respectively the nitro blue tetrazolium(NBT),guaiacol colorimetry,permanganate titration,and thiobarbituric acid(TBA)methods described previously[18],and soluble protein content by the Coomassie Brilliant Blue G-250 method of Jiang and Zhang[19].O2-and H2O2contents were measured using O2-and H2O2content determination kits,respectively,according to the manufacturer’s instructions(Suzhou Geruisi Biotechnology,Co.,Ltd.,Suzhou,Jiangsu,China).

2.6.Dry matter weight

Five representative plants were sampled from each plot on the day after each treatment and at the R6 stage.Samples were dried at 80 °C in a forced-draft oven(DHG-9420A,Shanghai Bilon Instruments Co.,Ltd.,Shanghai,China)to constant weight and weighed separately.

2.7.Grain yield

At R6,30 ears were harvested from three rows at the center of each plot.The kernels were air-dried and weighed,and grain yield was expressed at 14%moisture,according to the standard moisture content of maize for storage or sale in China.

2.8.Statistical analysis

Data used in this study were analyzed using SPSS 18(IBM Corporation,Armonk,NY,USA).The means were compared by ANOVA and least significant differences(LSD)tests at the 5% level.Sigmaplot 10(Systat Software,Inc.,Richmond,CA,USA)was used to draw the figures.

3.Results

3.1.Activity of antioxidant enzymes

The activities of leaf antioxidant enzymes of summer maize were decreased by shading,waterlogging and their combined treatments at several growth stages(Fig.1).Waterlogging at V3 and V6 stages reduced their activities more than did waterlogging at VT stage.However,shading impaired these activities more at VT stage than at the other stages.It can be seen that the activities of leaf SOD,POD,and CAT of summer maize under the simultaneous stresses of waterlogging and shading were significantly decreased,with the decrease greater than that under shading or waterlogging treatment at the three growth stages.For the activities of SOD and CAT,the interaction between waterlogging and shading stress was significant.

3.2.MDA content

Leaf MDA content of summer maize was increased by shading,waterlogging,and their combination at several growth stages(Fig.2).After waterlogging,shading and their combination at the V3 stage,leaf MDA content was increased by respectively 76.6%,41.8% and 91.4%,by 50.2%,23.3%,and 54.3% at the V6 stage,and by 32.0%,49.1%,and 105% at the VT stage compared with that of CK.Waterlogging increased the content of MDA more at the early stage than at a later stage,whereas shading at VT stage caused more severe damage than shading at V3 or V6 stages.There was a significant interaction between waterlogging and shading stress.Leaf MDA content of summer maize under simultaneous waterlogging and shading was significantly increased,by an amount greater than that caused by shading or waterlogging treatment.

3.3.Leaf soluble protein content

Leaf soluble protein content was decreased by shading,waterlogging and their combination at several growth stages.Significant interaction between waterlogging and shading at V3 was observed.The decrease of leaf soluble protein content activity under simultaneous waterlogging and shading was significantly higher than that under shading or waterlogging alone(Fig.3).Leaf soluble protein content afterwaterlogging,shadingandtheircombinationatV3wasdecreased by respectively 28.7%,24.7%and 40.5%,by 16.8%,18.5%,and 34.5%at V6,and by 13.8%,20.2%,and 28.2%at VT,compared with that of CK.

3.4.Contents of H2O2 and O2-

The contents of leaf H2O2and O2-of summer maize were increased by shading,waterlogging and their combination at several growth stages(Fig.4).Waterlogging,shading and their interaction caused significant damage to the ROS metabolism of summer maize.Waterlogging at early stage increased leaf H2O2and O2-contents more than waterlogging at a later stage,whereas shading at a later stage increased leaf H2O2and O2-content more than shading at an early stage.After the combination of waterlogging and shading at V3,V6,and VT stage,leaf H2O2content was increased by respectively 40.2%,54.3% and 29.1%,and O2-content by 34.8%,25.1%,and 22.9%compared with that of CK.The increase in H2O2and O2-content for the combined treatments was higher than that caused by shading or waterlogging treatment alone.

3.5.LAI

LAI was decreased by shading,waterlogging,and their combination at several growth stages(Fig.S3).The interaction between waterlogging and shading stress was significant.The LAI after shading,waterlogging,and their combination was decreased by respectively 23.8%,62.0% and 60.6% at V3,by 26.3%,45.0%,and 56.7% at V6,and by 11.7%,11.6%,and 14.4% at VT,compared with that of CK.Thus,the combined stress of waterlogging and shading at V3 caused the greatest decrease in LAI,followed by V6 and VT,and the decline of LAI caused by the combined treatment was significantly higher than that caused by shading or waterlogging.

3.6.SPAD

Leaf SPAD was decreased by shading,waterlogging and their combination at several growth stages(Fig.5).Leaf SPAD after shading,waterlogging and their combined stress at V3 was decreased by respectively 8.5%,23.4% and 25.8%,by 2.1%,9.3%,and 7.9% at V6,and by 7.3%,1.6%,and 9.6% at VT,compared with that of CK.The interaction between waterlogging and shading stress was significant.Thus,the combined stress of waterlogging and shading at V3 caused the greatest decrease in leaf SPAD,followed by V6 and VT,and the decline of SPAD caused by the combined treatment was significantly higher than that caused by shading or waterlogging.

3.7.Gas exchange parameters

Leaf Pnwas decreased by shading,waterlogging and their combination at several growth stages.The decrease of leaf Pnof summer maize under simultaneous waterlogging and shading was significantly higher than that under shading or waterlogging treatment(Table 1).Leaf Pnof summer maize under simultaneous stress of waterlogging and shading was decreased by respectively 36.4%,3.3%,and 29.4% at V3,V6,and VT stages,compared to that of CK.There was a significant interaction between waterlogging and shading.At V3 and V6 stages,there was no significant difference in Pnbetween waterlogging and the combined stress,whereas there was a significant difference between shading and the combined stress.There was also a significant difference in Pnbetween waterlogging and the combined stress at VT stage.Compared with leaf Pnof shading treatment,those of the combined treatment at V3 and V6 stage were decreased by 33.7% and 28.4%,respectively.Leaf Pnunder simultaneous stress of waterlogging and shading at VT stage was significantly decreased by 24.8%,compared to that under waterlogging.

Table 1 Effects of shading,waterlogging,and their combination on leaf gas exchange parameters in summer maize at R3 stage.

The variation trend of Trand Gswas largely consistent with Pnafter shading,waterlogging,and their combined treatment.For Trand Gs,the interaction between waterlogging and shading was significant.Leaf Trunder simultaneous stress of waterlogging and shading at V3,V6,and VT stages was decreased by 36.6%,33.5%,and 23.4%,whereas leaf Gswas decreased by respectively 38.7%,36.5%,and 33.4%,compared with those of CK.Leaf Trand Gsof shading at VT stage was decreased most,while leaf Trand Gsof waterlogging at V3 stage was decreased most.Waterlogging,shading,and their combined stress led to an increase in Ci.Leaf Ciwas increased by 23%–30%after waterlogging and the combined stress at V3 or V6 stage.However,there was no significant effect on leaf Ciat V3 and V6 stage.Leaf Ciwas increased by respectively 6.0%,7.2%,and 6.1%after waterlogging,shading and the combined stress at VT.

3.8.Dry matter

Fig.1.Effects of shading,waterlogging and their combination on leaf antioxidant enzyme activity in summer maize.V3,third leaf stage;V6,sixth leaf stage;VT,tasseling stage;CK,control treatment;S,shading treatment;W,waterlogging treatment;S+W,combined treatments of shading and waterlogging.Values followed by different lowercase letters within a column are different at the 0.05 probability level.*,P＜0.05;**,P＜0.01;ns,the effect is not significant at P＜0.05.

Fig.2.Effects of shading,waterlogging and their combination on leaf MDA content in summer maize.V3,third leaf stage;V6,sixth leaf stage;VT,tasseling stage;CK,control treatment;S,shading treatment;W,waterlogging treatment;S+W,combined treatments of shading and waterlogging.Values followed by different lowercase letters within a column are significantly different at the 0.05 probability level.*,P＜0.05;**,P＜0.01;ns,the effect is not significant at P＜0.05.

Fig.3.Effects of shading,waterlogging and their combination on leaf soluble protein content in summer maize.V3,third leaf stage;V6,sixth leaf stage;VT,tasseling stage;CK,control treatment;S,shading treatment;W,waterlogging treatment;S+W,combined treatments of shading and waterlogging.Values followed by different lowercase letters within a column are significantly different at the 0.05 probability level.*,P＜0.05;**,P＜0.01;ns,the effect is not significant at P＜0.05.

Dry matter weight of summer maize was decreased by shading,waterlogging,and their combination treatments at all three growth stages(Fig.S4).The interaction between waterlogging and shading was significant.Dry matter weight at the day after each after shading,waterlogging,and their combination at V3 was decreased by respectively 13.1%,63.3% and 70.0%,by 7.6%,18.6%,and 23.1% at V6,and by 11.1%,4.5%,and 14.5% at VT,compared with that of CK.At R6 stage,dry matter weight after shading,waterlogging and their combination at V3 was decreased by respectively 13.8%,48.8% and 52.9%,w by 22.4%,46.1%,and 46.0% at V6,and by 28.5%,14.3%,and 33.7% at VT,compared with that of CK.Thus,the combined stress of waterlogging and shading at V3 caused the largest decrease in dry matter weight of summer maize,followed by V6 and VT,and the decline of dry matter weight caused by the combined treatment was significantly higher than that caused by shading or waterlogging.

3.9.Yield

Grain yield of summer maize was decreased by shading,waterlogging and their combination(Fig.6).There was a significant interaction between waterlogging and shading stress.Grain yield after shading,waterlogging and their combination at V3 was decreased by respectively 7.5%,34.4%,and 39.4%,by 10.3%,28.8%,and 32.0% at V6,and by 27.1%,9.5%,and 30.0% at VT,compared with that of CK.Thus,waterlogging and the combination of waterlogging and shading at V3 led to the largest decrease in grain yield,followed by V6 and VT,while shading at VT led to the largest decrease in grain yield.The decline of grain yield caused by the combined treatment was significantly higher than that caused by shading or waterlogging.

3.10.Correlation analysis

The yield of summer maize was positively correlated with dry matter weight,photosynthesis characteristics(Pn,Tr,Gs),SOD activity,and soluble protein content.Yield was negatively correlated with MDA and H2O2content.The photosynthetic characteristics(Pn,Tr,Gs)were positively correlated with the activity of SOD,LAI,and SPAD,and was negatively correlated with the content of O2-and H2O2(Fig.S5).

4.Discussion

4.1.Responses of leaf senescence to waterlogging,shading,and their combined stress

Fig.4.Effects of shading,waterlogging and their combination on leaf H2O2 and O2-content in summer maize.V3,third leaf stage;V6,sixth leaf stage;VT,tasseling stage;CK,control treatment;S,shading treatment;W,waterlogging treatment;S+W,combined treatments of shading and waterlogging.Values followed by different lowercase letters within a column are significantly different at the 0.05 probability level.*,P＜0.05;**,P＜0.01;ns,the effect is not significant at P＜0.05.

Fig.5.Effects of shading,waterlogging,and their combination on leaf SPAD in summer maize.V3,third leaf stage;V6,sixth leaf stage;VT,tasseling stage;CK,control treatment;S,shading treatment;W,waterlogging treatment;S+W,combined treatments of shading and waterlogging.Values followed by different lowercase letters within a column are significantly different at the 0.05 probability level.*,P＜0.05;**,P＜0.01;ns,the effect is not significant at P＜0.05.

Fig.6.Effects of shading,waterlogging and their combination on grain yield in summer maize.V3,third leaf stage;V6,sixth leaf stage;VT,tasseling stage;CK,control treatment;S,shading treatment;W,waterlogging treatment;S+W,combined treatments of shading and waterlogging.Values followed by different lowercase letters within a column are significantly different at the 0.05 probability level.*,P＜0.05;**,P＜0.01;ns,the effect is not significant at P＜0.05.

Leaf senescence was closely related with the ability to scavenge the ROS[20].The findings of our study that the protective enzyme system of summer maize was damaged after shading or waterlogging,and that the activities of CAT,SOD and POD were decreased,are consistent with those of previous studies of waterlogging[18]and shading[14,21,22].As a result,the content of O2-and H2O2was significantly increased after shading or waterlogging,especially under the combined stress of shading and waterlogging.Excessive O2-and H2O2exposed plants to oxidative stress,which led to a loss of cell membrane integrity and to metabolic disorders,thus inhibiting maize growth[23,24].Soluble proteins are mainly enzymes involved in various metabolic activities,whose decrease was the main characteristic of leaf senescence.In our study,leaf soluble protein content was decreased after shading or waterlogging and their combination,resulting in the impaired photosynthetic performance.Shading,waterlogging,and their combined stress resulted in an increase of MDA content in summer maize,intensified membrane lipid peroxidation and plant chlorosis,and led to premature senescence,reducing photosynthetic activity.These results suggested that waterlogging,shading,and their combined stress significantly damaged the cell membrane integrity and metabolic processes,resulting in leaf senescence.Among these,the compound effects of shading and waterlogging on the activity of antioxidant enzymes and soluble proteins content were greater than those of single stress.However,there were no significant difference between waterlogging and the combined stress at V3 and V6 stage,and no significant difference was found between shading and the combined stress at VT stage for the activity of antioxidant enzymes and MDA content.It appears that waterlogging stress at an early stage had a greater influence on the antioxidant system than waterlogging stress at late growth stage,while shading stress at a late growth stage had a large effect on the antioxidant system.

4.2.Responses of photosynthetic characteristics to waterlogging,shading,and their combined stress

Plant senescence is characterized by a loss of chlorophyll and a reduction in green leaf area.Chlorophyll is a light-harvesting molecule whose content reflects the potential of the plants to produce photosynthetic compounds by light energy[25,26].Leaf area was positively correlated with photosynthetic capacity of maize[27,28].Chlorophyll loss was positively correlated with leaf senescence and decreased activities of SOD and POD enzymes[28,29].Stress damage to leaf protective enzyme systems leads to leaf chlorosis and premature plant senescence,impairing photosynthesis.In this study,waterlogging,especially at V3 and V6 stages,reduced the SPAD,suggesting that the ability of waterlogged summer maize to trap,transport,and use light energy was decreased.Waterlogging also reduced green leaf area,thus reducing light interception.Pn,Gs,and Trwere also reduced by waterlogging.Thus,waterlogging significantly reduced dry matter accumulation rate.Waterlogging at V3 stage led to the most severe damage,in agreement with previous studies[29,30].In previous studies[14,24,31,32],shading at early stage reduced leaf assimilation capacity and increased Ci,while shading at late stage reduced leaf Pn,Gs,and Trof maize,and shading at flower period caused a greater reduction in photosynthetic efficiency.Consistently,our study also showed that shading at VT stage did the most serious damages to SPAD and Pn,leading to the greatest loss of dry matter weight.The damage to various physiological characteristics of crops under the combined stress of shading and waterlogging was higher than that under either single stress.Leaf net photosynthetic rate and boll yield of cotton under combined stresses were lower than those under shading or waterlogging[25].The decrease of leaf net photosynthetic rate and yield of maize under the combined stress was higher than that under single stress[25,33].The combined stress of shading and waterlogging reduced LAI,SPAD,and Pn,and the reduction was higher than that from single stress.As a result,the production capacity of photoassimilates was weakened by the combined stress at any growth stage(especially at V3 stage,followed by V6 and VT),leading to a decrease in dry matter accumulation.The Civalue was increased under the combined stress,indicating that the compound effects of shading and waterlogging on photosynthetic efficiency were affected by stomatal and non-stomatal restriction at the same time.Waterlogging damage to photosynthesis of summer maize was greater than that of shading stress at an early growth stage,whereas that of shading stress was greater than that of waterlogging stress at a late growth stage,indicating that waterlogging stress was dominant at early growth stage,while shading stress was dominant at late growth stage.The combined stress of shading and waterlogging at V3 stage exerted the strongest effect on photosynthetic performance,followed by V6 and VT stages,and the combined influence of shading and waterlogging was greater than that of either single stress.

4.3.Responses of grain yield to waterlogging,shading,and their combined stress

Yield loss was closely correlated with photosynthetic activity and leaf senescence characteristics(Fig.S5).The reduction of photosynthate at an early growth stage inhibited panicle growth.An insufficient supply of photoassimilates compounds led to increased competition between panicle kernels,leading to kernel abortion or reduced kernel weight[12,34].Waterlogging,shading,and especially their combined stress accelerated leaf senescence,leading to the reduction of effective interception rate of light energy and impairment of photosynthetic performance.As a result,dry matter accumulation was inhibited,leading to yield reduction(Fig.6).The reduction of grain yield was due to different reasons at different stress stages(Table S1).The combined stress of waterlogging and shading reduced grain number,1000-kernel weight,and harvest ear number of summer maize,thus leading to reduced yield.However,shading at V3 and V6 stage affected mainly ear number and 1000-kernel weight,while shading stress at VT stage reduced mainly kernel number per ear.Waterlogging at V3 and V6 reduced mainly ear number and kernel number per ear,whereas waterlogging at VT reduced mainly 1000-kernel weight.

5.Conclusions

Combined stress of shading and waterlogging accelerated leaf senescence,impairing photosynthetic characteristics and thereby impeding the accumulation of photoassimilates,reducing yield of summer maize.V3 was most susceptible to the combined stress and waterlogging,followed by V6 and VT,while VT was most susceptible to shading,followed by V6 and V3,and the combined influence of shading and waterlogging was greater than that of either single stress.

CRediT authorship contribution statement

Baizhao Ren:Conceptualization,Funding acquisition,Supervision,Supervision.Weizhen Yu:Data curation,Writing–original draft.Peng Liu:Resources,Supervision.Bin Zhao:Project administration,Resources.Jiwang Zhang:Conceptualization,Funding acquisition,Writing–review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was funded by the National Natural Science Foundation of China(31801296),the Postdoctoral Innovation Program of Shandong Province(202003039),China Agriculture Research System of MOF and MARA(CARS-02-21).

Appendix A.Supplementary data

Supplementary data for this article can be found online at https://doi.org/10.1016/j.cj.2022.06.003.