Responses Of Leaf Subtending To Cotton Boll Sucrose Metabolism And Fiber Development To Combined Elevated Temperature And Soil Drought During Flowering And Boll Formation Period

With global warming,extreme weather outbreaks have become more frequent.In cotton production,the average daytime temperature of 32°C is the dividing line of the elevated temperature effect,when the maximum daily temperature exceeds 35°C,cotton production will subject to significant high temperature stress.If the global average air temperature is expected to increase by 1.8°C to 4.0°C at the end of this century,as IPCC reported,the risk of high temperature stress for cotton production will increase.What’s worse is that the flowering and boll formation stage is most sensitive to changes in the external environment,the high temperature occurring with drought during this time will affect the leaf subtending to cotton boll（providing about 60%-87%of the cotton boll biomass）development,which in turn affects cotton yield and fiber quality.In this paper,the co-effects of elevated temperature and soil drought on the yield and fiber quality of cotton were studied.A temperature-sensitive cultivar Sumian 15 and a temperature-tolerance cultivar PHY370WR were used in the experiment,and the experiment was carried out at Pailou Experimental Station（118°50’E,32°02’N）of Nanjing Agricultural University from 2016 to 2018.Two temperature regimes including ambient temperature（AT）and elevated temperature（ET,with an average day/night temperature of 33.4℃/28.9℃across 3years,which was 2.5-2.7℃higher than AT）,and three soil relative water content（SRWC）levels including（75±5）%,（60±5）%and（45±5）%were conducted.In the current study,we examined the physiological mechanisms of elevated temperature,drought and their combination on cotton yield and fiber quality,photosynthetic carbon metabolism of leaf subtending to cotton boll（LSCB）,and fiber development and fiber quality formation,which provides a basis for cotton production and high-quality cultivation under combined stresses.In addition,we further explored the effects of cotton cultivars with different temperature sensitivity under the combination of elevated temperature and soil drought.The main results are as follows:1.Responses of cotton yield and fiber quality to combined elevated temperature and soil drought during flowering and boll formation periodWhen the average daytime temperature of the environment was higher than 32.0℃,the ET（2.5-2.7℃）,drought,and both stress combined at flowering and boll formation stage were not conducive to the development of cotton yield and fiber quality.（1）The boll numbers,boll weight and seed cotton yield decreased significantly under drought,elevated temperature and stress combined,and declines were greater as SRWC decreased.Effects of combination on boll numbers,boll weight,and seed cotton yield were aggravated with drought deep,and were more obvious on the middle and upper fruiting branches.Among yield components,boll numbers were most sensitive to the co-occurred ET and drought,which was the main reason for seed cottonyield decline.（2）The drought at flowering and boll formation stage decreased fiber length,strength and micronaire,with micronaire level optimized.Under ET,fiber length shorted,but fiber strength and micronaire increased,and there were no significant changes in micronaire level.The combined ET and drought produced more obvious effects on middle fruiting branches,where the fiber length shorted more than any stress in isolation,especially under ET-SRWC（45±5）%.However,ET can partly reduce the drought stress effects by increasing fiber strength under combined stress,and was more significantly under ET-SRWC（60±5）%.（3）Under combined stress,the seedcotton yield and fiber quality of temperature-sensitive cotton cultivar had a greater effect than the temperature-tolerance cultivar.2.Responses of leaf subtending to cotton boll sucrose metabolism to combined elevated temperature and soil drought during flowering and boll formation periodWhen the average daytime temperature of the environment was higher than 32.0℃,the ET（2.5-2.7℃）,drought and combined treatments significantly affected the sucrose metabolism of LSCB.ET reduced the Rubisco and cy-FBPase activities and net photosynthetic rate（Pn）,which inhibited the formation of photosynthetic products,and was not conducive to the accumulation of sucrose.While,effects of combined stress on sucrose metabolism of LSCB showed a drought main effect,where the sucrose content increased as SRWC decreased.Still,the sucrose increases under co-effects were smaller than drought individually.That was because the increased cy-FBPase,SPS（because of the Gh SPS2expression up-regulated）activities and the decreased Su Sy activity under combined stresses were smaller than drought stress.Besides,compare with drought,Rubisco activity and Pn of LSCB significantly decreased under combination,especially under ET-SRWC（45±5）%,which also inhibited the formation of photosynthetic products.The cy-FBPase and SPS,and Gh SPS2 were the most sensitive enzymes and gene in response to combined ET and drought stress in the sucrose metabolism of LSCB,and reactions under combined stresses were more pronounced in temperature-sensitive cultivar than temperature-tolerance cultivar.3.Responses of cotton fiber elongation to combined elevated temperature and soil drought during flowering and boll formation periodWhen the average daytime temperature of the environment was higher than 32.0℃,the ET（2.5-2.7℃）,drought,and stress combined at flowering and boll formation perios were not conducive to fiber elongation,and will significantly shorted fiber length.Drops of fiber length were greater as SRWC decreased and were more obviously under stress combined than ET or drought individually.The ET and drought treatments down-regulated Gh SUT-1 expression and decreased fiber Su Sy activity,which played a decisive role in fiber elongation,resulted in lower fiber sucrose content and inhibited fiber elongation.What difference between them was that ET mainly shorted the duration of fiber rapid elongation（FRED）,but drought decreased both FRED and the maximum fiber elongation speed(V_max).Under combined ET and drought treatment,the co-effects showed a drought main effect,which decreased FRED and V_max in fiber elongation,but the decreases were greater than drought stress.Specifically,the expression of the Gh SUT-1 under ET-SRWC（45±5）%was significantly down-regulated than ET-SRWC（60±5）%and produced shorter fiber length.In addition,combined stress down-regulated the expression of Gh KT-1 and significantly reduced the content of K⁺and malate,further inhibited fiber elongation.The fiber elongation under combined ET and drought stress was co-regulated by the changes of sucrose,K⁺,and malate content;Susy and Gh SUT-1 were the most sensitive enzyme and gene that responded to the combined stress.In cultivars,sucrose,K⁺,and malate content during fiber elongation in temperature-sensitive cultivar had greater changes than temperature-tolerance cultivar and were more susceptible to the combined stress.4.Responses of cotton fiber cellulose synthesis to combined elevated temperature and soil drought during flowering and boll formation periodWhen the average daytime temperature of the environment was higher than 32.0℃,either the ET（2.5-2.7℃）,drought,or the combined stresses during flowering and boll formation period were not conducive to the fiber thickening,which reduced the fiber biomass,and a more pronounced reduction was observed under combined treatment than single factors.Co-effects on fiber thickening and fiber strength showed a drought main effect but were smaller than drought individually.Besides,fiber strength decreased as SRWC reduced.ET reduced fiber sucrose content and inhibited cellulose synthesis by changing the activity of fiber sucrose metabolizing enzymes.Drought,and the combined ET and drought affected the activity of sucrose metabolizing enzymes and changed the carbon flow direction of fiber thickening development.Under combined stress,the down-regulated expression of Gh Su Sys（especially Gh Su Sy-A and Gh Su Sy-B）and Gh SPSs resulted in reduced Su Sy and SPS activities,and then sucrose and cellulose content decreased;meanwhile,the direction of carbon flow was conducive to the synthesis ofβ-1,3-glucan and the synthesis of cellulose was hindered,because of the expression ofβ-1,3-glucanase down-regulated and Gh Cal S-5 upregulated.Under combination,reduction of the above indicators was significantly greater than single factor,and was decreased more under ET-SRWC（45±5）%than ET-SRWC（60±5）%.Under ET combined with drought stress,fiber thickening was controlled together by changes of sucrose,cellulose,andβ-1,3-glucan content;Su Sy was the most sensitive enzyme,Gh Su Sy-A and Gh Su Sy-B were the most sensitive genes.The sucrose responses,cellulose andβ-1,3-glucan for combined stresses were more pronounced in temperature-sensitive cultivar than temperature-tolerance cultivar.In summary,in terms of photosynthetic product supply,the ET and drought during the flowering and boll formation period affected the sucrose metabolism of LSCB by regulating the activities of cy-FBPase and SPS.At the same time,it inhibited the supply of photosynthesis products by reducing the activity of Rubisco and Pn,resulted in decreases in boll number,boll weight,and seedcotton yield,among which the decrease in boll number was the main reason for the decline in seedcotton yield.As for fiber development and fiber quality formation,the co-effects accelerated fiber development,resulted in the decline of mature fiber length,strength,and micronaire.Under combined stress,fiber elongation was co-regulated by sucrose,K⁺and malate,Su Sy was the most sensitive enzyme,Gh SUT-1was the most sensitive gene;fiber thickening was jointly regulated by sucrose,cellulose andβ-1,3-glucan,Su Sy was the most sensitive enzyme,Gh Su Sy-A and Gh Su Sy-B were the most sensitive genes.Effects on seed cotton yield,fiber development and fiber quality formation of ET-SRWC（45±5）%were significantly greater than ET-SRWC（60±5）%.Besides,the above-mentioned indicators in temperature-sensitive cultivar were more susceptible to the combined stress than the temperature-tolerance cultivar.