Determination Of Stem Xylem Water Potential-based Lower Irrigation Limit Of Cotton

Cotton is an important economic crop in China.It is mainly planted in areas with relatively short water resources,and is often affected by drought stress during production.The establishment of an appropriate irrigation system can help alleviate cotton drought and improve production efficiency.Determining the appropriate irrigation water limit is an important part of a rational irrigation system.Determining the irrigation system with the stem xylem water potential as the irrigation water limit can make full use of the water-saving capacity of the crop itself to serve agricultural production and achieve the purpose of high efficiency and water saving.In this study,the responses of different cotton varieties to short-term and alternate drought stress-rewatering were studied by a combination of barrel experiments and field experiments,so as to determine the appropriate irrigation water limit based on cotton xylem water potential.The 2020 barrel planting experiment mainly studies the physiological responses of two cotton varieties,Xinluzhong 67（XLZ-67）and Xinluzao 26（XLZ-26）,under conditions of short-term drought and different rewatering times;on this basis In 2021,the field experiment will mainly study two cotton varieties,Xinluzhong 67（XLZ-67）and Xinluzao 26（XLZ-26）,in the bud stage and the flowering and boll stage under full irrigation W1（95%FC,FC is the field capacity）,moderate water deficit W2（80%FC）and severe deficit W3（65%FC）,the responses of cotton growth,stem xylem water potential(Ψ_stem-pd),fluorescence characteristics and yield.The responses of cotton xylem stems were determined,and the irrigation water limit was determined cotton xylem stem water potential.The main research conclusions are as follows:（1）After the short-term water deficit reaches the lower limit and rewatering,the water potential can basically return to the initial level within 1 day.With the deepening of the degree of drought,the loss value PLC of cotton’s hydraulic conductivity gradually increases;the vulnerability curve of cotton presents an"S"shape,and the corresponding water potential values（P50）when the loss of xylem hydraulic conductivity of the two cotton varieties is 50%,respectively for-3.78MPa（XLZ-67）and-3.89MPa（XLZ-26）.（2）Gas exchange parameters of barrel-grown cotton changed significantly with increasing water deficit.When the stem water potential(Ψ_stem-pd)dropped to about-1.5 MPa before dawn,the net photosynthetic rate A and transpiration rate E of cotton both decreased by about 30%;when theΨ_stem-pd dropped to about-2.5 MPa before dawn,the A,E and G_sdecreased by about 90%;after rewatering,there will be an obvious compensation effect,and the main recovery period is within 1 day after rewatering.The light energy conversion efficiency Y（II）and steady-state photoluminescence decay rate（Rfd-Lss）of cotton PS II gradually decreased with the increase of water deficit.Under the same deficit conditions,the loss value of XLZ-26 was smaller than that of XLZ-67,and the recovery is faster.In general,the content of non-structural carbohydrates（NSC）increased first during the depletion-rewatering process.after subtraction.In order to ensure the normal progress of cotton physiological and biochemical reactions and avoid irreversible damage to cotton,the lower limit of water control should be kept at the soil water content of 65%-75%FC,and the corresponding lower limit of theΨ_stem-pd is-1.5 MPa.（3）On the basis of the barrel experiment,the better irrigation water limit in the range of[-0.5 MPa,-1.5 MPa]was determined in the field test.The three lower water potential limits of the field test:95%FC（W1）,80%FC（W2）,and 65%FC（W3）,respectively,correspond to the lower limits of the predawn stem water potential of-0.5 MPa（W1）,-0.9MPa（W2）,-1.5 MPa（W3）.Water deficit at bud stage and boll stage significantly decreased the relative chlorophyll content SPAD and increased leaf temperature.TheΨ_stem-pd dropped to about-0.9MPa（W2）,and the leaf temperature increased by 1.4°C on average compared with W1.It dropped to about-1.5 MPa（W3）,and the leaf temperature increased by 3.1°C on average compared with W1.For the steady-state fluorescence decay rate Rfd-Lss,The response of cotton to water deficit at the bud stage was greater than that at the flowering and boll stage.The loss value of W2 treatment was 4%-5%,and the loss value of W3 treatment was 16%-17%.After rewatering,W2 could recover completely,but W3 could not recover.（4）Under drought conditions in the growth period,the plant height and stem diameter of W2 treatment were less obvious than those of W1 treatment,but W3 treatment significantly restricted the plant height and stem diameter compared with W1 treatment.Water stress at the bud stage restricted the growth rate and peak value of leaf area,while water stress at the flowering and boll stage caused the leaf area peak to appear earlier,and leaves fell off earlier at the mature stage.The water deficit at the bud stage mainly reduced the dry matter mass of leaves,and the water deficit at the flowering boll stage mainly decreased the dry matter mass of cotton bolls.In terms of yield,maintaining 80%of the lower soil moisture limit（W2）at the cotton bud stage would not significantly affect seed cotton yield,but would significantly improve irrigation water use efficiency.lint yield and boll number per plant,and significantly reduce irrigation water use efficiency.But the opposite is true in the flowering&Boll period.In conclusion,water deficit at bud stage mainly affects the vegetative growth of cotton,while deficit at flowering and boll stage mainly affects its reproductive growth,which will significantly reduce cotton yield.It is suggested that the predawn stem water potential(Ψ_stem-pd)of-0.9 MPa in the bud stage and the predawn stem water potential(Ψ_stem-pd)of-0.5MPa in the flowering boll stage as the irrigation water limit,so as to achieve maximum yield and improve irrigation water use efficiency.