Effects Of K⁺ And Ca²⁺ Supplement On Plant Growth,Physiology And Yield Of Salt-Stressed And Drought-Stressed Cotton

Soil salinity and water scarcity are two major constraints to sustainable development of irrigated agriculture in arid and semi-arid areas.Consequently,a better understanding of plant responses to salt stress,drought stress and combined stress of salinity and drought,as well as sustainable irrigation and fertilization management strategies are essential for improving crop productivity while alleviating salinity stress in drought-prone and salt-affected regions.In this study,four pot experiment were performed on cotton（Gossypium herbaceum）plant in a natural light greenhouse.In experiment I,cotton plants were exposed to two salt stress levels（0 m M Na Cl and 150 m M Na Cl,respectively）,the effect of salt stress on leaf gas exchange,plant water relations,phytohormonal level,nutrient absorption,biomass partitioning and water use efficiency were investigated.In experiment II,cotton plants were subjected to two salt stress levels（0 m M Na Cl and 150 m M Na Cl,respectively）,two irrigation levels（90%and 70%of SWHC,respectively）and four fertilization regimes(without K⁺and Ca²⁺,0.6 m M K⁺,3.25m M Ca²⁺,0.6 m M K⁺and 3.25 m M Ca²⁺,respectively),the effect of K⁺or/and Ca²⁺supplement during fertigation on salt tolerance of cotton at full and deficit irrigation regimes was evaluated and the mechanism of salt tolerance was illustrated from photosynthesis stomatal-limitation.Finally,proposed optimal irrigation and fertilization management strategy based on the result of principle component analysis.In experiment III,cotton plants were subjected to two salt stress levels（0 m M Na Cl and 150 m M Na Cl,respectively）and two fertilization regimes(without K⁺and Ca²⁺,0.6 m M K⁺and 3.25 m M Ca²⁺,respectively),the effect of K⁺and Ca²⁺supplement during fertigation on leaf gas exchange both for stomatal and non-stomatal limitations of salt-stressed cotton plants were assessed.In experiment IV,cotton plants were exposed to two salt stress levels（0 m M Na Cl and 150 m M Na Cl,respectively）,two drought stress levels（well-watered and progressive soil drying,respectively）and two fertilization regimes(without K⁺and Ca²⁺,0.6 m M K⁺and 3.25 m M Ca²⁺,respectively),the response of stomatal behavior and growth of salt-stressed cotton plants with K⁺and Ca²⁺fertilization to progressive soil drying was detected.The major results of current study show as belows:（1）Investigated the effect of salt stress on cotton plant growth,physiology and yield formation.150 Mm Na Cl salinity significantly decreased relative water content（9.45%）,leaf water potential（43.24%）and leaf hydraulic conductance（63.01%）.Besides,150 m M Na Cl salinity significantly increased leaf ABA concentration（37.86%）,while significantly decreased leaf GA3 concentration（24.04%）and leaf ZR concentration（14.70%）.Thus,significant reduction in stomatal conductance（52.78%）,transpiration rate（49.84%）and plant water consumption（29.56%）.However,salt stress showed a less negative effect on net photosynthetic rate than stomatal conductance and transpiration rate,meanwhile,salt stress enhance biomass partitioning into boll（38.89%）.Therefore,the intrinsic water use efficiency,instantaneous water use efficiency and water use efficiency of boll biomass of cotton plants grown under salt stress were 99.97%,81.22%and 81.25%,respectively,higher than those grown under control.In addition,150 m M Na Cl salinity significantly increased leaf nitrogen content（12.39%）and relative chlorophyll content（11.65%）,delayed cotton leaf senescence,compared to control.（2）Supplement of K⁺and Ca²⁺effectively alleviated inhibition of plant growth and leaf gas exchange as affected by salt stress at full irrigation.Salt stress significantly increased leaf Na⁺content（31.76%）and stem Na⁺content（43.42%）,significantly decreased plant height（11.23%）,stem diameter（15.10%）,leaf area（16.05%）,stomatal conductance（47.62%）,transpiration rate（39.72%）,net photosynthetic rate（6.93%）,leaf K⁺/Na⁺ratio（9.03%）and stem K⁺/Na⁺ratio（29.85%）.However,at full irrigation,compared to no K⁺and Ca²⁺addition,application of K⁺and Ca²⁺significantly improved plant height,stem diameter,leaf area,stomatal conductance,transpiration rate,net photosynthetic rate,leaf K⁺/Na⁺ratio and stem K⁺/Na⁺ratio.Under salt stress,the plant height,stomatal conductance,transpiration rate,net photosynthetic rate,leaf K⁺/Na⁺ratio and stem K⁺/Na⁺ratio of cotton plants with K⁺and Ca²⁺application were 6.66%,50.00%,40.50%,19.50%,39.08%and 7.05%,respectively,higher than those grown with no K⁺and Ca²⁺addition.（3）Application of K⁺and Ca²⁺notably alleviated stomatal limitation of photosynthesis as affected by salt stress,while no significant effect on non-stomatal limitation.At full irrigation,supplement of K⁺and Ca²⁺significantly improved stomatal conductance and net photosynthetic rate in comparison with no K⁺and Ca²⁺addition.However,application of K⁺and Ca²⁺had no significant effect on maximum net photosynthetic rate under saturated CO2,carboxylation capacity of rubisco,electron transport rates,regenerate ribulose-1,5-bisphosphate and mesophyll conductance,indicating the enhanced net photosynthetic rate of salt-stressed cotton plants under K⁺and Ca²⁺application mainly due to alleviated stomatal limitation.150 m M Na Cl salinity had a less effect on photosynthesis than stomatal conductance due to enhanced capacity of photosynthesis at non-stomatal process under salinity.150 m M Na Cl stress significantly increased maximum net photosynthetic rate under saturated CO₂（25.63%）,carboxylation capacity of rubisco（48.45%）,electron transport rates（32.88%）,regenerate ribulose-1,5-bisphosphate（28.64%）and mesophyll conductance（37.96%）,thus showed a less negative effect on net photosynthetic rate（reduced 2.21%）than stomatal conductance（decreased 42.42%）.（4）Application of K⁺and Ca²⁺enhanced biomass accumulation in shoot of cotton plants grown under drought stress relative to those no K⁺and Ca²⁺application.150 m M Na Cl stress significantly reduced the transpiration rate and senescence rate of cotton plants grown under drought stress.Drought stress significantly decreased stem diameter（16.00%）,leaf area（18.53%）and biomass accumulation in shoot（25.31%）.In comparison with no K⁺and Ca²⁺addition,supplement of K⁺and Ca²⁺significantly increased plant height（20.60%）,leaf area（21.60%）and biomass accumulation in shoot（26.77%）of cotton plants grown under drought stress.Besides,along with progressive soil drying,stomatal of cotton plants grown under control was closed at day 8,while those grown under salinity environment was closed at day14.（5）Continue fertilization after flowering stage may exaggerate the risk of second soil salinization.Application of K⁺and Ca²⁺had no significant effect on soil soluble Na⁺content,soil exchangeable Na⁺content and soil electrical conductivity of soil at flowering and bolling stage,while continue supplement of K⁺and Ca²⁺significantly increased soil electrical conductivity of soil at boll opening stage（25.00%）.Collectively,appropriate utilization of brackish water in areas with limited freshwater resources is conductive to higher economic yield of cotton plants,meanwhile could improve cotton resistance to drought stress.At full irrigation,K⁺and Ca²⁺supplement during fertigation could alleviate inhibition of vegetative growth and stomatal limitation of photosynthesis of cotton plants as affected by salinity.Brackish water and exogenous K⁺and Ca²⁺shouldn’t be applied after flowering and bolling stage,otherwise,may aggravate the risk of secondary soil salinization.