Effects Of Three Exogenes Substances On Creeping Bentgrass Turf Under Heat Stress In Summer

Creeping bentgrass（Agrostis stolonifera）is an important cool-season turfgrass due to its longer green period,stronger creeping characteristic,fine texture,and low mowing height.It was widely used in landscape greening and golf course and other fields.However,creeping bentgrass has poor heat resistance.In summer,the decrease in turf quality caused by high temperature is increasingly outstanding worldwide.In our study,creeping bentgrass cultivar’penncross’was used as experimental material.The effects of exogenous application of water（CK）,sodium chloride（Na Cl）,gamma-aminobutyric acid（GABA）and chitosan（CTS）solutions on creeping bentgrass turf were investigated tduring summer（from May 28 to August 7,2017 and from June 9 to September 8,2018）.According to the degree of stress,it can be divided into three different stages:the early stage of stress.（28May 2017 and 9 June 2018）,middle stage of stress（11 July 2017 and 15 August 2018）,and late stage of stress（7 August 2017 and 8 September 2018）.Main results are as follows:（1）Under high temperature condition in summer,the application of three exogenous substances could significantly slow down the stress-induced decrease in turf quality and leaf firing percentage.In the middle and late stage of stress in 2017 and 2018,the application of three exogenous substances could significantly improve the turf quality,and these substances had the most significant effects in the late stage of stress.In the late stage of stress in 2017,the leaf firing percentage of CK,Na Cl,GABA or CTS treatments was53.33%,41.67%,38.33 or 35.00%,and the turf quality was 4.11,5.56,5.78 or 6.00,respectively.In the late stage of stress in 2018,the leaf firing percentages of CK,Na Cl,GABA or CTS treatments was 56.67%,40.00%,43.33%or 36.67%,and the turf quality was 4.44,5.56,5.78 and 6.00,respectively.（2）The application of three exogenous substances could significantly decrease stress-caused decreases in chlorophyll content,F_v/F_m,PI_abs,and photosynthesis.In the middle and late stage of stress in 2017 and 2018,the application of three substances significantly improved the chlorophyll content and PI_abs,and in the late stage of stress,these exogenous substances had the most significant effects.The application of three exogenous substances could significantly improve the F_v/F_min the late stage of stress in2017 and 2018.In the late stage of stress in 2017,the three exogenous substances treatments had 33.53%,41.14%and 38.07%higher PI_absas compared with the water treatment,respectively.In the late stage of stress in 2018,Na Cl,GABA or CTS treatment had 22.34%,32.43%or 34.77%higher PI_absthan the water treatment,respectively.In the late stage of stress in 2017 and 2018,the application of three exogenous substances could significantly enhance photosynthesis.In the late stage of stress in 2017,three exogenous substances treatments had 62.50%,88.82%and 99.34%higher net photosynthetic rate than the water treatment,respectively.In the late stage of stress in 2018,they had 77.90%,80.66%and 149.17%higher net photosynthetic rate than the water treatment,respectively.（3）During the period of high temperature in summer,the application of three exogenous substances could significantly promote the accumulation of total soluble sugar and free proline,slow down the stress-induced decrease in RWC,and enhance osmotic adjustment.The Na Cl,GABA and CTS treatments had higher RWC and total soluble sugar than CK treatment in the late stage of stress.They had higher free proline content than CK treatment in the middle stage of stress.In the late stage of stress in 2017,three exogenous substances treatments had 11.39%,12.70%,14.20%higher RWC than the CK treatment,and they had 15.84%,17.82%and 12.09%lower OP than the CK treatment,respectively.In the late stage of stress in 2018,the RWC in leaves of Na Cl,GABA and CTS treatment was 13.21%,17.34%or 17.87%lower than water treatment,and the OP was 18.49%,16.78%or 20.98%lower than water treatment,respectively.（4）The application of three exogenous substances significantly reduced the membrane lipid peroxidation and improved the cell membrane stability.In the middle and late stage of stress in 2017 and 2018,the Na Cl,GABA and CTS treatments maintained lower EL,H₂O₂and MDA content than CK treatment.In the late stage of stress,the three exogenous substances exhibited most pronounced effects on increasing the membrane stability.（5）Based on the membership function analysis of 13 indexes,the result showed that the heat resistance of four treatments was ranked:CTS>GABA>Na Cl>CK.