| A pot experiment was conducted to investigate the influence of different degree of drought stress and soil water after rewatering on chlorophyll content of one-year-old Platycladus orientalis seedlings.We exposed Platycladus orientalis seedlings to stress treatments using four relative soil water content(RSWC)with 87.84%,70%,52.16% and 40%;and four levels of stress duration in days(d)with 60 d,45d,30 d and 15 d,and adequate water supply(100% RSWC)was taken as a control,rewatering after different durations of drought stress and soil water stress of different degrees,changes in chlorophyll content before and after rehydration were determined on 1d,3d,5d,7d and 9d.The main result was as follows:(1)The growth rate and biomass of seedlings of Platycladus orientalis were affected by different durations and different degrees of soil drought stress.The growth rate of seedlings of Platycladus orientalis was higher than that of the control in the treatment of short duration of stress and high relative water content of soil,and the aboveground and underground biomass was higher than that of the control.The plant height increased by 20.5% and the ground diameter increased by 38.3% and 36.0% after two months of rehydration.Therefore,the plant height,ground diameter and biomass of Platycladus orientalis Seedlings have different recovery ability to rehydrate after drought.The treatments with high relative water content and short stress duration have compensatory growth,while the treatments with low relative water content and long stress duration have no compensatory growth.Therefore,moderate drought stress is beneficial to the compensatory growth of Platycladus orientalis Seedlings in the later stage of rehydration.(2)The relative water content of Platycladus orientalis Seedlings gradually recovered with the rehydration,and all treatments basically recovered on the third day of rehydration with compensation effect,among which the relative water content of soil was 87.84% The treatment lasted for 15 days,30 days,45 days and 60 days was higher than the control 3%,1.4%,0.1% and 4%.At the third day of rehydration,the relative water content of soil was 70% and the stress lasted for 15 days,30 days,45 days and 60 days,which were lower than the control 1%,8.3%,3.2% and 7.6%.Different duration and different degree of soil drought stress had an effect on the relative water content and water saturation deficit of Platycladus orientalis Seedlings,but the effect was not significant because of the strong drought tolerance of Platycladus orientalis.(3)The net photosynthetic rate and transpiration rate of Platycladus orientalis Seedlings decreased with the extension of stress duration and the aggravation of soil drought stress,and the water use efficiency increased gradually.The net photosynthetic rate and transpiration rate decreased by 52.2% and 62.1% respectively,and the water use efficiency increased by 18.8% compared with the control.With the rehydration process,the net photosynthetic rate,transpiration rate and water use efficiency of Platycladus orientalis Seedlings gradually recovered.All treatments basically recovered on the third day of rehydration and exceeded the control,and there was a compensation effect,but there was a fluctuation on the fifth day,and then gradually recovered and there was a compensation phenomenon on the ninth day of rehydration.Different duration and different degree of soil drought stress have obvious effects on the net photosynthetic rate,transpiration rate and water use efficiency of Platycladus orientalis Seedlings,but this effect can be recovered through later rehydration,and moderate drought stress makes later rehydration appear compensation effect.Therefore,it is beneficial for the growth of Platycladus orientalis Seedlings to recover water after moderate drought stress in the early stage.(4)The chlorophyll fluorescence parameters Fv/Fo,Fv/FM,Yield and ETR of Platycladus orientalis Seedlings decreased after different durations and degrees of soil drought stress,while NPQ increased.Fv/Fo,Fv/FM,ETR and Yield of Platycladus orientalis Seedlings after 60 days of stress decreased 3.7%,0.9%,26.5% and 0.9% respectively,while NPQ was 24.8% higher than that of the control.After rehydration,the seedlings of Platycladus orientalis under different stress treatments recovered with rehydration time,Fv/Fo,Fv/FM,Yield,ETR and NPQ,and basically recovered to the control on the third day of rehydration.With the extension of rehydration time,compensation effect appeared.On the 9th day of rehydration,the soil relative moisture content of 87.84% treatment Fv/Fo,Fv/FM,ETR and Yield increased by 7.3%?1.7%?17.1% and 1.5% respectively compared with the control.It can be seen that different durations and different degrees of soil drought stress can damage the PS ? of Platycladus orientalis Seedlings,reduce the conversion efficiency of primary light energy of PS?,inhibit the photochemical activity of PS? reaction center,damage the potential activity of PS? reaction center,and affect photosynthesis to a certain extent,but Platycladus orientalis Seedlings gradually adjust their light protection and light adaptation mechanism It adapts to this kind of arid environment and recovers each index after rehydration.(5)With the extension of drought stress duration,the chlorophyll content of Platycladus orientalis Seedlings decreased slowly,and with the aggravation of soil drought stress,it first decreased and then increased.The content of chlorophyll a,chlorophyll b,total amount of chlorophyll a/b and chlorophyll a/b of Platycladus orientalis Seedlings with relative water content of 40% for 60 days were 15.4%,14.0%,15.1% and 1.6% lower than those of the control respectively.After rehydration,the chlorophyll content of Platycladus orientalis Seedlings was restored.On the third day after rehydration,the chlorophyll content of Platycladus orientalis Seedlings basically recovered to the control,and then there was compensation effect.On the 9th day of rehydration,the total amount of chlorophyll a,chlorophyll b and chlorophyll increased by 24.8%,30.1% and 25.8% respectively,and the chlorophyll a/b was 4.0% lower than the control.Different duration and different degree of soil drought stress have influence on the chlorophyll content of Platycladus orientalis Seedlings,but Platycladus orientalis Seedlings gradually adapt to this drought environment by adjusting their physiological response mechanism,and recover in time after rehydration,showing compensation effect.(6)The longer the stress duration,the lower the soil relative water content,and the greater the cell membrane permeability of Platycladus orientalis Seedlings.With the rehydration,the cell membrane permeability of Platycladus orientalis Seedlings quickly responded and quickly recovered,and all the treatments appeared over compensation on the 5th day of rehydration.On the fifth day of rehydration,the relative water content of soil was 87.84% on the 15 th day,35.8% on the 30 th day,and 38.8% on the 40 th day.The soil relative water content of 52.16% for 60 days was 35.5% lower than that of the control.The membrane permeability of Platycladus orientalis Seedlings was obviously affected by different duration and different degree of soil drought stress,but the damage was reversible.With the process of rehydration,the membrane permeability gradually recovered and the phenomenon of over compensation appeared. |
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