Response Of Photosynthesis Of Vitex Negundo To Soil Drought And Water Supplement Effect

With the change of global climate,regional atmospheric drought and the phenomenon of soil drought occured frequently,which seriously affect the normal growth and development of plants.Vitex negundo was distributed widely in China,and it was a pioneer's tree species for greening barren hills and soil and water conservation.It played an important role in ecological restoration in arid area.In the past,many studies have focused on the chemical composition and medicinal value of Vitex negundo,and less study on the effect of soil drought stress on photosynthesis,especially on the time of severe drought stress and the effect of supplemental water on photosynthesis.Therefore,to explore the response of photosynthesis to soil drought and the effect of water supplement,to provide a reference and basis for understanding the water relationship of the photosynthesis of the Vitex negundo and the physiological mechanism of drought resistance.A potted experiment was carried out in the experimental station of Shandong Agricultural University,and we planted two years old seedlings of Vitex negundo as targets.The soil water content gradient was obtained by natural water consumption and artificial replenish irrigation,and studied the response laws of photosynthetic characteristic parameters and chlorophyll fluorescence parameters under multistage continuous gradient soil under light saturation,on this basis,we determined the key water threshold points of drought stress and the range of soil water content under severe drought stress,and we summarized the influences of the handles which was key threshold point,continuous severe drought and continuous water supplementation on the photosynthetic physiological indexes of Vitex negundo,we finally determined the photosynthetic physiological compensation indexes of Vitex negundo after water supplementation,the main research results are as follows:?1?Effects of soil drought stress on photosynthesis of Vitex negundo under saturated lightThe characteristic parameters of photosynthesis and chlorophyll fluorescence of the leaves of the Vitex negundo showed clear threshold value responses to the change of soil water content,and the high or low soil water content could reduce the photosynthetic physiological activity of the Vitex negundo.Under light saturation,the net photosynthetic rate?P_n?of Vitex negundo was maximum when the relative soil water content?RWC?was 81.1%,and the RWC corresponding to the maximum value of water use efficiency?WUE?was 63.4%.The RWC for the limitation of the synergistic use of Vitex negundo by stomatal factor to non stomatal factor was 46.8%,that is,the main reason for the reduction of photosynthesis of Vitex negundo in soil drought,when the RWC>46.8%,the supply of CO₂ was blocked because of the decrease of stomatal opening.,when the RWC<46.8%,the decrease of photosynthetic capacity caused by the damage of the photosynthetic mechanism,and it showing declined in the maximum fluorescence?F_m?,the maximum photochemical efficiency?F_v/F_m?,the actual photochemical efficiency?PSII?,and the photochemical quenching coefficient?qP?.Therefore,the RWC?46.8%was the soil water content range of Vitex negundo photosynthesis under severe drought stress.?2?Effects of soil drought stress time on photosynthesis of Vitex negundoThe photosynthetic physiological parameters of Vitex negundo were both determined by the degree of drought stress and duration of stress under severe drought stress.It can be seen from the two different stress levels of threshold values A and threshold values B?the threshold values A was critical value of photosynthesis restriction from stomatal factor to non-stomatal factor,which the RWC was 46.8%;the threshold values B was critical value with net photosynthetic rate close to 0,which the RWC was 16.5%?that the effect of threshold B on the physiological parameters of Vitex negundo was more serious than that of threshold A in the same duration of constant stress.That is,during the continuous stress period?0³0days?of threshold values B,the P_n,WUE,leaf transpiration rate?T_r?,stomatal conductance?G_s?,F_m,F_v/F_m,P_n,PS II and qP are smaller than that of threshold values A,especially in the 15 days of the persistent stress,the threshold values B photosynthetic physiological parameters was much smaller than than of threshold values A,but the photosynthetic parameters of threshold values A and threshold values B were close to each other in 30days.The results showed that threshold values B had more serious damage to photosynthetic structure in 15 days before continuous stress,but the damage degree of threshold A and threshold B was similar at the end of stress.It can be seen that under the same degree from the duration of stress,P_n and WUE which were stable around 0 during the duration of the threshold B stress,During the 0⁵ days of the threshold B stress,the respiration rate exceeded the photosynthetic rate?Pn<0?,and when the stress lasted for 10 to 20 days,Pn was more than 0,the photosynthetic rate might recover or the respiration rate would decrease more.The photosynthetic rate was higher than the respiration rate,but the Pn was negative again after the continuous stress for more than 20days,indicating that the phenomenon of"starvation"may occur under the threshold B long-term stress,but the specific reasons need to be further proved.the rest of the photosynthetic parameters of the threshold A and threshold B were decreased or increased with the duration of stress,in which the NPQ was briefly rising when the threshold values A continues to stressed 5days?threshold A-5?.The F_v/F_m,PSII and qP declined slowly,but all of them decreased significantly after 5days,The results showed that Vitex negundo in the initial stage of of severe drought?threshold A-5?would reduce the damage of photosynthetic apparatus from excessive excitation energy through heat dissipation,thus reduced the reduction of light energy conversion efficiency.The threshold A-5 and threshold values B sustained stress 0d?threshold B-0?caused serious damage to the photosynthetic apparatus of Vitex negundo.at this time,the non stomatal limitation is still the dominant factor of limiting photosynthesis.?3?Effects of water restoration on photosynthesis of Vitex negundo under severe soil droughtThe symptoms of water deficiency of Vitex negundo leaves was obviously under drought stress,it showed gradual wilting and even partial abscission.After the water restoration,the photosynthetic parameters had a great difference,which was closely related to the drought stress and the duration of drought stress.That is,while the threshold values A-0,the P_n,WUE,F_m,qP,F_v/F_m and PSII have an over-compensation effect?QR/QC>1?around the water restoration 10days,and the T_r,G_s,C_i,L_s,and NPQ achieve equal compensation?QR/QC=1?.After water restoration 20 days,they were basically stable in equal compensation.While threshold A-10,the P_n,WUE,F_m,F_v/F_m,?_PS?,NPQ?qP were equally compensate,while threshold values A-20 and threshold values A-30 did not reach the control level?CK?even after 30d water restoration,and only partial compensated was obtained?QR/QC<1?.While the threshold values B-0,the P_n,WUE,T_r,C_i,F_v/F_m,?_PS?and NPQ appear equivalent compensation effects after water restoration 15 days.The threshold values B-10,threshold values B-20 and threshold values B-30 failed to reach CK after water restoration 30days,.And while threshold values B-30,NPQ and?_PS?were respectively compensated for injury at water restoration 0-5days??QR/QC<0?and no compensation effect??QR/QC=0?,but partial compensation effect appeared after water restoration 5 days.In summary,it was beneficial to make up for the damage or obtain the overcompensation effect of Vitex negundo in a short time?0¹0days?heavy drought stress and the appropriate time of replenishment treatment?about 10days?,if the water restoration time was too long?about 20days?and it would not have over compensation effect.For along time?20³0days?heavy drought stress,it would not reach contrast after the water restoration,especially after 30 days of continuous drough stresst,and short-term?0⁵days?water restoration,it may lead to no compensation effect and damage compensation effect.for?_PS?and NPQ.