Physiological And Biochemical Responses Of Phyllostachys Edulis To Drought, Heat And Synergistic Stress

In order to investigate the physiological and biochemical responses of 3-year-old Phyllostachys edulis seedlings under drought stress(i.e.control check(70%-80% of field water-holding capacity(FC)),light stress(60%-70% FC),medium stress(40%-50% FC)and heavy stress(20%-30% FC)),heat stress(i.e.25°C and 40°C)and synergistic stress.Photosynthetic rate,chlorophyll fluorescence parameters,and the contents of photosynthetic pigment,reactive oxygen species(ROS),malondialdehyde(MDA),ascorbic acid and glutathione,and the changes in enzyme activity of antioxidative enzymes and ascorbate-glutathione(AsA-GSH)cycle in the leaves of P.edulis seedlings were analyzed by using artificial climate chamber method under different treatment conditions.Through the comprehensive analysis of test data,the results are as follows:1.The contents of Chl a and total chlorophyll,the ratio of Chl a/b decreased with the increase of drought stress,while the content of carotenoids(Car)increased.The chl a/b decreased by 12.50%(P < 0.01)compared with CK under heat stress(40°C,the same below).The Chl a,Chl(a+b)content and Chl a/b ratio were further reduced after synergistic stress,Chl a and Chl a/b decreased by 15.06% and 22.12%(P < 0.01)compared with CK,respectively,and the total chlorophyll content decreased by 9.09%(P < 0.05)compared with CK under light synergistic stress,while Car was not significant under synergistic stress.2.With the increase of drought stress,the maximum net photosynthetic rate(Pnmax),apparent quantum efficiency(AQY)and light saturation point(LSP)of P.edulis seedlings were reduced,while(light compensation point)LCP increased.Compared with CK,Pnmax increased and LCP decreased(P < 0.01)under heat stress,significantly.After the synergistic stress,the change range of light response parameters increased.The Pnmax,AQY and LSP were reduced by 60.19%,50.00% and 31.87%(P < 0.01)compared with CK,respectively,and LCP was 165.04%(P < 0.01)higher than CK under light synergistic stress.3.The minimal fluorescence(Fo)increased and maximal fluorescence(Fm)decreased gradually under the single stress.The Fo,relative variable fluorescence at the J-step(VJ)and approximated initial slope of the fluorescence transient(Mo)were further increased under synergistic stress,increased by 114.88%,86.84% and 101.41%(P < 0.01)compared with CK under severe synergistic stress,respectively,while Fm was 36.20% lower than CK(P < 0.01).The fluorescence quantum yield ratio and the specific activity parameters showed that the activity of the unit excited state area decreased under synergistic stress more than those of single stress.Density of reaction centers(RC/CSO)gradually increased,while performance index on absorption basis(PIABS)gradually decreased under the single stress.The overall change of RC/CSO was not significant,but it increased by 96.25%(P < 0.01)compared with CK under the synergistic stress.And PIABS decreased gradually,which decreased by 97.52%(P < 0.01)compared with CK under severe synergistic stress.4.The contents of superoxide radical(O2·-),hydrogen peroxide(H2O2)and MDA in leaves of P.edulis increased under heat and drought stress,and these contents were higher than those of single stress.The contents of O2·-?H2O2 and MDA were 67.56%,28.77% and 170.47% higher than CK(P < 0.01)under mild synergistic stress,respectively,which indicated that the damage of synergistic stress of P.edulis seedlings was greater than that of single stress.5.The activities of superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT)were elevated under the single stress.But the activities of SOD and POD increased first at the interaction of moderate synergistic stress,which were 2.92 times and 3.21 times higher than CK(P < 0.01),respectively,then decreased.The activity of CAT showed an increasing trend under synergistic stress.The results indicated that the leaves of P.edulis could be against the damage of ROS by adjusting the activity of antioxidant enzymes,and CAT efficiency in the clearance of H2O2 was more stable than POD under synergistic stress.6.The contents of AsA,DHA,GSH and GSSH were not significantly changed compared with CK under the interaction of synergistic stress.The activities of ascorbate peroxidase(APX),monodehydroascorbate reductase(MDHAR),dehydrogenated ascorbate reductase(DHAR)and glutathione reductase(GR)increased first and then decreased,which were 1.23 times,3.94 times,1.89 times and 1.86 times higher than CK(P < 0.01),respectively,at the interaction of moderate synergistic stress,then decreased.However,the change of ?GSH /?GSSG ratio was not significant,and ?AsA/?DHA ratio decreased slightly under the interaction of heavy synergistic stress,which indicated the removal of H2O2 by GSH cycle was more lasting than AsA cycle.In conclusion,the chlorophyll content and photosynthetic efficiency decreased,PSII reaction center electron transfer was inhibited,carbon assimilation capacity decreased,and the accumulation of ROS was increased and the degree of membrane lipid peroxidation was enhanced under synergistic stress of heat and drought.In order to improve the ability of adapting to the environment,P.edulis could effectively eliminate ROS and protect itself effectively from oxidative damage by reducing light capture,increasing heat dissipation,regulating the antioxidant enzyme system and nonenzymatic system AsA-GSH cycle synergies,which maintained the physiological balance of physiological function and showed strong drought and heat resistance under a certain drought and heat stress extent.However,its antioxidant capacity decreased significantly with the further increase in synergistic stress.After severe synergistic stress,photosynthetic pigment content of P.edulis decreased,the photosynthetic rate and chlorophyll fluorescence performance index decreased sharply,the activity of antioxidant enzyme and the activity of AsA-GSH cycle related were decreased,which led to the dynamic balance of physiological function to break,the physiological function disorder and defensive ability decreased.Therefore,the synergistic stress of heat and moderate drought conditions may be the critical point for P.edulis seedlings to resist double stress of heat and drought.This experiment provides a theoretical basis for cultivation of P.edulis in heat and drought weather,and provides the data base for the artificial breeding of P.edulis provenances with drought resistance and heat.