Physiological Response And Molecular Basis Of Tea Plant (Camellia Sinensis) Exposed To Water Stress

Drought is one of the most important environmental limited factors in plant growth and crop yield.It may cause unrecoverable damage and even death to plant under serious condition.Fifty-two cultivars of tea plant,the one of important cash crops in China,were used as materials to conduct on numerical taxonomy of photosynthetic character.On the basis,two tea cultivars(cv.Tieguanying and Fuding Dabaicha)with marked difference of drought resistance were screened to study their responses of photosynthesis,osmotic regulation and reactive oxygen species(ROS)scavenging system to water stress.And different protein expression profiles were also investigated in tea seedling exposed to water stress by two-dimensional electrophoresis(2-DE)methods.The results were as follow:The importance of carbon input in tea productivity can be appreciated from the fact that 95% of the dry weight of the plant is derived from photosynthesis.Therefore,an increase in net photosynthesis and more efficient partitioning are logical steps for increasing productivity,which was subject to the tea genotypes.The photosynthetic parameters in leaves of 52 different tea (Camellia sinensis)cultivars were measured in the field using CID-301 photosynthetic analyzer. The numerical taxonomy was adopted for the data analysis via principal component,cluster and discrimination program from the DPS statistical package(Ver.3.01).The results showed that there were greatly significant differences(p＜0.01)in the net photosynthesis rate(Pn),transpiration rate (Tr),stomata conductance(Gs),intercellular CO₂ concentration(Ci),water use efficiency(WUE) among the tested cultivars.The first three principal components were screened on the basis of 96.98%of their cumulative contribution in variances.Pn,Tr and Gs were the dominating elements in the first principal component.WUE and Ci were the dominating elements in the second and third principal components.Fifty-two tea cultivars were clustered into five groups based on Pn,Tr,Gs and WUE.Group I including 15 tea cultivars with high photosynthesis rate,moderate transpiration rate,low stomata resistance and high water use efficiency could be used for the genetic improvement in tea cultivars.Then,photosynthetic parameters(Pn,Tr,Gs,WUE)greatly affecting the classification were selected for the discrimination analysis,and five discrimination models with better distinguishing abilities were setup.Based on the above analysis,two cultivars with difference in drought resistance were screened to study the effect of photosynthetic physiology,light response characteristics and chlorophyll fluorescence characteristics in leaves of two-year-old tea seedlings(Cv. TieGuanyin and Fuding Dabaicha)exposed to different soil water stress in the pot of greenhouse.The results showed significantly diurnal variations of Pn,Tr,Gs,WUE in leaves of tea seedlings under different soil water stress;And Pn,Tr,Gs and WUE decreased,but the WUE decrease in daily mean was less than that of Pn under drought stress.The maximum net photosynthetic rates(Pn_max),apparent quantum yields(AQY)and light saturation points(LSP)decreased both in cv.TieGuanyin and Fuding Dabaicha,but light compensation points(LCP)and dark respiration rates(Rd)increased under drought stress. The results also showed that the minimal fluorescence(Fo)in leaves increased significantly under drought stress,and the maximal fluorescence(Fm),the variable fluorescence(Fv), the efficiency of light energy transformation of PSâ…¡(Fv / Fm)and the potential activities of PSâ…¡(Fv / Fo)decreased obviously,suggesting that PSâ…¡reaction center was destroyed in the leaves exposed to water stress.The rate of photochemical reaction(P_rate),the time of fluorescent increase(T_1/2)and the rate of electron transport(ETR)decreased in response to water stress,resulted in the increase of the rate of thermal dissipation(D_rate),the non-photochemical quenching(qN)and the relative limit of photosynthesis(L_(PFD)),but in the decrease of the photochemical quenching(qP)and the actual photochemical efficiency of PSâ…¡(Yield).Compared with cv.Fuding Dabaicha,the higher values in Pn,Pn_max,LSP and AQY,but lower in LCP and Rd in cv.TieGuanyin revealed that cv.Tie Guanyin had higher capacity of ecological adaptation to different soil water supply and light intensity. Different changes of chlorophyl fluorescence in leaves of cv.Tie Guanyin and cv.Fuding Dabaicha under drought stress indicated that there were stronger stress tolerance and higher photochemical efficiencies of PSâ…¡in leaves of cv.Tie Guanyin than those of cv.Fuding Dabaicha.As for the osmotic variation,the results also indicated the decrease of relative water content(RWC)and the increase of soluble protein content in leaves of tea seedling with the intensity of serious water stress.However,with significantly negative correlation to RWC, the soluble protein content decreased and the soluble sugar contents increased in the leaves of cv.Fuding Dabaicha exposed to serious water stress.The free proline content obviously increased with intensity of drought stress,but decreased in leaves of cv.Fuding Dabaicha under heavy drought stress.Under normal water supply,soluble protein content and soluble sugar contents in leaves of cv.Tie Guanyin were higher than those of cv.Fuding Dabaicha, and free proline content lower.But the increment of soluble protein content,soluble sugar contents and free proline content in leaves of cv.Tie Guanyin were higher than those of cv. Fuding Dabaicha under drought stress. As for the response of reactive oxygen species(ROS)scavenging system,the results showed that accumulation of ROS,cell membrane permeability and content of malondialdehyde(MDA)increased in both of cultivars' leaves;and the activities in superoxide dismutase(SOD),peroxidase(POD),catalase(CAT),ascorbate peroxidase(APX) activity and glutathione reductase(GR)increased in the leaves of cv.Tie Guanyin under slightly stress and mild stress,but decreased under heavy stress.The content of ascorbic acid(AsA)and glutathione(GSH)in leaves had the same profiles as the enzyme activities. But in leaves of cv.Fuding Dabaicha,the enzyme activities,the content of ascorbic acid(AsA)and glutathione(GSH)increased under slightly stress,while decreased under mild stress and heavy stress.Gene expression of the 3 mail protective enzymes genes related to the metabolism of ROS was analyzed under PEG stress by fluorescence quantitative polymerase chain reaction(FQ-PCR).The results indicated that comparing with control,the Cu/Zn SOD gene expression presented with up-down-up regulation as the time of PEG stress prolonged.And the relative expression ratios ranged from 0.49 to 3.06.CAT gene expression increased significantly at the early stage of PEG stress(after treated 3 and 6 hours),and then decreased.The relative expression ratios ranged from 0.15 to 3.45.APX gene showed upregulation expression in the 24 h after treated with PEG;but increased slowly in the first 12 hours and reached peak value after treated 24 hours.And the relative expression ratio ranged from 0.86 to 1.98.The different expression characteristics of the 3 major protective enzymes genes under PEG stress showed that these 3 genes differed from each other in response to water stress.Two-dimensional electrophoresis(2-DE)were carried out to study the leaves different protein of tea in responsive to PEG stress.And expression profiles of 18 different proteins were analyzed under PEG stress.Sisteen different proteins were identified by MS/MS using 4700 Proteomics Analyzer(Applied Biosystems,USA),and 14 proteins were identified and represented 10 kinds of proteins.In these proteins,5 were the same proteinribulosel, 5-bisphosphate carboxylase/oxygenase large subunit(RubisoLSU),other 9 proteins were response proteins to PEG stress including photosystemâ…¡23 kDa polypeptide, chloroplast chaperonin 21,triose phosphate isomerase(TPI),ATP synthase subunit, S-adenosylmethionine synthetase 1(SAMS1),centroradialis(CEN),nascent polypeptide associated complex alpha chain(α-NAC),20S proteasome alpha subunit E2(PAE2)and translation initiation factor 5A(eIF5A).These proteins will play an important role in many life activity of plant,such as chloroplast composition,sugar metabolism,energy metabolism,sulfur metabolism,protein metabolism,signal transduction,gene expression regulation and programmed cell death.