The Response Of Populus Euphratica To Initial Salinity And The Relevance To Salt Tolerance

Using a salt-resistant woody species,Populus euphratica Oliv.and a salt-sensitive woody species, Populus 'pupularis 35-44'(P.popularis),we investigated the genotypic difference in the response to salt stress.Results showed that P.euphratica seedlings were sensitive to the initial salinity,which contributing to the salt tolerance in a longer-term of stress.Furthermore,we found that the initial salinity-induced stomatal closure was the result of increased endogenous abscisic acid(ABA)and calmodulin(CAM).1.A longer-term of increasing salinity(19 days,NaCl increased from 111.8 mmol/L to 250.0 mmol/L)(1)Leaf stomatal conductance(Gs),unit transpiration rate(EVAP)and net photosynthetic rate (Pn)markedly decreased in P.euphratica after the onset of NaCl treatment.The salt-induced decline of gas exchange in P.popularis took place on day 6,indicating that P.popularis leaves were less sensitive to the initial soil salinity.Moreover,P.euphratica maintained higher Gs,EVAP and Pn throughout the duration of salt stress.However,leaf gas exchange were severely restricted in P.popularis after day 13, Meanwhile,symptom of salt damage was clearly seen in P.popularis leaves.(2)Endogenous leaf ABA and CaM contents in P.euphratica significantly increased at the beginning of treatment,and P.euphratica maintained higher leaf ABA and CaM during the period of salinity,e.g.ABA and CaM content was 1.2 times and 1.9 times of that in controls after 19 days of salt stress.However,leaf ABA and CaM in P.popularis was markedly reduced by salt stress,which were 52% and 55% of that in controls at the end of experiment.Results suggest that P.euphratica had a higher capacity to synthesize stress signal—ABA and CaM under a longer-term of increasing salinity. Moreover,the increased endogenous ABA and CaM may contribute to the salt-induced stomatal closure.2.A short-term of salinity(50 mmol/L NaCl,4h) Endogenous abscisic acid(ABA)and calmodulin(CAM)in P.euphratica leaves significantly increased after the onset of NaCl treatment(50 mmol/L NaCl,4h),stomatal conductance,unit transpiration rate and net photosynthetic rate markedly declined correspondingly,suggesting that the increased concentrations of ABA and CaM both contribute to the salt-induced stomatal closure.Gs,EVAP and Pn in control plants were not changed after plants subjected to 24 hours of 5 mmol/L tungstate(sodium form)(ABA synthesis inhibitor),10mmol/L EGTA(extracellular calcium chelator)or 5mmol /L LaCl₃(calcium channel inhibitor)treatment,but leaf gas exchange of NaCl-stressed plants(50 mmol/L NaCl,4h)was significantly elevated.Results showed that tungstate (sodium form)application decreased ABA synthesis but increased CaM levels in salt-stressed plants,in contrast,EGTA or LaCl₃ treatment decreased CaM levels but increased ABA synthesis in stressed P. euphratica.Accordingly,application of tungstate(sodium form),EGTA or LaCl₃ exhibits an inhibitory effect on salt-induced stomatal closure.3.Extraneous ABA introductionExtraneous ABA was applied via xylem feeding to mimic the salt-induced elevation of ABA in P. euphratica leaves.Endogenous abscisic acid(ABA)and calmodulin(CAM)in P.euphratica leaves significantly increased after ABA application(10μmol/L,4h,8h),and Gs,EVAP and Pn correspondingly declined.The introduction of 5mmol/L EGTA or 5mmol/L LaCl₃ prior to ABA-feeding decreased CaM levels and the inhibitory effect of ABA on leaf gas exchange was significantly reduced although ABA levels were increased by EGTA or LaCl₃ application.Moreover,we examined the concentration effect of ABA(10μmol/L and 100μmol/L)on leaf gas exchange in P.euphratica.Collectively,application of tungstate(sodium form),EGTA or LaCl₃ exhibits an inhibitory effect on ABA-induced stomatal closure,suggesting that ABA and CaM both participate in the process of salt-induced stomatal closure,which induces the increase of cytosol Ca²⁺concentration and activates the outward K⁺ and Cl^- channels,causing stomatal closure and subsequently reduces the root-to-shoot salt transport via transpiration stream.