Abstract:

In this study we investigated the response of leaf gas exchange to the initial salt stress in a saltresistant woody species, Populus euphratica. Endogenous abscisic acid (ABA) and calmodulin (CaM) in P. euphratica leaves significantly increased after the onset of NaCl treatment (50 mmol·L^－1 NaCl, 4 h), and stomatal conductance (G_s), unit transpiration rate (EVAP) and net photosynthetic rate（P_n） markedly declined correspondingly. Those changes suggested that the increased concentrations of ABA and CaM both were involved in the saltcaused stomatal closure. G_s, EVAP and P_n in control plants were not changed after plants were subjected to 5 mmol·L^－1tungstate (sodium form) (ABA synthesis inhibitor), 10 mmol·L^－1 EGTA (extracellular calcium chelator) or 5 mmol·L^－1 LaCl₃ (calcium channel inhibitor) treatment, but leaf gas exchange of NaClstressed plants (50 mmol·L^－1 NaCl, 4 h) increased significantly. Tungstate (sodium form) application decreased ABA synthesis, however increased CaM levels in the saltstressed plants, in contrast, EGTA or LaCl₃ treatment decreased CaM levels but increased ABA synthesis in stressed P. euphratica. Extraneous ABA was applied via xylem feeding to mimic the saltinduced elevation of ABA in P. euphratica leaves. ABA and CaM in P. euphratica leaves significantly increased after ABA application (10 μmol·L^－1), and G_s, EVAP and P_n correspondingly declined. The introduction of 5 mmol·L^－1 EGTA or 5 mmol·L^－1 LaCl₃ prior to ABAfeeding decreased CaM levels and the inhibitory effect of ABA on leaf gas exchange significantly although ABA levels were increased. In general, application of tungstate (sodium form), EGTA or LaCl₃ exhibited an inhibitory effect on saltinduced stomatal closure. The results suggest that both ABA and CaM participate in the process of saltcaused stomatal closure and hence reduce the roottoshoot salt transport via transpiration stream probably by inducing the increase of cytosol Ca²⁺ concentration and activating the outward K⁺ and Cl^－ channels.

Key words: Populus euphratica,  leaf, ABA, CaM, gas exchange