Abstract: The effects of salt (NaCl) stress on diurnal course of photosyntIlesis of young ‘Hosui’ pear trees on two rootstocks with different salt tolerence。Pyrus betulaefolia and P.caueryana were studied by portable photosynthesis system CIRAS-1 with pot-grown grafting seedlings in soil salt-containing 0.3％,0.6％ or no(control)．The results showed that under natmal conditions(control)，diumal course of net photosynthetic rate(Pn) in pear leaves presented two peaks，and daily variation of stomatal conductance(Gs)and transpiration rate(Tr)applied single peak；while salt stress resulted in changes of diumal variation of photosynthetic properties，especially whole decline of diurnal curve of Pn；and changes of diumal curve of photosynthetic chareactefistics(or the decline of Pn)were sharply afected by diferent rootstocks or salinity as wel1．On the other hand．under salt stress，diurnal variation of Tr had linear relations with variation of Gs in leaves of pear tres with rootstock P．betulaefolia，while diurnal variation of Tr had linear relations with variation of VPD in pear trees with rootstock P.caueryana ．In addition。in the forenoon or aftemoon (under rather suitable environment)stomatal limitation was main inhibition factor ofphotosynthesis of‘ pear，while in midday(under higher light intensity and temperature and arid air) nonstomatal limitation was tnain inhibition factor.

Key words: Pear, Rootstock, Salt stres, Ph0tosynthesis