The physiological response of two soybean varieties to salt stress was examined. The results showed that salt stress induced a significantly (p<0.01) lower decrease of the net photosynthetic rate (P N ) in salt-tolerant S111-9 than in saltsensitive Glycine max. P N decrease was positively related to the decrease of stomatal conductance (g s ) and intercellular CO 2 concentration (C i ) in S111-9, while with g s in G. max. a threshold of relative water content (RWC) was found, above which a slight decrease in RWC lead to a sharp reduction in g s . The photochemical quenching (q P ), the efficiency of open PSII centers(Φ PSII )and the Rubisco activity (RA) significantly decreased with increasing salinity level in G. max. The maximum PSII quantum yield(F v /F m )decreased significantly under the highest NaCl in both varieties. The higher reduction of RA in G. max was attributed to Rubisco content, which was mainly regulated at LSU expression level rather than at rbcL transcript level. These findings led us to conclude that the salt-induced reduction in P N was mainly due g s and RA for S111-9 and G. max, respectively.

The somatic hybrid descendants between a cultivated soybean Glycine max Melrose and a wild species Glycine cyrtoloba ACC547 were found to possess some salinity-resistant traits of the wild soybean. Under salt stress, two of the descendants as well as their wild parent grew better than their cultivated parent. In addition, salinity-induced decline in the net photosynthetic rate and the maximum photochemical efficiency was much less in the wild species and the descendants than in Melrose when stressed for more than 5 days. Analysis of the postillumination transient increase in chlorophyll fluorescence and the dark rereduction of the oxidized primary electron donor in photosystem I (PSI) (P700 1 ) indicated that salinity induced a significant upregulation of the cyclic electron flow around PSI (CEF1) in the wild species and the hybrid descendants. Similar to their wild parent, the descendants maintained higher nonphotochemical dissipation of excess excitation energy than their cultivated parent under salt stress. As a consequence, there were lower levels of superoxide radical and membrane lipid peroxidation in the plants of the descendants and the wild species. Based on these results, we proposed that the high salinity resistance of the descendants might be because of, at least partially, the trait inherited from the wild species of the enhanced CEF1 which contributed to the sufficient dissipation of excess excitation energy to protect photosynthetic apparatus from the damage of reactive oxygen species.