Photosynthetic response of salt-tolerant and sensitive soybean varieties

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 salt-sensitive Glycine max. P (N) decrease was positively related to the decrease of stomatal conductance (g (s)) and intercellular CO2 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 (I broken vertical bar(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, respective.