Ionic effects of Na+ and Cl- on photosynthesis in Glycine max seedlings under isoosmotic salt stress

Ion-specific stress effects of Na(+) and Cl(-) on photosynthesis of seedlings of two soybean (Glycine max) cultivars (the salt-tolerant 'Lee68' and the salt-sensitive 'N23674') were studied and compared under isoosmotic (-0.53 MPa) solutions [PEG-6000, NaCl, Na(+) (without Cl(-)) and Cl(-) (without Na(+))] for 6 d. The results showed that the chlorophyll contents and the ribulose bisphosphate carboxylase/oxygenase (Rubisco) activities of seedlings of both cultivars were inhibited to a less degree by PEG-6000 than by NaCl, Na(+) (without Cl(-)) or Cl(-) (without Na(+)) solutions. The maximum photochemical efficiency of photosystem II (PSII) (F(v)/F(m)), electron transfer rate (ETR) and effective quantum yield of PSII photochemistry (F(v)'/F(m)') were lowered significantly by PEG-6000 treatment for 2 d and 6 d. But when treated with the three isoosmotic salt stresses, most of the above three indexes significantly declined. Leaf stomatal conductance (G(s)) and net photosynthetic rate (P(n)) in both cultivars were significantly decreased under the four isoosmotic treatments, and much more drops were observed in the three salt stresses, but the intercellular CO(2) concentration (C(i)) decreased by PEG-6000 treatment and increased by the three salt stresses. Higher decreases including chlorophyll content, Rubisco activity, F(v)/F(m), ETR, F(v)'/F(m)', P(n) and G(s), and higher contents of Cl(-) as well as the content of Cl(-) plus Na(+) in chloroplasts were all observed under Cl(-) (without Na(+)) treatment than those under Na(+) (without Cl(-)) treatment, especially for the salt-sensitive cultivar 'N23674'. The results indicate that, among the adverse effects of NaCl stress on photosynthesis of G. max seedlings, the ionic toxicity was stronger than osmotic stress, and the toxicity of Cl(-) was more severe than that of Na(+).