Adaptation responses in C4 photosynthesis of maize under salinity

The effect of salinity on C4 photosynthesis was examined in leaves of maize, a NADP-malic enzyme (NADP-ME) type C4 species. Potted plants with the fourth leaf blade fully developed were treated with 3% NaCl solution for 5 d. Under salt treatment, the activities of pyruvate orthophosphate dikinase (PPDK), phosphoenolpyruvate carboxylase (PEPCase), NADP-dependent malate dehydrogenase (NADP-MDH) and NAD-dependent malate dehydrogenase (NAD-MDH), which are derived mainly from mesophyll cells, increased, whereas those of NADP-ME and ribulose-1,5-bisphosphate carboxylase, which are derived mainly from bundle sheath cells (BSCs), decreased. Immunocytochemical studies by electron microscopy revealed that PPDK protein increased, while the content of ribulose-1,5-bisphosphate carboxylase/oxygenase protein decreased under salinity. In salt-treated plants, the photosynthetic metabolites malate, pyruvate and starch decreased by 40, 89 and 81%, respectively. Gas-exchange analysis revealed that the net photosynthetic rate, the transpiration rate, stomatal conductance (gs) and the intercellular CO2 concentration decreased strongly in salt-treated plants. The carbon isotope ratio (δ13C) in these plants was significantly lower than that in control. These findings suggest that the decrease in photosynthetic metabolites under salinity was induced by a reduction in gas-exchange. Moreover, in addition to the decrease in gs, the decrease in enzyme activities in BSCs was responsible for the decline of C4 photosynthesis. The increase of PPDK, PEPCase, NADP-MDH, and NAD-MDH activities and the decrease of NADP-ME activity are interpreted as adaptation responses to salinity.