Effects of exogenous glycinebetaine on growth, CO2 assimilation, and photosystem II photochemistry of maize plants

Effects of exogenous glycinebetaine (GB, 2-50 mM) on growth, photosynthetic gas exchange, PSII photochemistry, and the activities of key enzymes involved in CO2 fixation in maize plants were investigated. Growth, CO2 assimilation rate, and stomatal conductance increased at low GB concentrations (2-20 mM) but decreased significantly at high GB concentrations (30-50 mM). Leaf relative water content and water potential remained unchanged at low GB concentrations but decreased at high GB concentrations. The maximal efficiency of PSII photochemistry was unchanged either at low or high GB concentrations. The actual PSII efficiency ((Phi(PSII)) and photochemical quenching (q(P)) increased at low GB concentrations but decreased at high GB concentrations. At low GB concentrations, there were no significant changes in the efficiency of excitation energy capture by open PSII reaction centres (F-v'/F-m') and non-photochemical quenching (q(N)). At high GB concentrations, F-v'/F-m' decreased while qN increased significantly. There were no changes in the activities of phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, and ribulose-1, 5-bi sphosphate carboxylase in control and GB-fed plants. However, there was a linear correlation between CO2 assimilation rate and stomatal conductance in control and GB-fed plants. Moreover, there were no significant differences in O-2 evolution rate between control and GB fed-plants under saturated CO2 conditions. The results suggest that exogenous GB application at certain concentrations can enhance CO2 assimilation rate, which can be explained by an increased stomatal conductance.