Photosynthesis is improved by exogenous glycinebetaine in salt-stressed maize plants

The effects of exogenous application of glycinebetaine (GB) (10 mM) on growth, leaf water content, water use efficiency, photosynthetic gas exchange, and photosystem II photochemistry were investigated in maize plants subjected to salt stress (50 and 100 mM NaCl). Salt stress resulted in the decrease in growth and leaf relative water content as well as net photosynthesis and the apparent quantum yield of photosynthesis. Stomatal conductance, evaporation rate, and water use efficiency were decreased in salt-stressed plants. Salt stress also caused a decrease in the actual efficiency of PSII (Phi(PSII)), the efficiency of excitation energy capture by open PSII reaction centers (F-v'/F-m'), and the coefficients of photochemical quenching (q(P)) but caused an increase in non-photochemical quenching (NPQ). Salt stress showed no effects on the maximal efficiency of PSII photochemistry (F-v/F-m). On the other hand, in salt-stressed plants, GB application improved growth, leaf water content, net photosynthesis, and the apparent quantum yield of photosynthesis. GB application also increased stomatal conductance, leaf evaporation rate, and water use efficiency. In addition, GB application increased Phi(PSII), F-v'/F-m', and q(P) but decreased NPQ. However, GB application showed no effects on F-v/F-m. These results suggest that photosynthesis was improved by GB application in salt-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the actual PSII efficiency.