Differential blockage of photosynthetic electron flow in young and mature leaves of Arabidopsis thaliana by exogenous proline

摘要

Responses of the photosynthetic electron transport system of chloroplasts to exogenous proline application were evaluated in young and mature leaves of Arabidopsis thaliana plants under optimal growth conditions. Exogenous proline application (10 mM) during the 4(th) week of growth increased proline accumulation in young leaves more than in mature leaves, and possibly due to its degradation producing NADPH, decreased significantly the ratio of NADP(+)/NADPH in both leaf types compared with controls (without proline). However, the ratio of NADP(+)/NADPH remained significantly higher in the young leaves, suggesting lower proline degradation which resulted in less reduced plastoquinone pool than that in the mature leaves, under both low light [130 mu mol(photon) m(-2) s(-1)] and high light [1,200 mu mol(photon) m(-2) s(-1)] treatments. The young leaves seemed to adjust nonphotochemical fluorescence quenching in order to maintain a better PSII quantum yield. We concluded that under optimal growth conditions exogenous proline results in overreduction of the plastoquinone pool and blockage of photosynthetic electron flow due to accumulation of NADPH. We suggest that optimum concentrations of proline are required for optimal PSII photochemistry.