Exogenous malate application inhibits the photochemical activity of photosystem II in rice leaves

Abstract

Malate is a carrier that transfers CO2 and NADPH from mesophyll cells (MC) to vascular bundle sheath cells (BSC) in C4 plant photosynthesis. It likely plays a key role in regulating photosystem II (PSII) photochemical activity. We used the C3 plant rice to study the effect of exogenous malate (200 μM) from roots on PSII activity. The malate treatment increased leaf malate content, NADPH/NADP ratio, and the initial fluorescence yield (F0) and decreased photosynthetic oxygen evolution, the maximum fluorescence (Fm), the maximal efficiency of PS II photochemistry (Fv/Fm), and the number of active PSII reaction centers per excited cross section (RC/CSm). Malate treatment also increased the initial fluorescence yield (F0') and decreased the maximum fluorescence (Fm') and maximal efficiency of PS II photochemistry (Fv'/Fm') from light-adapted leaves. Based on these results, we concluded that higher concentrations of exogenous malate inhibit photosynthesis and PSII photochemical activity, perhaps in part because of the rise in NADPH/NADP ratio in chloroplasts in C3 rice plants.