Influence of Manganese Toxicity on Photosynthesis in Ricebean (Vigna Umbellata) Seedlings

Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna umbellata) was studied by the measurement of gas exchange characteristics and chlorophyll fluorescence parameters. The net photosynthetic rate (P N), transpiration rate (E), and stomatal conductance (g s) were reduced with increasing Mn concentration in nutrient solution. The reduction in g s and E was more pronounced at 6 d of Mn treatment. However, P N declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to the direct effect of Mn on stomatal regulation. Mn did not affect the maximum efficiency of photosystem 2 (PS2) photochemistry (Fv/Fm). A reduction in photochemical quenching (qP) and excitation capture efficiency of open PS2 (Fv′/Fm′) with a concomitant increase in qN was observed. This implies that reduced demand for ATP and NADPH due to the reduction in photosynthesis causes a down-regulation of PS2 photochemistry and thus a high pH gradient (increase in qN) and limited electron transport (decreased qP).