Influence of manganese toxicity on photosynthesis in ricebean (Vigna umbellata) seedlings

Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna u mbellata) was studied by the measurement of gas exchange characteristics an d chlorophyll fluorescence parameters. The net photosynthetic rate (PN). tr anspiration rate (E), and stomatal conductance (g(s)) were reduced with inc reasing Mn concentration in nutrient solution. The reduction in g(s) and E was more pronounced at 6 d of Mn treatment. However, PN declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to t he direct effect of Mn on stomatal regulation. Mn did not affect the maximu m efficiency of photosystem 2 (PS2) photochemistry (F-v/F-m). A reduction i n photochemical quenching (qp) and excitation capture efficiency of open PS 2 (F-v(1)/F-m(1)) with a concomitant increase in qN was observed. This impl ies that reduced demand for ATP and NADPH due to the reduction in photosynt hesis causes a down-regulation of PS2 photochemistry and thus a high pH gra dient (increase in qN) and limited electron transport (decreased qP).