Effects of succinate on manganese toxicity in pea plants

Pea (Pisum sativum cv. Citrine) plants were grown in nutrient solution containing various manganese (Mn) concentrations in the presence or absence of succinate to evaluate the potential role of succinate in the plant tolerance to Mn excess. Supplying pea plants with excess Mn led to a reduction in the relative growth rate (RGR), chlorophyll a and b content, photosynthetic O-2 evolution activity, and photosystem II (PSII) activity, as measured in the light-adapted state (phiPSII) in comparison to the control. The primary photochemical efficiency of PSII, estimated by the F-v/F-m ratio, was less affected by increasing Mn concentration. Chloroplasts from Mn-treated leaves exhibited significant changes in their ultrastructure, depending on the strength of Mn toxicity. The concentration of Mn in roots, stem, and leaves increased with the increase of Mn in the nutrient solution. Addition of succinate before and after Mn treatment did not reduce the inhibitory effect of Mn on the plant growth, chlorophyll fluorescence parameters, photosynthetic O-2 evolution activity, and chloroplast structure of the pea plants. It was found that supply of exogenous succinate at a high Mn concentration (over 1500 /muM) in the nutrient solution led to an increase of Mn uptake in the roots accompanied by a decrease in a Mn translocation to the leaves and stems compared to Mn-treated pea-plants. However, differences in the toxicity effect of Mn in both Mn and Mn/Succinate-treated pea plants were not detected. Thus, such changes in Mn distribution within the Mn/succinate-treated plant did not confer tolerance of Mn excess to pea plants. These results suggest that succinate probably has an affinity for Mn and may function as a "terminal acceptor" of large amounts of Mn, decreasing Mn transport to the stem and leaves, but does not contribute to Mn tolerance.