Leaf Gas Exchange of Bean (Phaseolus vulgaris L.) Seedlings Subjected to Manganese Stress

Manganese (Mn) is a microelement required for optimal growth of plants and involved in several metabolic processes, mainly in photosynthesis. In the present work, the phytotoxicity of Mn was studied in bean plants (Phaseolus vulgaris L., var. ‘Coco-blanc’) treated with various concentrations of MnCl2: 10 (control), 100 and 300 µM (excess). After 20 days, treatment with 300 µM Mn affects plant fresh biomass. Nevertheless, shoot/root ratios were similar in all treatments. Inhibition of growth in 300 µM Mn-treated plants is concomitant to a reduction in chlorophylls and carotenoids contents. Furthermore, there was no significant difference in light-saturated carbon assimilation (Amax) between leaves grown in 10 and 100 µM Mn. In contrast, the presence of 300 µM decreased significantly Amax, which suggest an inability of bean plants to maintain its photosynthesis in response to Mn excess. Simultaneously, stomatal conductance (gs) and transpiration (E) declined at 300 µM Mn as compared to other treatments. This inability to maintain photosynthesis did not result from a decrease in leaf thickness since specific leaf area (SLA) was similar in all treatments. Probably, photosynthesis inhibition in plants grown under Mn excess is the consequence of their lower stomatal conductance.