Seedlings of Citrus volkameriana (L.) were grown hydroponically for 43 days in order to study the effect of Mn concentration (0, 2, 14, 98 and 686 microM) in the nutrient solution on leaf anatomy and mesophyll chloroplast ultrastructure. Increasing Mn concentration stimulated leaf lamina thickness. The size of mesophyll chloroplasts decreased and increased under 0 and 686 microM Mn, respectively, compared to the intermediate Mn concentrations, similar with regard to the number of chloroplasts per mesophyll cell area. Thylakoid membranes of plants grown under 0 microM Mn were somewhat swelled, while those in other Mn treatments did not present any visible malformation. The relative volume of starch grains per chloroplast was significantly smaller under 0-98 microM Mn (12.8-16.0%) than in the treatment with 686 microM Mn (67.6%). Further, under 686 microM Mn, dark deposits were found in vacuoles. The existence of a cell adaptation mechanism to excessive Mn availability (686 microM Mn) by increasing the size of chloroplasts as well as their number per cellular area, is discussed.