Gas exchange, chlorophyll a fluorescence kinetics and lipid peroxidation of pecan leaves with varying manganese concentrations

Changes in pigment content, photosynthetic CO2 assimilation rate, photosystem II photochemical efficiency and lipid peroxidation of pecan [Carya illinoinensis (Wangenh.) C. Koch, cv. Kiowa] leaves induced by Mn deficiency were examined. Mn limitation was found to cause extensive declines in total chlorophyll and carotenoid contents in leaves, but it only slightly affected the chl a:b and chl:car ratios. Photosynthetic CO2 assimilation rates per unit leaf area decreased with the decreasing chlorophyll content in such a manner that the photosynthetic rates of Mn-deficient leaves were identical to those of controls when expressed on a chlorophyll weight basis. Chlorophyll fluorescence induction measurements showed that the ratio of variable to maximum (F-v:F-m) fluorescence underwent only small decreases with increasing manganese deficiency, indicating that the intrinsic quantum efficiency of the PSII units remaining in Mn-deprived leaves was not significantly decreased. Further, the net photosynthetic rates in Mn-deprived leaves were linearly correlated with the concentration of these remaining PSII units. Electron microscopy revealed no apparent change in the number and organization of thylakoids in chloroplasts from Mn-deficient leaves. From these results it is concluded that Mn deficiency depresses leaf photosynthetic capacity primarily by reducing the number of PSII units per unit leaf area, and that those remaining possess photochemical abilities similar to those of control leaves. It is also concluded that such decrease in PSII units results form a decrease in the number of chloroplasts per unit leaf area, rather from a decrease in the number of PSIIs per individual chloroplast. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.