Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch ( Betula platyphylla var. japonica ) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO 2 (C a ) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO 2 assimilation rate at intercellular CO 2 pressures attained at C a of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO 2 (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O 2 evolution at saturating CO 2 was not affected. The maximum efficiency of PSII photochemistry (F v /F m ) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g −1 dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m −2 s −1 , the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F′ v /F′ m ) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO 2 reduction cycle rather than the potential efficiency of photochemistry, leading to increases in Q A reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.