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Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings
Authors:Mitsutoshi Kitao  Thomas T. Lei  Takayoshi Koike
Affiliation:Forestry and Forest Products Research Institute. Hokkaido Research Center, Sapporo 062, Japan.
Abstract: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 CO2 (Ca) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO2 assimilation rate at intercellular CO2 pressures attained at Ca of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO2 (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O2 evolution at saturating CO2 was not affected. The maximum efficiency of PSII photochemistry (Fv/Fm) 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 CO2 reduction cycle rather than the potential efficiency of photochemistry, leading to increases in QA reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.
Keywords:Betula platyphylla var. japonica    carboxylation efficiency    chlorophyll fluorescence    Mn toxicity    photosynthesis    quantum yield    white birch
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