Direct and indirect effects of Cd2+on photosynthesis in sugar beet (Beta vulgaris)

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Abstract
Sugar‐beet plants (Beta vulgarisL. cv. Monohill) were cultivated for 4 weeks in a complete nutrient solution. Indirect effects of cadmium were studied by adding 5, 10 or 20 μMCdCl2to the culture medium while direct effects were determined by adding 1, 5, 20, 50 or 2 000 μMCdCl2to the assay media. The photosynthetic properties were characterized by measurement of CO2fixation in intact plants, fluorescence emission by intact leaves and isolated chloroplasts, photosystem (PS) I and PSII mediated electron transport of isolated chloroplasts, and CO2‐dependent O2evolution by protoplasts. When directly applied to isolated leaves, protoplasts and chloroplasts. Cd2+impeded CO2fixation without affecting the rates of electron transport of PSI or PSII or the rate of dark respiration. When Cd2+was applied through the culture medium the capacity for, and the maximal quantum yield of CO2assimilation by intact plants both decreased. This was associated with: (1) decreased total as well as effective chlorophyll content (PSII antennae size), (2) decreased coupling of electron transport in isolated chloroplasts, (3) perturbed carbon reduction cycle as indicated by fluorescence measurements. Also, protoplasts isolated from leaves of Cd2+‐cultivated plants showed an increased rate of dark respiration.