As already mentioned above, inhibition of photosynthesis is one effect that most of the heavy metals have in common when present at toxic concentrations. It is a very sensitive response. Measuring the photosynthetic activity is a good screening method for detecting possible stress situations, perhaps including those involving heavy metals. Direct effects of heavy metals on light and dark reactions and indirect effects caused by them decreasing the photosynthetic pigment content are involved, as well as changes in stomata function (Mysliwa-Kurdziel et al. 2004). It seems that nearly all of the components of the photosynthetic apparatus are influenced by almost all heavy metals, including chlorophyll and carotenoid content, chloroplast membrane structure, light-harvesting and oxygen-evolving complexes, photosystems and constituents of the photosynthetic electron transport chain (Barcelo and Poschenrieder 2004). Several enzymes involved in the Calvin cycle are also inhibited, especially Rubisco and PEPcarboxylase (Mysliwa-Kurdziel et al. 2004).

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