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In the previous works from this laboratory, it was reported that singlet oxygen(¹O₂, ¹△g) is efficiently photoproduced by thylakoid-bound iron proteins, primarily iron-sulfur proteins, plus intense blue light: ¹O₂ so produced causes severe photodamage not only to the light phase reaction system of cbloroplasts [Jung and Kim (1990) Photochem. Photobiol. 52,1003-1009] but also to the dark phase reaction system through the inactivation of the Calvin cycle enzymes [Jung and Kim (1991) Photochem. Photobiol, 54,833-839], leading to loss of photosynthetic capacity of plant leaves. Recently we prepared and used an artificial nonheme iron protein from bovine serum albumin as a model of ironsulfur proteins to demonstrate that the Fe-S clusters indeed play a crucial role in photodynamic reaction acting as endogenous sensitizers in cells[Kim and Jung (1992) Photochem. Photobiol. 55,856-861]. In addition, we presented evidence that chloroplast cytochromes also take part, less efficintly than Fe-S proteins though, in photosensitization in plant cells. Based on these observations, it seems reasonable to assume that, at least in the same species of plants grown under the identical conditions except for the amount of iron contained in a growing medium, the higher the content of iron in chloroplast membranses, the more sensitive the photosynthetic organelles are to high intensity light. If such is the case, the iron toxicity of plant leaves can be viewed, at least in part, as a conseqence of ¹O₂-mediated photodynamic reaction occurring at higher rate in chloroplasts with excessive content of iron uptaken than in those with low iron content. This report presents some prelliminary results supporting the aforementioned assumption.
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