Inhibition of the Chloroplast Photochemical Reactions by Treatment of Wheat Seedlings with Low Concentrations of Cadmium: Analysis of Electron Transport Activities and Changes in Fluorescence Yield

Abstract

2+ ions for 24 h in dark, and after this treatment the plants were grown in the light until the primary leaves were fully developed. Cd 2+ ions (30-120 fiu) induced a significant decrease in activities of both photosystem II and photosystem I. The extent of the decrease in PS II activity was much greater than that in the PS I activity. Analysis of changes in the fluorescence yield of chlorophyll a also indicated that Cd 2+ ions drastically affect the photochemistry of photosystem II. Cd 2+ ions induced decrease in the rates of photoreduction of 2,6-dichlorophenol indophenol even in the presence of the exogenous electron donor, hydroxylamine, both in Tris-treated and untreated chloroplasts. This result suggests that the site of inhibition is near the site of donation of electrons by hydroxylamine. Treatment with Cd 2+ ions impairs the electron transport system on the reducing side of PS II. The decrease in the fluorescence yield of Chi a is less than that in the evolution of O2 mediated by oxidized phenylenediamine. This difference may be a result of inhibition on the reducing side of PS II. In addition to inhibition on the reducing side, Cd 2+ ions may affect the oxidizing side of PS II. A comparative study of the rates of evolution of O2 with /7-benzoquinone and dichloro-^7-benzoquinone as electron acceptors was performed since the halogenated benzoquinones have been shown to accept electrons from both active and inactive centers of photosystem II while some of the benzoquinones accept electrons only from active centers. The results suggest that Cd 2+ ions induced a marginal increase in the number of inactive reaction centers in PS II. Analysis of light-saturation-kinetics of the evolution of O2 catalysed by PS II indicates a reduction in the size of the antennae as well as in the concentration of the active (a-type) reaction centers of PS II. Thus, the Cd 2+ -induced effects on the photochemistry of PS II involve changes on the reducing side of PS II as well as possible changes in the sizes of the populations of active and inactive centers. Thus, short-term exposure to Cd 2+ ions during establishment of seedlings has a severely detrimental effect on photochemical activities in chloroplasts.