The photosynthetic pigments and photochemical efficiency of photosystem 2 (PS2) were studied in four constitutive species (Achillea millefolium L., Festuca pseudovina Hack. ex Wiesb., Potentilla arenaria Borkh., and Thymus degenianus Lyka) of a semiarid grassland in South-eastern Hungary. Every species displayed typical sun-adapted traits and substantial plasticity in the composition and functioning of the photosynthetic apparatus. The contents of chlorophylls (Chls) and carotenoids (Cars) on a dry matter basis declined from May to July, however, the amount of total Cars on a Chl basis increased. This increase was the largest in Potentilla (48 %) and the smallest in Achillea (14 %). The pool of xanthophylls (VAZ) was between 25 % and 45 % of the total Car content and was larger in July than in May. The content of β-carotene increased by July, but lutein content did not change significantly. The Chl fluorescence ratio Fv/Fm was reduced by 3-10 % at noon, reflecting the down-regulation of PS2 in the period of high irradiance and high temperature. The occurrence of minimal values of ΔF/Fm' showed close correlation to the de-epoxidation rate of violaxanthin. Hence in natural habitats these species developed a considerable capacity to dissipate excess excitation energy in the summer period in their photosynthetic apparatus through the xanthophyll cycle pool and a related photoprotective mechanism, when the photochemical utilization of photon energy was down-regulated. and S. Veres ... [et al.]
We investigated the effect of chromium (20-40 g m-3, 8-72 h) on the photosystem 2 (PS2) activities of Chlorella pyrenoidosa cells. By using chlorophyll fluorescence transients, thermoluminescence, oxygen polarography, and Western blot analysis for D1 protein we found that inhibition of PS2 can be accounted for by the enhanced photodestruction of the reaction centres in the cells cultivated in the presence of Cr(VI) at 25 °C in "white light" (18 W m-2). Hence photodestruction of D1 is caused by an enhanced oxidative stress and lipid peroxidation, as indicated by the appearance of a high-temperature thermoluminescence band. and Z. T. Hörcsik ... [et al.].