Characterization of the complex interaction between the electron acceptor silicomolybdate and Photosystem II

Silicomolybdate (SiMo) and its effects on thylakoids have been characterized to evaluate its use as a probe for Photosystem II (PS II). It can accept electrons at two places in the electron transport chain: one at PS II and the other at PS I. In the presence of 1 μM 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) only the site at PS II is available. It is suggested that SiMo must disp;ace bicarbonate from its binding site to be able to function as an electron acceptor. This displacement is non-competitive. The binding of SiMo is inhibited differentially by PS II inhibitors: dinoseb>ioxynil> diuron. This difference is determined by the different positions of the inhibitors within the QB binding niche and their interaction with bicarbonate. The experimental results show that the SiMo-binding niche is located between the parallel helices of the D1 and D2 proteins of PS II, close to the non-heme iron. We conclude that SiMo is an electron acceptor with unique characteristics useful as a probe of the acceptor side of PS II.

Mode of action of photosynthesis inhibiting 4-hydroxy-benzonitriles containing nitro group

The effects of 3,5-disubstituted 4-hydroxy-benzonitriles - bromoxynil and compounds containing nitro group - on some photosynthetic parameters of spinach and wheat were investigated. There is considerable difference in the effectivity of the compounds even at the level of chloroplasts. On the basis of their effect on the development of CO2 fixation of greening wheat seedlings bromoxynil and 3,5-dinitro-4-hydroxy-derivative were ineffective, while the asymmetric - 3-nitro-5-halogenic-substituted - compounds showed a strong effectivity. The bromine and iodine containing asymmetric compounds reduced the intensity of the short wavelength fluorescence. All asymmetric compounds shifted the long wavelength fluorescence maximum with 5-8 nm to shorter wavelength. These results refer to inhibition of formation of chlorophyll forms absorbing at longer wavelengths and to the change in the conditions of energy migration. The ultrastructure of the chloroplasts in the treated seedlings became similar to the shade adapted chloroplasts.