Oxygen can afFect the photosynthetic processes in two antagonistic (protective and destructive) ways. The protection is represented by draining off of the electron transport systém, by utilisation of NADPH and ATP, and production of carbon dioxide in the process of photorespiration, oxygen reduction in the Mehler reaction, and also by regeneration of monodehydroascorbate, one of the fmal products of the Mehler-peroxidase reaction. The subsequent building up of the proton gradient in the Mehler and Mehler-peroxidase reactions also helps protéct the photosynthetic apparatus. The production of harmful oxygen radicals is accompanied by the Mehler reaction. This is in contrast to the fact that the Mehler reaction can also protéct the photosynthetic apparatus. Nevertheless, the scavenging mechanisms in plants are efficient enough for protection against the active oxygen species. In some cases the disproportion between the production and scavenging of active oxygen can result in the destruction of thylakoid membrane. Singlet oxygen, another toxic form of oxygen, can also significantly increase the inhibition of photosynthesis in the presence of oxygen. None of these processes works alone or independently, they are in a dynamic equilibrium and each of them contributes to the regulation of photosynthesis.
Needle yellowing is a typical symptom of declíning spruce trees [Picea abies (L.) Karst.] which grow at altitudes over 700 m a.s.l. In tiie chloroplasts of yellowing needles the thylakoidal systém was reduced. The contents of chlorophyll (Chl) a, b, carotene (a + |3) and neoxanthin in the needles decreased simultaneously, while lutein and the pigments of the xanthophyll cycle were less affected. Activities of the xanthophyli cycle were inhibited only in the advanced stages of yellowing. Yellowing processes with comparable symptoms could be induced in artificial atmospheres, simulating realistic conditions of pollution. Possible mechanisms accounting for the yellowing process were examined. Fast yellowing events occurred not only under full sunlight, but also imder red radiation (X > 600 ran). Therefore, the excited Chl probably sensitized pigment destruction. As the light-harvesting C\ň-a/b- protein complex of Photosystem 2 lost its photo- and acid-stability before yellowing occiured, chloroplasts were changed from a photostable to a photolabile state.
Ability of detergents digitonin (Dg), Triton X-IOO (TX), sodium dodecylsulphate (SDS), cetylpyridinium chloride (CPC), and Zwittergents 3-12 (Z-12), 3-14 (Z-14), and 3-16 (Z-16) to fragment unstacked thylakoid membranes was tested in French beán (Phaseolus vulgaris L.) leaves differing in age. The power of selected detergents to initiate fragmentation did not differ with different Chemical structure and molecular size of detergents, but it was controlled more probably by their molecular charge and shape. Unfavourable was the presence of positive charge and the absence of alkyl chain in detergent molecule. Membrane fragments obtained after the action of individual detergents on agranal thylakoids were separated on a Percoll gradient. Separated fractions were characterized by their chlorophyll (Chl) content, Chl a/b and Chl/protein ratios, position of absorption maximum (A^gx) i” spectral region and ratios of intensities of fluorescence emission bands. Since separation of no Chl-protein was achieved, no priority of detergent binding to any membrane component occurred under the conditions ušed. Fractions of similar density, products of the action of low concentrations of different detergents on the chloroplasts from leaves of the same age, differed in their characteristics. Low concentrations of detergents fragmented agranal membranes in a pattem which changed during leaf ontogeny. Only Z-16 had an increased capacity to extract proteins from membrane; this was probably promoted by the presence of 16-C alkyl chain.