[Studies on the light-induced reversible xanthophyll-conversions in Chlorella and spinach leaves].

1. Using new methods in thin-layer chromatography, experiments were carried out to prove the light-induced changes in the quantity of various xanthophylls in Chlorella and spinach leaves. The probable connection of these interconversions to electron transport in photosynthesis was demonstrated. 2. The kinetics of these xanthophyll conversions were investigated during strong illumination and in the succeeding dark period (Chlorella). Already after illumination of 1 min one can detect a decrease of the di-epoxide xanthophyll violaxanthin and a corresponding increase of the epoxide-free zeaxanthin. The intermediate of this interconversion is the mono-epoxide antheraxanthin. Neoxanthin exhibits no change in concentration under the given light intensity and an illumination time of 60 min and more; the same result can be observed with the other carotenoids (α-carotene, β-carotene, lutein, lutein-5,6-epoxyd) and the chlorophylls a and b. 3. The light-induced formation of zeaxanthin is not correlated with those pigment interconversions which are photooxidative in their nature and which may be detected only after long illuminations. However, by using damaged, e.g., briefly heated Chlorella cells, a photooxidative-induced decrease of carotenes and chlorophyll a and a smaller decrease of xanthophylls and chlorophyll b could already be demonstrated after illumination of 15 min. In this case the ratio xanthophylls/ carotenes increases. 4. The transformation violaxanthin → antheraxanthin → zeaxanthin ("forward-reaction") is induced not only by an illumination with white light (point 2) but also with red light (>600 nm); that means the reaction proceeds at a wavelength which cannot be absorbed by the xanthophylls themselves. Chlorophyll acts as light-acceptor. 5. The "forward-reaction" does not proceed after the cells have been heated for a short time. The presence of inhibitors of light-reaction II in photosynthesis such as o-phenanthroline, hydroxylamine and DCMU entirely suppresses the above reaction. The inhibition by DCMU can be reversed by substances (in Chlorella) which initiate or increase the cyclic electron transport at chlorophyll aI: vitamin K5 and hexylresorcinol. In contrast to its effect in chloroplasts (unpublished results), salicylaldoxime is only a very weak inhibitor of xanthophyll-conversion. Cyanide does not influence the "forward-reaction"; furthermore the reaction can be observed under aerobic and anaerobic conditions. The light-induced formation of zeaxanthin is entirely suppressed by the uncouplers CCCP and methylamine in concentrations of 10(-4) M and 5×10(-4) M, respectively. 6. The light-independent backward-reaction zeaxanthin → antheraxanthin → violaxanthin, which normally prevents a high accumulation of zeaxanthin, does not proceed under anaerobic conditions. Therefore under such conditions accumulation of zeaxanthin can be observed even in dim light. 7. The results indicate that the light-induced transformation violaxanthin → antheraxanthin → zeaxanthin, which consists in the light-induced splitting of the epoxide oxygen from violaxanthin, is not identical with the process which cases the release of oxygen in photosynthesis. There is evidence, however, that the xanthophyll-conversion is coupled with that electron-transport which goes on between reduced plastoquinone and oxidized chlorophyll aI; energy-rich compounds which are formed in this step of electron transport or ATP itself apparently is responsible for the cleavage of the oxygen from violaxanthin and for the resulting formation of zeaxanthin.