The oxidation of manganese by illuminated chloroplast preparations.

It has long been known that manganese is an essential micronutrient of plants. Although the precise functions of manganese are unknown there is evidence that it may be concerned in chlorophyll synthesis, photosynthesis, respiration and nitrate assimilation. Decreased photosynthesis under conditions of manganese deficiency was originally attributed to the diminished chlorophyll content, but more recent investigations suggest the possibility of a direct effect of manganese on photosynthesis. The evidence, which is conflicting, is reviewed by Ruck & Bolas (1954). Arnon (1954) and Gerretsen (1937, 1949, 1950) obtained evidence in favour of a direct effect of manganese on photosynthesis. Gerretsen (1950) concluded from potentiometric measurements that H202 was formed when chloroplast preparations were illuminated in the presence of Mn2 . It was suggested that Mn2+ functions as an activator of oxygen and that a photosensitive oxidation-reduction system is present in chloroplasts in which there is equilibrium between Mn2+ and Mn3+ or Mn4+, this equilibrium moving in the direction of oxidation during illumination. Mehler (1951 a, b) demonstrated the formation of H202 by illuminated chloroplast suspensions by showing that addition of catalase to such suspensions resulted in the coupled oxidation of added ethanol. Kenten & Mann (1950) showed that peroxidase systems catalyse the oxidation of Mn2+. If chloroplast preparations contain peroxidase the hydrogen peroxide formed on illumination might be used for the oxidation of Mn2+. The present work was undertaken to find out whether illuminated chloroplast preparations oxidize M-n2+ and whether such oxidation is due to peroxidase systems.