Photo-oxidation of ascorbic acid in leaves

Abstract

Ascorbic acid is rapidly oxidized to dehydro-ascorbic acid in strawberry leaves when these are illuminated. The oxidation, the rate of which increases with increase in the illumination, occurs both under aerobic and anaerobic conditions. The phenomenon has been observed with leaves of cabbage, iris, poplar, spinach and privet. On removal of light the process is reversed dehydroascorbic acid decreases to the steady-state concentration observed in darkened leaves. The rate of the reduction is more rapid in N than in air. The photo-oxidation of ascorbic acid in light leads to an increase in the steady-state concentration of dehydroascorbic acid in the leaf, the extent of this increase depending on the intensity of illumination. The steady-state concentration may be increased by increasing, or decreased by decreasing, the illumination. Evidence that light also stimulates the reduction of dehydroascorbic acid has been obtained from cyanide-poisoned leaves. Cyanide in low concentrations appears to suppress the photo-oxidation of ascorbic without inhibiting that of photoreduction. The steady-state concentration of dehydroascorbic acid in illuminated leaves is explicable on the basis of an equilibrium between photo-oxidation and photoreduction reactions. No relation was found between the concentration of ascorbic acid in the leaf and the steady-state concentration of dehydroascorbic acid produced on illumination. Inhibitors of the photo-synthetic process, 1[image]-(3,4-dichlorophenyl)-l,l-dimethylurea, hydroxyl-amine and o-phenanthroline, suppress both photooxidation of ascorbic acid and photoreduction of dehydroascorbic acid, and as a result the steady-state concentration of dehydroascorbic acid in the illuminated leaves is indentical with that found in darkened leaves.