Dark Activation of NADP-Malate Dehydrogenase in Maize Leaf Discs

NADP-malate dehydrogenase (NADP-MDH) is normally activated in the light and inactivated in the dark in plant leaves, including the C₄ species maize. In the present study, leaf discs of maize were taken from illuminated plants and the state of activation of the enzyme followed in darkness under various conditions. After 30 min of incubation of the leaf discs in the dark, NADP-MDH was largely inactivated, but during the next hour of incubation in the dark the enzyme was reactivated. Bubbling air into the solution containing the leaf discs or addition of 100 μM Antimycin A prevented the reactivation of the enzyme in the dark. It is concluded that leaf discs of maize in solution in the dark become deficient in O₂ due to respiration and that the depletion of O₂ leads to activation of NADP-MDH.