THE EFFECT OF THIOCTIC ACID ON THE QUANTUM EFFICIENCY OF THE HILL REACTION IN INTERMITTENT LIGHT

Abstract

The yield of O2 per flash was measured in Scenedesmus in flashing light. At dark inter-vals between flashes of 0.05 second the yield per flash is limited by the concentrations of both quinone and thioctic acid in the medium. When the dark interval is lengthened to 0.2 second, these limitations disappear and are replaced by light limitation. The yield per flash under these latter conditions is nearly equal to the total amount of thioctic acid in the plant on a molar basis, which is consistent with the proposal that thioctic acid is participating in the quantum conversion process in photosynthesis, provided that the lifetime for thermal decay of the photochemically active state (of chlorophyll) is at least of the order of 0.2 second in vivo. This, together with existing data, leads to the suggestion of 2 distinct physical stages in the conversion of the photon energy into chemical potential, only the second of which may involve thioctic acid.