A circadian rhythm in the rate of light-induced electron flow in three leguminous species

Abstract

Three legume species, Pisum sativum L., Glycine max (L.), and Phaseolus vulgaris L., were grown in light-dark cycles and then maintained in constant dim light. During the constant conditions, chloroplasts were isolated throughout the day and assayed for various light-reaction activities. Similar results were found for all three species. The rate of whole-chain, light-induced electron flow (H(2)O to methyl viologen) was rhythmic over a 24-hour period provided an uncoupler of photophosphorylation was present. Chloroplasts varied in their response to uncouplers on a 24-hour basis and the per cent stimulation of electron flow was rhythmic. Neither PSII activity (H(2)O to DCPIP or light-induced pH changes in the presence of K(3)Fe(CN)(6)), PSI activity (DCPIPH(2) ascorbate to methyl viologen) or the rate of oxidation of hydroquinone (TMQH(2) to methyl viologen) could be identified as a rate-limiting step for the rate of electron flow. The capability to photophosphorylate as measured by a photosynthetic control assay was also constant with time. A rhythm in oxygen evolution was also observed with leaf mesophyll cell suspensions isolated from Pisum. The possible involvement of dynamic changes in the composition or configuration of the thylakoid membrane is discussed.