A Circadian Rhythm in the Rate of Light-Induced Electron Flow in Three Leguminous Species

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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 3 spp. The rate of whole-chain, light-induced electron flow (H2O to methyl viologen) was rhythmic over 24 h provided an uncoupler of photophosphorylation was present. Chloroplasts varied in their response to uncouplers on a 24 h basis and the percent stimulation of electron flow was rhythmic. Neither PSII [photosystem II] activity (H2O to DCPIP [2,6-dichlorophenol indophenol] or light-induced pH changes in the presence of K3Fe(CN)6), PSI [photosystem I] activity (DCPIPH2 ascorbate to methyl viologen) or the rate of oxidation of hydroquinone (TMQH2 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 O2 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.