Rhythmic oscillations in wheat chloroplast photochemical activity. I. Oscillations in whole chain, photosystem II and photosystem I electron transport activities

Abstract

Chloroplasts isolated from the primary leaves of wheat seedlings grown under different light in the laboratory as well as from the leaves of field grown seedlings exhibited distinct rhythmic oscillations in photosynthetic electron transport activities. Chlorophyll content in the primary leaves of continuous light grown wheat seedlings did not oscillate. Whole chain electron transport and partial reactions measured as photosystem II and photosystem I activities estimated at regular intervals oscillated with a periodicity of about 12–16 h. The periodicity of less than 22 h suggested that the rhythms are of a non-circadian type. The oscillations in photochemical activity persisted whether the seedlings were raised under continuous light or under light-dark cycles, indicating their endogenous nature. The photochemical activities attained maximal levels during both the light and dark phases of the cycles. The rhythm pattern persisted in the presence of photophosphorylation uncouplers. A time dependent sensitivity of uncouplers to stimulate electron transport activity was observed. The timings of the activity maxima for photosystem II and photosystem I mediated reactions alternated each other during the day. The ratios of photosystem II to photosystem I activities were out of phase of photosystem I and closely resembled the rhythm pattern in photosystem II activity indicating a possible oscillation in the population of active photosystem units. The results suggest that the temporal modulation of the photochemical activity in wheat chloroplasts is brought about by some sort of reversible conformational change in the thylakoid membranes.