Phosphorylation of the D1 photosystem II reaction center protein is controlled by an endogenous circadian rhythm.

The light dependence of D1 phosphorylation is unique to higher plants, being constitutive in cyanobacteria and algae. In a photoautotrophic higher plant, Spirodela oligorrhiza, grown in greenhouse conditions under natural diurnal cycles of solar irradiation, the ratio of phosphorylated versus total D1 protein (D1-P index: [D1-P]/[D1] + [D1-P]) of photosystem II is shown to undergo reproducible diurnal oscillation. These oscillations were clearly out of phase with the period of maximum in light intensity. The timing of the D1-P index maximum was not affected by changes in temperature, the amount of D1 kinase activity present in the thylakoid membranes, the rate of D1 protein synthesis, or photoinhibition. However, when the dark period in a normal diurnal cycle was cut short artificially by transferring plants to continuous light conditions, the D1-P index timing shifted and reached a maximum within 4 to 5 h of light illumination. The resultant diurnal oscillation persisted for at least two cycles in continuous light, suggesting that the rhythm is endogenous (circadian) and is entrained by an external signal.