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] ϩ [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.Photosynthetic oxygen evolution involves a supramolecular protein-pigment complex, PSII (Ort and Yocum, 1996;Mattoo et al., 1999). The PSII reaction center, which includes the D1 and D2 protein heterodimer, binds most of the nonprotein components of the PSII electron transport chain (Nanba and Satoh, 1987;Michel and Deisenhofer, 1988;Mattoo et al., 1989; Hankamer et al., 1997). Light is central to the metabolism of the D1 protein, regulating its synthesis (Mattoo et al., 1984), intramembrane translocation (Mattoo and Edelman, 1987; Callahan et al., 1990), posttranslational phosphorylation Elich et al., 1992) and acylation (Mattoo and Edelman, 1987;Mattoo et al., 1993), and its rate of degradation (Mattoo et al., 1984; Greenberg et al., 1987; Aro et al., 1993). The posttranslational phosphorylation of D1 occurs at its N-terminal Thr residue, catalyzed by a light-dependent redox-regulated kinase Elich et al., 1992).Protein phosphorylation is a mechanism used by eukaryotes to regulate cellular activity (Stone and Walker, 1995). In plants, protein phosphorylation is a key response to environmental signals such as wounding (Usami et al., 1995) and light (Allen, 1992). The greatest concentration of phosphoproteins in plants is found in the chloroplast membranes (Bennett, 1991). Phosphorylation and dephosphorylation of the D1 protein are strictly light dependent (Elich et al., 1993(Elich et al., , 1997. Reversible, redox-sensitive phosphorylation of the light-harvesting chlorophyll apoprotein is thought to be a mechanism maximizing quantum yield by equalizing electron flow through PSII and PSI (Allen, 1992); however, the role of phosphorylation of D1 or other PSII proteins is largely unknown. It has variously been suggested that phosphorylation regulates D1 degradation, maintaining it as a storage form prior to its replacement (Rintamäki et al., 1995a), or that it regulates dimerization of the reaction center...