Long-term imbalance in light absorption and electron transport by photosystem I (PSI) and photosystem II (PSII) in chloroplasts brings about changes in the composition, structure, and function of thylakoid membranes. The response entails adjustment in the photosystem ratio, which is optimized to help the plant retain a high quantum efficiency of photosynthesis (W.S. Chow, A. Melis, J.M. Anderson [1990] Proc Nat Acad Sci USA 87: [7502][7503][7504][7505][7506]. The dynamics of photosystem ratio adjustment were investigated upon the transfer of pea (Pisum sativum) plants from a predominantly PSI-light to a predominantly PSII-light environment and vice versa. The concentration of functional components (primary electron accepting plastoquinone of PSll [QA], P700) and that of constituent proteins were monitored during acclimation by A difference spectrophotometry and immunoblot analysis, respectively. Fully reversible changes in photosystem ratio occurred with a half-time of about 20 h. They involved closely coordinated changes in the concentration of the QA, reaction center protein D1, D2, and the 9-kD apoprotein of the cytochrome b,,, for PSII. Similarly, closely coordinated changes in the relative concentration of P700 and reaction center proteins of PSI were observed. The leve1 of chlorophyll b and that of the light-harvesting complex II changed in accordance with the concentration of PSll in the acclimating thylakoids. Overall, adjustments in the photosystem ratio in response to PSI-or PSII-light conditions appeared to be a well-coordinated reaction in the chloroplast. The response was absent in the chlorophyll b-less chlorina f2 mutant of barley (Hordeum vulgare) and in a phycobilisomeless mutant of Agmenellum quadruplicatum, suggesting that photosystem accessory pigments act as the lightquality perception molecules and that PSI and PSll themselves play a role in the signal transduction pathway.Under limiting intensity of illumination, the efficiency of photosynthesis depends on the coordinated interaction of two photosystems in the electron-transport chain. PSII is involved in the oxidation of water and reduction of plastoquinone, whereas PSI enables electron transport from plastohydroquinone and from the Cyt b-f complex to Fd. The quantum yield of photosynthesis in many plant species from diverse light habitats is about 0.106 & 0.001 mo1 of O2 evolved mol-' of photons absorbed (Ley and Mauzerall, 1982; Bjorkman and Demmig, 1987; Evans, 1987). This value is very close to a theoretical upper limit of 0.125 mo1 of O2 evolved mol-' of photons absorbed, translating in a quantum efficiency of about 85%, independent of the light climate in ' This work was supported by a grant from the National Science * Corresponding author; fax 1-510-642-4995.Founda tion. which plants grow. This is a remarkable feature of the photosynthetic apparatus, given the contrasting light qualities that prevail in different plant ecosystems (Bjorkman and Ludlow, 1972; Kirk, 1983;Terashima and Saeki, 1983) and the fact that substantially d...