Light-induced development of photosystem (PS)-II activity was followed using chlorophyll a fluorescence during irradiance of etiolated Helianthus annuus (sunflower) cotyledons. Cotyledons from seedlings grown in continuous darkness for 6 d were exposed to 100 umol photons m-2 s-1 for time periods of 1 h, 3 h, 6 h, and 12 h. Associated with increased time of irradiance exposure were significant increases in the concentration of PS II, increases in the quantum efficiency of PS II, and decreases in the ratio of PS II quinoneB (QB)-nonreducing centers to total PS-II centers (PS II QB-nonreducing + PS II QB-reducing centers). The results support the hypothesis that development of PS II involves assembly of complexes which cannot initially reduce QB and that QB-reducing heterogeneity within PS-II pools represents different developmental states and, in mature chlorophyllous tissue, the continuous turnover of the QB-binding protein.

Key words: chlorophyll a fluorescence, etioplast, Helianthus, photosystem-II development, quinoneB.