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Light-induced development of photosystem (PS)-II activity was followed during irradiance of etiolated Helianthus annuus (sunflower) cotyledons using chlorophyll a fluorescence. Cotyledons from seedlings grown in continuous darkness for 6 d were exposed to 100 μmol photons·m(-2)·s(-1) for time periods of 1, 3, 6, and 12 h. Associated with increased time of irradiance exposure were significant: (1) increases in concentration of PS II, (2) increases in quantum efficiency of PS II, (3) decreases in the ratio of PS-II quinone(B) (Q(B))-nonreducing centers to total PS-II centers (PS-II Q(B)-nonreducing centers + PS-II Q(B)-reducing centers), and (4) decreases in the ratio of slow PS-II Q(B)-reducing centers to total PS-II Q(B)-reducing centers (fast PS-II Q(B)-reducing centers + slow PS-II Q(B)-reducing centers). The results support the hypotheses that development of PS II involves assembly of complexes which initially cannot reduce Q(B) and that heterogeneous aspects of PS-II pools during chloroplast maturation may represent different developmental states.

Water oxidation, or the photolysis of water in oxygenic photosynthesis, is the foundation of autotrophy. It is the source of O2, which makes the Earth habitable for higher life forms. It is the source of electrons, which generate reductants and ATP, which, in turn, sustain all other biological activities. About two thirds of global water oxidation occurs in leaves of land plants and the effectiveness of water oxidation in leaves is governed by access to CO2, the terminal electron acceptor of photosynthesis. Access to atmospheric CO2 in most leaves is limited by stomata. These pores are variable valves which effect a fundamental compromise between CO2 uptake and H2O loss, described by Stocker (1952) as the path plants must steer on their Homeric journey between the Scylla of death by hunger, and the Carybdis of death by thirst (see Steudle, this Vol.).