Etiolated sunflower cotyledons developed in complete darkness and lacking photosystem (PS) 2 were exposed to continuous 200 µmol(photon) m-2 s-1 "white light" for 1, 3, 6, 12, and 18 h prior to evaluations of excitation-energy dissipation using modulated chlorophyll a fluorescence. Photochemical potential of PS2, measured as the dark-adapted quantum efficiency of PS2 (FV(M)/FM), and thermal dissipation from the antenna pigment-protein complex, measured as the Stern-Volmer non-photochemical quenching coefficient (NPQ), increased to 12 h of irradiation. Following 12 h of irradiation, thermal dissipation from the antennae pigment-protein complex decreased while the efficiency of excitation capture by PS2 centers (F'V/F'M) and light-adapted quantum efficiency of PS2 (ΦPS2) continued to increase to 18 h of irradiation. The fraction of the oxidized state of QA, measured by the photochemical quenching coefficient (qP), remained near optimal and was not changed significantly by irradiation time. Hence during the development of maximum photochemical potential of PS2 in sunflower etioplasts, which initially lacked PS2, enhanced thermal dissipation helps limit excitation energy reaching PS2 centers. Changes of the magnitude of thermal dissipation help maintain an optimum fraction of the oxidized state of QA during the development of PS2 photochemistry. and J. G. Lebkuecher ... [et al.].