Nothofagus nitida (Phil.) Krasser (Nothofagaceae) regenerates under the shade. Nonetheless, older seedlings are commonly found at full sun. We tested the hypothesis that light capture and photochemical and non-photochemical energy dissipation of both photosystems PSI and PSII adjust with ontogeny and brighter environment. Light energy partitioning in both photosystems was studied in seedlings of different developmental stages (small 9.7 cm, tall 36 cm) under contrasting light environments (8-200 and 1,800-2,043 mu mol photons m(-2) s(-1)) in the Chilean evergreen temperate forest. Higher A (max), dark respiration, and light compensation and saturation points in sun seedlings of both developmental stages were accompanied by higher rates of electron transport. These seedlings also showed a high fraction of open PSII reaction centres and similar non-photochemical quenching at high-light in both photosystems, showing no effect of developmental stage in these parameters. Conversely, light capture, total thermal dissipation after photoinhibition, active down-regulation of antenna efficiency, and state transitions were higher in smaller seedlings than in taller ones. These changes maintain photostasis, preventing photodamage, while favouring a more oxidized quinone pool. There is an independent effect of seedling development and light acclimation on this transition from shade to sun during early ontogeny. This transition reflects short-term responses of the photosynthetic apparatus to light and longer term responses that depend on seedling developmental stage.