Changes during early development in photosynthetic light acclimation capacity explain the shade to sun transition in Nothofagus nitida.

Nothofagus nitida (Phil.) Krasser, an emergent tree of the Chilean evergreen forest, regenerates under the canopy. Nonetheless, it is common to find older saplings in clear areas. We hypothesized that this transition from shade to sun during the early developmental stages is made possible by an ontogenetic increase in the light acclimation capacity of photosynthesis. To test our hypothesis, we studied photosynthetic performance and photoprotection in N. nitida saplings at different developmental stages corresponding with three different height classes (short: 16.2 cm; medium-height: 48.0 cm; and tall: 73.7 cm) grown under contrasting light conditions (photosynthetic photon flux (PPF) of 20, 300 or 600 micromol m(-2) s(-1)) until newly expanded leaves had developed. Light-saturated CO(2) assimilation rate and light compensation and saturation points increased with increasing PPF. Medium-height and tall saplings acclimated to high light had higher electron transport rates and higher proportions of open Photosystem II reaction centers than shorter plants acclimated to high light. Short saplings showed higher thermal dissipation and contents of xanthophylls than taller saplings. Only medium-height and tall saplings acclimated to high light recovered after photoinhibition. State transitions were higher in short plants growing in low light, and decreased with plant height and growth irradiance. Thus, during development, N. nitida changes the balance of light energy utilization and photoprotective mechanisms, supporting a phenotypic transition from shade to sun during its early ontogeny.