High-light acclimation in Quercus robur L. seedlings upon over-topping a shaded environment

Abstract

High developmental plasticity at the seedling-level during acclimation to the light environment may be an important determinant of seedling establishment and growth in temperate broadleaf forests, especially in dense understories where spatial light availability can vary greatly. Pedunculate oak (Quercus robur L.) seedlings were raised beneath a range of artificial light environments (high light, partial high light and low light) to examine morphological and photosynthetic acclimation to vertically stratified light availability. Acclimation observed at the seedling level included changes in proportional distribution of biomass and leaf area ratio to enhance either light gathering under low light availability or reduction of moisture stress under high light availability. Seedling-level acclimation was partially driven by plasticity at the flush level, but plasticity of traits determining flush morphology, such as leaf number, area, and mass, was largely controlled during bud formation rather than during shoot development. Therefore, flush-level acclimation was restricted when shoots elongated from a shaded environment into a high light environment. In contrast, traits influencing leaf-level acclimation, such as leaf thickness, specific leaf area, and pigment concentrations appeared to be driven primarily by the prevailing light environment during leaf development. The plastic response in leaf traits to light environments during shoot development enabled immediate acclimation of photosynthetic capacity to the prevailing light environment. In conclusion, oak seedlings displayed a large phenotypical plasticity on multiple levels that maximized whole seedling performance.