Light acclimation in nursery: morphoanatomy and ecophysiology of seedlings of three light-demanding neotropical tree species

Plants can acclimate to environmental changes by physiological and morphoanatomical responses. We aimed to evaluate the influence of high-light acclimation on the hardiness of neotropical tree species seedlings, through analysis of the gas exchange, morphology and anatomy of Aegiphila integrifolia (Jacq.) Moldenke, Guazuma ulmifolia Lam. and Heliocarpus popayanensis Kunth. Seedlings were grown in a shaded sector (40 % of photosynthetic photon flux density) of a nursery. After the growing period, part of the seedlings was kept in the shaded sector (shade) and another part was transferred to full sunlight (sun). The seedlings remained in the respective sectors for 168 days. H. popayanensis sun seedlings presented increases in net photosynthesis together with reduced stomatal conductance and transpiration, resulting in higher water-use efficiency. In contrast, the transfer to full sunlight led to a decrease in net photosynthesis of A. integrifolia and G. ulmifolia seedlings, but this parameter was recovered after development of new leaves in the sun. The hardening process under high irradiation levels induced morphoanatomical responses in all species, such as increased palisade parenchyma thickness and lower total leaf area (all species), higher stomatal density (A. integrifolia and G. ulmifolia), higher biomass allocation to roots (H. popayanensis and G. ulmifolia), and higher Dickson quality index (H. popayanensis). Thus, when submitted to a high radiation environment, the physiological and morphoanatomical acclimation increased seedlings hardiness and, in consequence, the probability of survival after planting in the field.