Nitrogen supplementation improves the high-light acclimation of Guazuma ulmifolia Lam. seedlings

Key message High nitrogen supply prevents dynamic photoinhibition in Guazuma ulmifolia seedlings and improves their morphophysiological quality during the high-light acclimation in the nursery. Acclimation under full sun is an important step in the production of tree seedlings in nurseries because it increases plant resistance to stressful conditions. Thus, the development of management techniques that improve high-light acclimation could facilitate the production of higher-quality seedlings. Given the importance of nitrogen (N) for plant growth and photosynthetic metabolism, here we evaluated whether supplementation with high N levels (HN) could positively affect the high-light acclimation of seedlings of Guazuma ulmifolia Lam., a tree species widely used in reforestation programs in the neotropics. We compared the morphophysiological parameters of seedlings transferred from the shaded sector of the nursery to full sun that were supplemented with or without HN. HN supply prevented the midday depression of both photosystem II (PSII) activity and the CO 2 assimilation rate, suggesting the avoidance of dynamic photoinhibition by HN. This effect was related to HN-induced increases in the electron transport rate of PSII, carboxylation efficiency and mesophyll conductance to CO 2 and not to reduced stomatal limitation. A decrease in excess excitation energy in PSII resulting from energy-demanding N assimilation, as suggested by increases in the light saturation points of the CO 2 assimilation and PSII electron transport rates, might also contribute to the protective effect of N. In addition, HN treatment increased seedling biomass accumulation and stem base diameter, enhancing the Dickson quality index of the acclimated seedlings. Overall, these results indicate that HN supply during high-light acclimation in the nursery improves the morphophysiological quality of G. ulmifolia seedlings.