Changes in stomatal density and leaf area per mass induced by sunlight acclimation in Bertholletia excelsa and Carapa guianensis

Light is essential for photosynthesis and plant growth, and although physiological changes in response to light acclimation have been the subject of much research, the ecophysiology of Amazonian trees under light acclimation is still under investigation. The aim of this study was to evaluate how seedlings of andiroba ( Carapa guianensis Aubl.) and Brazil nut ( Bertholletia excelsa Bonpl.) acclimate to full sunlight. We exposed andiroba and Brazil nut seedlings to six full sunlight acclimation times for 30 days: 0 h (control kept under greenhouse), 1.5, 3.0, 6.0, 9.0 and 12.0 h. Then, the seedlings were transferred to an open area to receive 12 h (the whole day) of full sunlight for 120 days, for a total experimental period of 150 days. We evaluated 30 plant trait parameters, including biomass, leaf gas-exchange, leaf mass per area (LMA), stomatal density (StoD) and leaf nutrient concentrations. Light and CO 2 saturated photosynthesis increased in sunlight-exposed seedlings. Biomass accumulation, LMA and StoD were responsive to sunlight acclimation, but the size of the effect varied between species. StoD slightly rose in Brazil nut, but it greatly increased in andiroba. Conversely, LMA moderately changed in andiroba, but it substantially rose in Brazil nut. Andiroba significantly increased biomass accumulation relative to Brazil nut, but the intrinsic water use efficiency was greater in the latter. This study highlights the remarkable ability of Brazil nut to increase LMA and WUEi, as well as the significant phenotypic plasticity of andiroba to increase stomatal density and biomass production in response to full sunlight acclimation.