Drought impacts on photosynthesis and thermoprotective mechanisms in tropical hardwood seedlings

Global demand for tropical hardwood is increasing, necessitating a deeper understanding of species-specific resilience to climate stressors. While drought impacts on tree physiology are well-documented, the interplay and potential trade-offs between key thermoprotective mechanisms, stomatal regulation via abscisic acid (ABA) and biochemical protection via isoprenoid emissions, remain poorly understood, especially in tropical hardwoods. We conducted a leaf-gas exchange and chlorophyll fluorescence experiment with seedlings of Astronium fraxinifolium (Anacardiaceae), Cariniana legalis (Lecythidaceae), and Handroanthus serratifolius (Bignoneaceae), grown under well-watered and water-stress conditions in a Atlantic rainforest located in the region of southern Bahia (Brazil). All species showed reduced net CO2 assimilation and stomatal conductance under drought. However, only H. serratifolius accumulated significant ABA, correlating with severe photosynthetic decline and poor recovery. In contrast, A. fraxinifolium maintained isoprene emissions independent of photosynthesis, indicating a costly but effective non-stomatal protection strategy. C. legalis showed an intermediate response with low ABA and moderate isoprene emissions. These findings highlight a trade-off: reliance on ABA-mediated stomatal closure conserves water but limits carbon gain and recovery, while sustained isoprenoid investment may enhance drought resilience. This insight into species-specific strategies aids in selecting trees for reforestation amid increasing climate change.