Contrasting responses of three tropical hardwood saplings species to a short-term drought experiment in Bahia, Brazil

The global demand for tropical hardwood continues to rise. However, exacerbated by a warming climate, high temperatures, and drought conditions during the dry season in many tropical regions is likely a contributing factor in the low survival rates of some planted hardwood tree seedlings grown under natural field conditions without watering. Here, we present a leaf-gas exchange and chlorophyll fluorescence experiment with tree seedlings of three species ( Astronium fraxinifolium - AF, Cariniana legalis - CL, and Handroanthus serratifolius - HS) under well-watered and water stress conditions. Following the cessation of watering, leaf temperatures increased as soil water content and transpiration rates decreased. A gradual reduction of soil water content over 4-days negatively impacted assimilation net CO ₂ rates ( A _net ), stomatal conductance ( g _s ) and transpiration (E) with CL showing the greatest reduction in A _net (94%), HS (90%), and AF the smallest reduction (77%). Moreover, the decline in A _net was not solely attributed to partial stomatal closure, as F _v /F _m photosynthetic parameters derived from chlorophyll fluorescence also declining throughout the drought. While HS did not show detectable emissions of volatile isoprenoids, AF and CL maintained leaf isoprene emissions in the light throughout the drought. Drought induced the leaf accumulation of absiscic acid in HS, although an unknown interference following ABA leaf extraction prevented its quantification in AF and CL. The results indicate that common tropical hardwood species in Brazil are highly sensitive to water stress, with partial stomatal closure and isoprenoid synthesis playing an important role in the thermotolerance of photosynthesis during moisture stress.