Impact of different levels of water deficit on physiological and fluorescence responses in <i>Eucalyptus urophylla × Eucalyptus grandis</i>

This study evaluated the physiological responses of Eucalyptus urophylla × Eucalyptus grandis under varying water stress levels and their recovery post-rehydration. Plants were exposed to control, light (LS), and moderate (MS) water stress treatments in an open area, with continuous monitoring of meteorological conditions. Physiological parameters, including net photosynthesis, stomatal conductance, transpiration, and the Crop Water Stress Index (CWSI), were assessed. Mini-lysimeters measured cumulative plant transpiration. Results showed that predawn leaf water potential ( Ψ pd ) declined by approximately 196% under LS and 599% under MS compared to the control (−0.2), reflecting significant stress, alongside an increased CWSI due to stomatal closure and higher leaf temperatures. Water stress significantly reduced gas exchange variables, notably photosynthesis, which showed quicker recovery after rehydration compared to transpiration and stomatal conductance. Changes in the photochemical fluorescence parameter q P (3), defined as the coefficient of photochemical quenching measured at the third saturation pulse during fluorescence induction, reflected adjustments in photochemical activity. Five days post-rehydration, plants exhibited partial recovery, with photosynthesis regaining more rapidly than other gas exchange processes. Overall, the study underscores that water stress heavily impairs in E. urophylla × E. grandis physiological processes, though photosynthesis recovers more readily than transpiration and stomatal conductance, depending on stress severity.