Physio-biochemical regulation of drought resistance in tree species within dryland plantations.

Drought-resistance strategies play a crucial role in determining tree resilience and mortality. During drought, stomatal closure limits photosynthesis and triggers the overproduction of reactive oxygen species. This process may decrease carbohydrate availability, which is key for antioxidant defense and osmoregulation. However, the mechanisms by which different species coordinate physiological and biochemical responses to drought, particularly for dryland tree plantations, remain poorly understood. To this end, we analyzed gas exchange, leaf water potential, and biochemical variables in apple (Malus pumila Mill.) and black locust (Robinia pseudoacacia L.) during their growing seasons at sub-humid (Changwu) and semi-arid (Mizhi) sites in northwest China. Apple trees exhibited a partially isohydric behavior, which was accompanied by a corresponding increase in net photosynthetic rate (Pn) under combined soil and atmospheric drought (hereafter, compound drought). The synergistic effects of this water-use strategy, enhanced antioxidant capacity, and active osmoregulation collectively contributed to their drought resilience. In contrast, black locust displayed an extremely anisohydric behavior, which did not lead to a significant increase in Pn under severe compound drought. Black locust maintained osmoregulation at the expense of elevated malondialdehyde levels, indicating enhanced oxidative stress. This study elucidates how stomatal regulation, osmoregulation, and antioxidant defenses interact in response to compound drought in these two species, offering insights into the mechanisms of drought resistance and potential drivers of mortality in dryland plantations.