The response of ABA and hydraulic indicator-mediated leaf gas exchange and nonstructural carbohydrate of Ginkgo biloba saplings to drought and rehydration

The Ginkgo ( Ginkgo biloba L. ) is a nonconiferous gymnosperm tree species and an enigmatic living fossil that has recently been globally redistributed. In this study, we conducted a drought-rehydration experiment to examine the physiological responses of ginkgo saplings. We aimed to integrate stomatal behavior, photosynthesis, abscisic acid (ABA), hydraulics, turgor loss, and nonstructural carbohydrate (NSC) content into a comprehensive sequence of physiological responses to drought and rehydration. Leaf ABA concentrations increased continuously under drought and were high even at the early stage of rehydration. The ABA concentration was negatively correlated with stomatal conductance and the net photosynthetic rate during drought and rehydration. In the late stage of rehydration, leaf gas exchange rapidly recovered with decreasing ABA during rehydration, suggesting that high concentrations of ABA inhibited gas exchange. The reduction in photosynthesis during drought and rehydration was attributed not only to stomatal limitation but also nonstomatal limitation. There was a negative correlation between ABA and NSC concentrations in leaves, regardless of drought or rehydration conditions. During drought, physiological thresholds, such as the turgor loss point, stomatal limitation and nonstomatal limitation transition and stomatal closure point, occurred sequentially, and the water potentials of these thresholds were all higher than P 50 (the water potential corresponding to 50% loss of xylem conductivity). In particular, ginkgo closed stomata well before the saplings significantly lost xylem hydraulic conductivity, indicating that ginkgo trees have a high safety margin and resistance to drought.