Plant leaf drought tolerance is regulated by the coordinated effects of water transport efficiency, transpirational water loss, and hydraulic safety. Although cotton is considered drought-tolerant, the mechanisms that coordinate water transport and gas exchange to confer drought tolerance remain incompletely understood. In this study, four soil moisture gradients were established under field conditions and maintained consistently throughout the growing season. The relationships among leaf turgor loss point (Ψtlp), gas exchange, and hydraulic traits were examined in two cotton cultivars at the peak flowering stage. With increasing drought treatments, Ψtlp, stomatal aperture ratio (gratio), leaf hydraulic conductance (Kleaf), leaf hydraulic conductance inside the xylem (Kx) and leaf hydraulic conductance outside the xylem (Kox) declined significantly, with Kx showing the greatest reduction. Both Kx and gratio were strongly positively correlated with Ψtlp. Anatomically, vein density (Dv) and vessel number (Np) increased, whereas xylem vessel area (Ap) decreased. The reduction in Ap was the primary structural factor driving the decline in Kx and contributing to lower Ψtlp. We conclude that cotton enhances drought tolerance through a coordinated hydraulic and osmotic strategy, by modifying xylem anatomy (reducing Ap) to downregulate Kx and by adjusting osmotically to depress Ψtlp. The synergistic reduction in Kx and gratio slows the decline in leaf water potential, thereby delaying Ψtlp and enhancing leaf hydraulic safety during drought. This integration optimizes stomatal regulation and water transport while ensuring hydraulic safety. The findings provide a key theoretical basis and potential breeding targets for the targeted improvement of drought tolerance and water use efficiency in cotton.
Nan et al. (Sat,) studied this question.