Abstract
Hydraulic theory predicts a positive coupling between leaf hydraulic conductance (Kleaf) and stomatal conductance (gs); however, this theory has not been fully supported by observations, and the underlying mechanisms remain unclear. Currently, subdividing Kleaf into leaf hydraulic conductance inside xylem (Kx) and outside xylem (Kox) offers a new perspective for elucidating the regulatory mechanism of Kleaf on gs. Optimal planting density can enhance water use efficiency (WUE) by optimizing gs; however, the changes in leaf hydraulic properties during this process and its regulation of gs and WUE remain unclear. We examined the relationships between Kx and Kox with gs, photosynthetic rate (AN), and WUE, and investigated the structural basis determining Kox in cotton under eight planting densities of 12, 18, 24, 36, 48, 60, 72, and 84 plant m-². The results showed that as the increase of planting density, Kleaf and AN remained consistent while Kox and gs decreased significantly. Kox was significantly influenced by leaf thickness and the volume fraction of inter-cellular air space. Kleaf and Kx showed no correlation with AN or gs, but Kox exhibited a significant positive correlation with gs. Furthermore, Kox is significantly negatively correlated with WUE. These findings suggest that Kox modulates gs to reduce water loss while maintaining AN, thereby enhancing WUE in cotton under various planting densities.
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