Critical role of outside xylem hydraulic conductance in regulating stomatal conductance and water use efficiency in cotton across different planting densities

1. New Perspective on the Relationship between K leaf and g s : This study offers a novel perspective by subdividing leaf hydraulic conductance ( K leaf ) into xylem inside ( K x ) and outside ( K ox ), elucidating the regulatory mechanisms of K leaf on stomatal conductance ( g s ). 2. Optimizing Planting Density to Enhance Water Use Efficiency: The research reveals that K ox is significantly positively correlated with g s , allowing cotton to reduce water loss while maintaining photosynthetic rate ( A N ), thereby improving water use efficiency (WUE) across various planting densities. 3. Structural Basis and Influencing Factors of K ox : K ox is significantly influenced by leaf thickness and the volume fraction of inter-cellular air space, with a notable negative correlation with WUE as planting density increases, highlighting the importance of changes in leaf hydraulic properties for effective water management. Hydraulic theory predicts a positive coupling between leaf hydraulic conductance ( K leaf ) and stomatal conductance ( g s ); however, this theory has not been fully supported by observations, and the underlying mechanisms remain unclear. Currently, subdividing K leaf into leaf hydraulic conductance inside xylem ( K x ) and outside xylem ( K ox ) offers a new perspective for elucidating the regulatory mechanism of K leaf on g s . Optimal planting density can enhance water use efficiency (WUE) by optimizing g s ; however, the changes in leaf hydraulic properties during this process and its regulation of g s and WUE remain unclear. We examined the relationships between K x and K ox with g s , photosynthetic rate ( A N ), and WUE, and investigated the structural basis determining K ox 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, K leaf and A N remained consistent while K ox and g s decreased significantly. K ox was significantly influenced by leaf thickness and the volume fraction of inter-cellular air space. K leaf and K x showed no correlation with A N or g s , but K ox exhibited a significant positive correlation with g s . Furthermore, K ox is significantly negatively correlated with WUE. These findings suggest that K ox modulates g s to reduce water loss while maintaining A N , thereby enhancing WUE in cotton under various planting densities.