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

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 underlying mechanisms are poorly understood. Partitioning K leaf into inside-xylem ( K x ) and outside-xylem ( K ox ) components offers a refined framework for elucidating the regulation of g s by leaf hydraulics. While optimal planting density may enhance water use efficiency (WUE) through modulation of g s , corresponding changes in leaf hydraulic properties and their influence on gas exchange remain unclear. We examined relationships among K x , K ox , g s , leaf photosynthetic rate ( a N ), and WUE, and analyzed the structural determinants of K ox in cotton grown under eight planting densities: 12, 18, 24, 36, 48, 60, 72, and 84 plants m –2 . Results showed that as planting density increased, K leaf and a N remained stable, whereas K ox and g s declined significantly. Leaf thickness and the volume fraction of inter-cellular air space were key structural factors influencing K ox . Neither K leaf nor K x correlated with a N or g s ; however, K ox exhibited a significant positive correlation with g s . Furthermore, K ox was negatively correlated with WUE. These findings indicate that K ox modulates g s to minimize water loss without compromising a N , thereby enhancing WUE in cotton across varying planting densities.