The cell wall plays a key role in determining mesophyll conductance (gm) and photosynthetic capacity. While the impact of wall thickness (Tcw) on gm is well understood, the influence of wall composition and structural interactions on Tcw and gm remains unclear, and it is unknown whether these factors have been affected during crop domestication. In this study, we examined 25 wild and 13 domesticated Gossypium genotypes to investigate whether variations in Tcw, composition, and structure affected gm and photosynthesis. X-ray diffraction was used to analyze internal cell wall structure. Cotton domestication reduced Tcw by modifying the pectin-to-(cellulose+hemicellulose) ratio and increasing cellulose crystallinity. However, cell wall composition and structure regulate gm differently in wild and domesticated genotypes. In wild genotypes, the pectin-to-(cellulose+hemicellulose) ratio influences CO2 diffusion, while in domesticated genotypes, the pectin matrix may be more compact, making 1/(pectin+cellulose+hemicellulose) a better predictor, reflecting the internal property differences of the cell wall. We suggest that the exceptionally low Tcw values reported in cotton domesticated genotypes indicate that they have reached the lower limit, which may impose physical constraints on component proportions and conformation.

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 (Ψ_tlp), gas exchange, and hydraulic traits were examined in two cotton cultivars at the peak flowering stage. With increasing drought treatments, Ψ_tlp, stomatal aperture ratio (g_ratio), leaf hydraulic conductance (K_leaf), leaf hydraulic conductance inside the xylem (K_x) and leaf hydraulic conductance outside the xylem (K_ox) declined significantly, with K_x showing the greatest reduction. Both K_x and g_ratio were strongly positively correlated with Ψ_tlp. Anatomically, vein density (D_v) and vessel number (Np) increased, whereas xylem vessel area (Ap) decreased. The reduction in Ap was the primary structural factor driving the decline in K_x and contributing to lower Ψ_tlp. We conclude that cotton enhances drought tolerance through a coordinated hydraulic and osmotic strategy, by modifying xylem anatomy (reducing Ap) to downregulate K_x and by adjusting osmotically to depress Ψ_tlp. The synergistic reduction in K_x and g_ratio slows the decline in leaf water potential, thereby delaying Ψ_tlp 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.

The cell wall plays a key role in determining mesophyll conductance ( g _m ) and photosynthetic capacity. While the impact of wall thickness ( T _cw ) on g _m is well understood, the influence of wall composition and structural interactions on T _cw and g _m remains unclear, and it is unknown whether these factors have been affected during crop domestication. In this study, we examined 25 wild and 13 domesticated Gossypium genotypes to investigate whether variations in T _cw , composition and structure affected g _m and photosynthesis. X-ray diffraction was used to analyze internal cell wall structure. Cotton domestication reduced T _cw by modifying pectin-to-(cellulose+hemicellulose) ratio and increasing cellulose crystallinity. In contrast, this ratio only correlated with g _m in wild genotypes, while in domesticated ones, 1/(pectin+cellulose+hemicellulose) ratio better explained g _m variation, reflecting internal property differences of cell wall. Cell wall composition and structure regulate g _m differently in wild and domesticated genotypes. In wild genotypes, pectin-to-(cellulose+hemicellulose) ratio influences CO ₂ diffusion, while in domesticated ones, pectin may behave structurally, making 1/(pectin+cellulose+hemicellulose) a better predictor. We suggest that the exceptionally low T _cw values reported in cotton domesticated genotypes indicate they have reached the lower limit, which may impose physical constraints on component proportions and conformation. Keywords: cell wall component, cell wall thickness, cellulose crystallinity, domestication, mesophyll conductance, pectin