Integrated Effects of Soil Moisture on Wheat Hydraulic Properties and Stomatal Regulation

The development of water-saving management relies on understanding the physiological response of crops to soil drought. The coordinated regulation of hydraulics and stomatal conductance in plant water relations has steadily received attention. However, research focusing on grain crops, such as winter wheat, remains limited. In this study, three soil water supply treatments, including high (H), moderate (M), and low (L) soil water contents, were conducted with potted winter wheat. Leaf water potential (Ψ_leaf), leaf hydraulic conductance (K_leaf), and stomatal conductance (g_s), as well as leaf biochemical parameters and stomatal traits were measured. Results showed that, compared to H, predawn leaf water potential (Ψ_PD) significantly reduced by 48.10% and 47.91%, midday leaf water potential (Ψ_MD) reduced by 40.71% and 43.20%, K_leaf reduced by 64.80% and 65.61%, and g_s reduced by 21.20% and 43.41%, respectively, under M and L conditions. Although g_s showed a significant difference between M and L, Ψ_leaf and K_leaf did not show significant differences between these treatments. The maximum carboxylation rate (V_cmax) and maximum electron transfer rate (J_max) under L significantly decreased by 23.11% and 28.10%, stomatal density (SD) and stomatal pore area index (SPI) under L on the abaxial side increased by 59.80% and 52.30%, respectively, compared to H. The leaf water potential at 50% hydraulic conduction loss (P₅₀) under L was not significantly reduced. The g_s was positively correlated with Ψ_MD and K_leaf, but it was negatively correlated with abscisic acid (ABA) and SD. A threshold relationship between g_s and K_leaf was observed, with rapid and linear reduction in g_s occurring only when K_leaf fell below 8.70 mmol m^-2 s^-1 MPa^-1. Our findings demonstrate that wheat leaves adapt stomatal regulation strategies from anisohydric to isohydric in response to reduced soil water content. These results enrich the theory of trade-offs between the carbon assimilation and hydraulic safety in crops and also provide a theoretical basis for water management practices based on stomatal regulation strategies under varying soil water conditions.