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 (<i>Ψ</i>_leaf), leaf hydraulic conductance (<i>K</i>_leaf), and stomatal conductance (<i>g</i>_s), as well as leaf biochemical parameters and stomatal traits were measured. Results showed that, compared to H, predawn leaf water potential (<i>Ψ</i>_PD) significantly reduced by 48.10% and 47.91%, midday leaf water potential (<i>Ψ</i>_MD) reduced by 40.71% and 43.20%, <i>K</i>_leaf reduced by 64.80% and 65.61%, and <i>g</i>_s reduced by 21.20% and 43.41%, respectively, under M and L conditions. Although <i>g</i>_s showed a significant difference between M and L, <i>Ψ</i>_leaf and <i>K</i>_leaf did not show significant differences between these treatments. The maximum carboxylation rate (<i>V</i>_cmax) and maximum electron transfer rate (<i>J</i>_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 <i>g</i>_s was positively correlated with <i>Ψ</i>_MD and <i>K</i>_leaf, but it was negatively correlated with abscisic acid (ABA) and SD. A threshold relationship between <i>g</i>_s and <i>K</i>_leaf was observed, with rapid and linear reduction in <i>g</i>_s occurring only when <i>K</i>_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.