Stomatal Behaviour and Its Regulatory Mechanisms in Wheat ( <i>Triticum aestivum</i> L.) in Response to Combined Elevated Temperature and Water Deficit Conditions

ABSTRACT Understanding the patterns and regulatory mechanisms of wheat stomatal responses to elevated temperature and reduced water availability is fundamental for adapting to changing global climate. This knowledge forms the basis for simulating and quantifying the impact of dry hot winds during the late growth stage on yield losses of wheat in northern China. Using potted wheat plants, we established four treatments at the flag leaf stage: ambient temperature with well‐watered conditions (control, CK), elevated temperature (H), mild drought (D), and combined elevated temperature and drought (HD). After 25 days of treatment, gas exchange parameters, light response curves, and stomatal morphology of the flag leaves were measured. Stomatal conductance models were fitted, and differences in the parameters among treatments were analysed, with an attempt to identify the sources of variability in each parameter. The results showed that (1) the H treatment induced moderate stomatal closure (stomatal opening reduced by 20.1%), maintaining stable net photosynthetic rate ( A n ) while improving CO 2 assimilation efficiency (16.4% decrease in intercellular CO 2 concentration), thereby forming an efficient water use strategy; (2) under D and HD treatments, the maximum net photosynthetic rate ( P max ) decreased by 19.9%–25.5%. Additionally, the photosystem adapted to low‐light conditions by downregulating the light saturation point ( I sat : 41.9%–42.5%) and light compensation point ( I c : 38.0%–51.6%); (3) Morphological parameters such as stomatal density and stomatal area remained stable, while stomatal aperture significantly decreased by 20.1%–37.8% under H, D, and HD treatments, and the theoretical maximum stomatal conductance ( g smax ) significantly decreased by 17.6%–18.4%, revealing the rapid response characteristics of stomatal dynamic regulation; (4) The slope parameters ( m and g 1 ) of the Ball‐Berry and Medlyn models showed a significant positive correlation with stomatal aperture, with a significant decrease in parameters under the H treatment (41.1% and 32.7%), confirming that stomatal movement is the core regulatory factor for model parameters. The relevant results provide key parameterization schemes and theoretical support for modelling the mechanisms of crop responses to combined elevated temperature and lower water supply conditions.