The Variations in Leaf‐Level Photosynthesis and Intrinsic Water Use Efficiency of Different Spike Types Winter Wheat in North China

ABSTRACT In the context of water scarcity, enhancing water use efficiency (WUE) of winter wheat has become a crucial objective in the advancement of water‐saving agriculture. This study aimed at comparing the changes in WUE in winter wheat of different spike types, and to elucidate the factors influencing intrinsic water use efficiency (WUEi) of leaf characteristics and photosynthetic traits. Field experiments involved two winter wheat spike types: large‐spike (SN30, TN18) and multi‐spike (JM22, QH001). We assessed genotypic variations in photosynthetic parameters, WUEi, instantaneous water use efficiency (WUEn), and leaf stable carbon isotope discrimination (Δ 13 C) across major growth stages. The results demonstrate that the average yield of the large‐spike (10.81 × 10 3 kg ha −1 ) was 18.04% higher than that of the multi‐spike. The photosynthetic rate of winter wheat was highest at anthesis stage (between 16.68 and 24.88 μmol m −2 s −1 depending on genotypes); the Δ 13 C values exhibited a range of 20.59‰–21.68‰ in the large‐spike. Significant inter‐annual differences emerged in transpiration rates (Tr), WUEi, and WUEn. Overall, large‐spike wheat demonstrated superior photosynthetic capacity and water use efficiency. The results indicated a negative correlation between WUEi and Δ 13 C and stomatal conductance (Gs), which suggests that the decline in WUEi is primarily limited by stomatal conductance. These findings emphasise the interaction between leaf photosynthetic characteristics and WUEi acclimation strategies.