Relationship Between Photosynthetic Gas Exchange Parameters, Carbon Isotope Composition and Water Use Efficiency of Amygdalus pedunculata Pall. Grown in the Mu Us Sandy Land

Against the backdrop of intensifying global climate change and increasing water scarcity in arid and semiarid regions, identifying ecological and economically valuable tree species with high water-use efficiency has become essential for vegetation restoration and sustainable management in sandy ecosystems. In this study, 14 cultivars of Amygdalus pedunculata Pall. were investigated along the southwestern margin of the Mu Us Sandy Land. Leaf gas exchange parameters, chlorophyll fluorescence parameters, relative chlorophyll content, and carbon isotope composition (δ13C) of leaves and current-year shoots were measured during the peak growing season from May to July. The relationships between δ13C, intrinsic water-use efficiency (WUEi), and photosynthetic physiological traits were analyzed, and principal component analysis (PCA) was performed to comprehensively evaluate cultivar performance and identify cultivars with high water-use efficiency suitable for the Mu Us Sandy Land. The results showed that net photosynthetic rate (An), stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (E), and the ratio of intercellular to atmospheric CO2 concentration (Ci/Ca) generally decreased significantly from May to July (p < 0.05), whereas WUEi increased from 64.34 to 169.60 μmol mol−1, indicating changes in the relationship between carbon assimilation and stomatal regulation. Leaf δ13C was significantly and positively correlated with WUEi only in May (r = 0.67, p < 0.01), with no significant correlations observed in June or July. Shoot δ13C showed no significant correlation with WUEi in any of the three months. Significant positive correlations between shoot and leaf δ13C were observed in June and July, with correlation coefficients of 0.63 and 0.82, respectively, suggesting differences in the seasonal dynamics of δ13C and its potential as an indicator of water-use efficiency between organs. PCA identified three principal components that collectively explained 77.43% of the total variance. Cultivars 13065, 13071, and 12037 achieved relatively high comprehensive evaluation scores, indicating favorable overall physiological performance. These findings provide a scientific basis for cultivar screening and site-specific selection of A. pedunculata in the Mu Us Sandy Land.