Evaluation of Drought Resistance of Apple (Malus domestica Borkh.) Rootstocks Based on Leaf Anatomical and Physiological Characteristics in Arid and Semi-Arid Regions

Water deficit is a major constraint on apple production in arid and semi-arid regions, and the selection of drought-adapted rootstocks is an effective strategy for improving orchard sustainability. This study evaluated the drought resistance of ten apple rootstocks and identified promising germplasm for apple production in the arid region of the Loess Plateau in Ningxia, China. A field experiment was conducted in Yanchi County, where annual precipitation is approximately 200 mm. Twelve leaf anatomical traits, nine stomatal traits, and ten physiological and biochemical parameters were measured and integrated for multivariate analysis. Significant differences were observed among rootstocks in epidermal, mesophyll, and stomatal traits. Qingzhen No. 1 exhibited the greatest upper and lower epidermal thicknesses, Qingzhen No. 2 had the greatest leaf thickness, and Pajam had the greatest mesophyll thickness. Stomatal traits also differed substantially among rootstocks, with SH40 showing the highest stomatal density, whereas Qingzhen No. 1 showed the largest stomatal size and opening characteristics. Pajam also exhibited relatively high proline content and POD activity, indicating coordinated osmotic and antioxidant responses under severe aridity. Principal Component Analysis (PCA) of 21 leaf anatomical and stomatal traits extracted five principal components explaining 91.888% of the total variance, while PCA of the 10 physiological and biochemical indicators extracted four principal components explaining 87.394% of the total variance. Membership function and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) analyses consistently ranked Pajam highest, followed by M26 and B9. The rootstocks were classified into high (Pajam and M26), moderate (B9, Qingzhen No. 1, SH40, JM7, T337, and Qingzhen No. 2), and low (M7 and Nic29) drought-resistance categories. These findings support environment-specific rootstock selection for apple production under water-limited conditions.