Effects of Foliar Application of Calcium, Magnesium, and Boron on Leaf Nutritional and Physiological Traits and Fruit Yield of Korla Fragrant Pear

Eight- to nine-year-old Korla fragrant pear (Pyrus sinkiangensis Yü) trees were used to evaluate the dose effects of a combined Ca–Mg–B foliar formulation on leaf nutritional and physiological traits and yield. A distilled-water control (CK) and three foliar spray treatments were established: L-CaMgB (0.1% Ca + 0.3% Mg + 0.15% B), M-CaMgB (0.2% Ca + 0.4% Mg + 0.25% B), and H-CaMgB (0.3% Ca + 0.5% Mg + 0.35% B). Foliar application increased leaf concentrations of N, P, Ca, Mg, Mn, Cu, Zn, and B, as well as SPAD values, leaf area index (LAI), single fruit weight, and yield, to varying degrees; these variables showed an increasing-then-decreasing trend with increasing concentration, with M-CaMgB performing best. Under M-CaMgB, leaf N, P, K, Ca, Mg, Mn, Cu, Zn, and B concentrations increased significantly by 59.09%, 21.04%, 49.38%, 51.41%, 40.27%, 73.53%, 53.29%, 26.35%, and 160.73%, respectively; SPAD and LAI increased significantly by 20.01% and 47.88%; Rubisco, SOD, CAT, and PAL activities were significantly enhanced by 171.41%, 127.26%, 128.90%, and 79.31%; and yield increased significantly by 33.78%. Correlation analysis revealed that Rubisco activity was highly significantly and positively correlated with N, P, Ca, and B (p < 0.01); yield showed the strongest correlations with LAI, PAL, CAT, and Rubisco, was highly significantly and positively correlated with N, P, K, Ca, Cu, Zn, and B (p < 0.01), and was significantly and positively correlated with Mn (p < 0.05). Mantel tests indicated that the association between mineral nutrients and leaf physiological traits was the strongest (r = 0.85, p < 0.001), followed by those between physiological traits and yield traits (r = 0.75, p < 0.001) and between mineral nutrients and yield traits (r = 0.68, p < 0.001). These exploratory results indicate strong associations among mineral nutrient accumulation, leaf physiological traits, and yield formation, suggesting that improved mineral nutrient status under the suitable Ca–Mg–B treatment may accompany enhanced leaf physiological performance and yield, whereas this coordinated association weakened at the higher application level. Based on these preliminary results, foliar co-application of 0.2% Ca fertilizer, 0.4% Mg fertilizer, and 0.25% B fertilizer is recommended for 8–9-year-old Korla fragrant pear trees, as it effectively improved leaf nutritional and physiological traits and achieved gains in both efficiency and yield.