Effect of Foliar Fertilization on the Antioxidant Activity of Walnut ( <i>Juglans regia L</i> .) Kernels

Among the various cultivation techniques for fruit trees, foliar fertilization is an effective method of delivering nutrients directly to the leaves to improve nutrient status and ultimately enhance yield and fruit quality. In walnut trees (Juglans regia L.), foliar fertilization can enhance physiological activity by enabling rapid nutrient absorption through the foliage. Accordingly, this study evaluated how different foliar nutrient formulations influence the antioxidant capacity and bioactive compound profiles of walnut kernels. The foliar fertilization treatments included B + Zn (T1), P + K (T2), N + K + Ca + Mg (T3), N + P + K + Mg + micronutrients (T4), and a control group without treatment. Walnut kernels were analyzed for total phenolic and flavonoid contents, antioxidant activities (ABTS and DPPH), and ellagic acid concentration. The total polyphenol contents for T1, T2, T3, T4, and the control were 31.40, 26.13, 23.70, 29.03, and 22.25 mg GAE·g−1, respectively. Corresponding ABTS radical-scavenging activities were 96.0%, 87.2%, 79.6%, 92.2%, and 77.1%, with the T1 treatment exhibiting the highest values for both total polyphenols and ABTS activity. Additionally, the ellagic acid content was highest in the T1 group at 0.695 mg·g−1, representing a significant increase compared with the other treatments. Overall, the B + Zn treatment (T1) most effectively enhanced antioxidant-related traits in walnut kernels, indicating that foliar nutrient composition is a key determinant of kernel functional quality. In conclusion, it is deemed that the antioxidant activity and antioxidant content of walnut kernels were affected by the method of foliar fertilization when cultivating walnut trees.