Effects of Different Foliar Selenium Spray Frequencies on Fruit Quality, Sugar-Acid Profiles, Aroma, and Phenolic Compounds of ‘Luli’ Apples

To elucidate the effects of different selenium (Se) spray frequencies on the comprehensive quality of apple fruits, this study was conducted using “Luli” apples to investigate the influences of various Se application times on basic quality attributes, sugar-acid composition, aroma profiles, and phenolic compounds. The results indicated that with an increase in the number of Se sprays (100 mg/L), fruit firmness, soluble solid content, vitamin C content, and peel color parameters exhibited a non-linear response, with their values peaking under the T2 treatment regime (two applications on June 1 and June 21). The total sugar, sorbitol, and fructose contents in fruits from treatments T1 (one application on June 1) and T2 were significantly higher than those from T3 (three applications on June 1, June 21, and July 11) and T4 (four applications on June 1, June 21, July 11, and July 31). In contrast, the levels of total organic acids and malic acid gradually decreased with increasing Se application frequency. Se treatment significantly enhanced the total content of aromatic substances, particularly esters and ketones. Furthermore, the phenolic content exhibited an initial increase followed by a decrease in response to increasing spray frequency. A comprehensive evaluation using the membership function method ranked the overall fruit quality under different treatments in the order: T2 > T3 > T1 > T4 > CK (water only). In summary, applying Se twice during the apple growing season yielded the best integrated fruit quality, while excessive spray frequencies adversely affected fruit quality development.