Foliar application of zinc-glycine and zinc-sulfate differentially affects leaf biochemical attributes and fruit quality in Newhall orange trees

Zinc (Zn) is an essential nutrient that impacts fruit quality, improves human health, and can help plants biochemically adapt to the adverse effects of climate change. This study evaluated the potential effects of foliar applications of two sources of Zn (Zn-glycine (Zn-G) and Zn-sulfate (Zn-S)), at three rates (150, 225, and 300 mg-Zn/L), on leaf biochemical attributes and fruit quality in Newhall orange over two consecutive years (2022–2023). The results showed that leaf biochemical and fruit quality traits increased dose-dependently for both Zn compounds. Most leaf biochemical traits showed a gradual increase with the increase in Zn rate, depending on the Zn source. Leaf biochemical attributes, in terms of chlorophyll a, chlorophyll b, soluble carbohydrate, starch, soluble protein, antioxidant capacity, and superoxide dismutase enzyme activity, were increased by 85, 75, 14, 62, 87, 42, and 14%, respectively, through superior treatment (G-300). Additionally, the findings revealed that the G-225 and G-300 treatments remarkably enhanced the colour intensity of Newhall peels, as evidenced by a notable increase in the colour index (6.10). Zn application increased the TSS/TA ratio in Zn-G and Zn-S treatments, especially with foliar application of Zn-S (8.7–10.2). The highest fruit antioxidant capacity and total phenolic contents were found in the foliar application of the G-300 treatment. These results suggest that the foliar application of Zn-G as a beneficial source, particularly in a concentration of 300 mg-Zn/L, can be considered an effective strategy to improve biochemical leaf traits and the external and internal quality of fruits in Zn-deficient soils.