Effects of Foliar Calcium and Magnesium Application on Peach (Prunus persica L.) Nutrition Status and Fruit Quality Formation

Peach fruit quality—including sweetness, firmness, and nutritional value—is strongly determined by mineral nutrition, yet limited soil availability of calcium (Ca) and magnesium (Mg) under production activities often constrains high-quality fruit formation in juicy peach production. This study aimed to evaluate the effects of foliar application of Ca and Mg on leaf nutrient status and fruit quality in peach (Prunus persica L. Batsch). Seven treatments were established, including 0.1% (T1), 0.2% (T2), and 0.3% (T3) CaCl2 solutions, 0.1% (T4), 0.2% (T5), and 0.3% (T6) MgSO4 solutions, and a water-sprayed control (CK). All treatments were applied during the fruit development stage, and RNA sequencing analysis was performed on the highest-concentration MgSO4 treatment group. The results of this study showed that high-concentration treatments (0.3% CaCl2, T3; 0.3% MgSO4, T6) significantly increased leaf chlorophyll and Ca/Mg accumulation relative to CK. For Ca treatments, T3 significantly increased sucrose content, whereas T2 enhanced glucose accumulation and total phenolic content, indicating that appropriate CaCl2 supplementation may improve fruit quality through regulation of carbohydrate metabolism and antioxidant responses. T6 produced the greatest gains in soluble sugars, soluble solids, vitamin C, and antioxidant capacity, indicating a close link between leaf Mg status and carbohydrate accumulation. Transcriptomic analysis of peach leaves further revealed that Mg supplementation modulated genes involved in starch and sucrose metabolism, contributing to sugar accumulation. This study revealed that foliar application of 0.3% MgSO4 offers an effective strategy for improving peach fruit quality.