Blue and green shade nets differentially regulate organic acid accumulation in citrus fruit

Colored shade nets modify orchard microclimate and may influence citrus fruit metabolism and quality. We evaluated their effects on organic acid accumulation in 'Newhall' sweet orange (Citrus sinensis) over two consecutive growing seasons. In the first season, red, yellow, green, and blue nets at 40% and 60% shading were compared with open-field conditions; green 40% and blue 60%, which showed contrasting acidity responses, were evaluated further in the second season. Shade nets reduced photosynthetically active radiation, light intensity, and canopy temperature, increased relative humidity, maintained PSII efficiency, and generally lowered stress-related indicators. Blue 60% maintained the highest citric acid, malic acid, and titratable acidity, whereas green 40% produced the lowest acidity. These patterns were associated with distinct transcriptional responses: blue 60% showed sustained higher CsPH8 expression and stage-specific increases in CS2 and CS1, while green 40% showed higher expression of several organic-acid degradation-related genes, including ACO, ACL3, FUM, and NAD-ME. Correlation analysis further linked lower radiation and temperature, higher relative humidity, and improved PSII performance with greater organic acid accumulation and expression of acid synthesis- and storage-related genes. Collectively, the findings suggest that blue 60% shading is associated with improved organic acid retention through coordinated microclimatic, physiological, and transcriptional responses, supporting its potential for maintaining citrus fruit acidity under warm, high-radiation conditions.