Long-Term Structural and Physiological Responses of Mediterranean ‘Fuji’ Apple Trees to Shading and Photoselective Nets

Mediterranean apple orchards are increasingly exposed to high solar radiation, elevated temperatures, and recurrent summer droughts. This study evaluated the long-term responses of ‘Fuji’ apple trees to five protective net systems (Black, Grey, White, Red, and Yellow) and an uncovered control over five consecutive growing seasons (2020–2024). Leaf gas exchange was analyzed using linear mixed-effects models, whereas chlorophyll content index (CCI), chlorophyll fluorescence, and spectral reflectance indices were analyzed using factorial analysis of variance (ANOVA) or mixed-effects models, depending on the response variable and model estimability. Specific leaf area (SLA) was analyzed descriptively because the sampling procedure did not retain tree-level identity. CCI, Normalized Difference Vegetation Index (NDVI), and Photochemical Reflectance Index (PRI) showed variation associated with net treatment, while gas exchange was driven primarily by interannual climatic variability and chlorophyll fluorescence mainly by season during the 2022–2024 quantitative assessment period. Leaf surface temperature and photosynthetically active radiation (PAR) measurements further characterized differences in the environmental conditions associated with the net systems. Overall, the results indicate that interannual climatic variability was a major source of variation in physiological responses, whereas differences among net environments were more evident for leaf structural and spectral traits than for instantaneous gas exchange and chlorophyll fluorescence. These findings highlight the potential of protective net systems to modify the radiation and thermal environment and to be associated with long-term structural and physiological responses while also emphasizing the need for replicated net structures and treatment-specific microclimate measurements in future studies.