High Environmental Irradiance Induces Anatomical and Ultrastructural Damage in Citrus Leaves.

Climate change is intensifying episodes of high irradiance and thermal stress, posing a major threat to the sustainability of citrus production. Although reflective particle films have emerged as mitigation tools, their protective effects at the anatomical and ultrastructural levels under field conditions remain insufficiently understood. Here, sweet orange plants (Citrus sinensis (L.) Osbeck cv. Valencia) were exposed to full sunlight, shaded or treated with kaolin or calcium carbonate particle films, and leaves were examined using light and electron microscopy, alongside foliar nutrient analysis. Exposure to full sunlight caused disruption of thylakoid membranes and mitochondrial ultrastructural alterations consistent with stress-associated metabolic imbalance, increased plastoglobule accumulation, and alterations in oil cavities, indicating sustained photo-oxidative pressure. These effects were accompanied by marked anatomical changes, including cell hyperplasia and reduced intercellular air spaces, together with elevated foliar K and Mg concentrations that may reflect stress-driven changes in leaf structure and ion homeostasis. In contrast, both particle film treatments preserved cellular and organelle integrity comparable to that of shaded plants, demonstrating that the structural injury observed under full sunlight is directly attributable to excess irradiance. This study provides mechanistic evidence of the limits of citrus leaf structural tolerance to high irradiance and establishes a detailed anatomical and ultrastructural baseline to support the development of more effective mitigation strategies under adverse field climate conditions.