Optimizing Chloride Fertilization Enhances Kiwifruit ( Actinidia spp.) Photosynthetic, Carbon–Nitrogen Metabolic, and Antioxidant Physiological Characteristics

Chloride (Cl⁻) is essential for kiwifruit growth, but the mechanisms through which Cl⁻ regulates physiological processes and gene expression remain unclear. We conducted a two-year field experiment to investigate the effects of different Cl⁻ application rates on the growth of kiwifruit cultivars HS2 and JS1, aiming to elucidate the mechanisms. The Cl-180 treatment enhanced photosynthetic parameters and chlorophyll content, and increased levels of soluble sugars, sucrose, starch, NO₃⁻, free amino acids, and soluble proteins. The Cl-180 treatment also enhanced the activities of SOD and CAT, which reduced H₂O₂ content. Consequently, this comprehensive regulation of growth resulted in higher kiwifruit biomass. Transcriptomic analysis revealed that the Cl-180 treatment primarily affected growth through pathways related to photosynthesis, carbon fixation via the Calvin cycle, starch and sucrose metabolism, and phenylpropanoid biosynthesis. Further analysis identified 13 key genes. This study offers new insights into Cl⁻ adaptation mechanisms in crops.