Antioxidant, osmotic, and ionic responses of citrus rootstocks to salinity stress under in vitro conditions

Soil salinity is a major abiotic stress affecting citrus cultivation globally. This study evaluates salt stress responses in two citrus rootstocks, Citrus aurantium and Poncirus trifoliata, under 0, 50, and 100 mM NaCl. Key physiological and biochemical traits were assessed, including K⁺/Na⁺ ratio, proline content, and antioxidant enzyme activities (SOD, CAT, POD). Results showed that P. trifoliata maintained a more stable K⁺/Na⁺ ratio under salinity, with only a 13.3% and 20.4% decrease at 50 and 100 mM NaCl, respectively. In contrast, C. aurantium exhibited sharper reductions (31.6% and 35.6%), indicating weaker ion homeostasis. Proline accumulation was more pronounced in C. aurantium (+27.6% at 50 mM), suggesting compensatory osmotic adjustment, whereas P. trifoliata maintained lower proline levels, owing to its ion regulation. Antioxidant responses differed between the genotypes; C. aurantium exhibited stress-induced increases in enzyme activity, while P. trifoliata maintained consistently higher basal levels regardless of salinity. The activities of SOD, CAT, and POD increased in P. trifoliata by 18.7%, 20.0%, and 15.6%, respectively, under 50 mM NaCl, while the corresponding increases in C. aurantium were limited to 10.5%, 9.3%, and 7.4%, respectively. These results indicate that the enhanced salt tolerance of P. trifoliata is closely linked to efficient ion sequestration and a pre-existing robust antioxidant system, while C. aurantium relies more on compensatory osmotic mechanisms activated under stress.