Salicylic Acid and Ascorbic Acid Alleviate Salinity Stress by Improving Antioxidant Capacity and Ion Homeostasis in Tobacco (Nicotiana tabacum L.)

Salinity is a major abiotic stress that restricts plant growth and quality by causing osmotic stress and ion toxicity. Ascorbic acid (AsA) and salicylic acid (SA) are plant growth regulators known to enhance plant tolerance to salt stress. This study explored the effects of foliar application of 1 mM AsA and 1 mM SA on Burley tobacco grown under four saline irrigation waters (0.25, 3.0, 6.0, and 10 dS m–1). The effects on yield, growth traits, chlorophyll content (SPAD), chemical composition, mineral nutrient status, and antioxidant enzyme activities were evaluated. Our findings show that increasing salinity remarkably reduced yield, shoot diameter, leaf area, relative water content, SPAD values, and reducing sugar content. However, both AsA and SA mitigated these reductions compared with untreated controls, with responses varying according to salinity level. Notably, the AsA application markedly enhanced SOD and GPX activities under saline conditions. On average, foliar AsA and SA reduced leaf Na+ content by 25.36% and 4.70%, respectively, and increased K+/Na+ ratios by 30.20% and 7.23%, relative to the non-sprayed control (C). Besides, AsA and SA reduced nicotine content by 22.29% and 15.48%, respectively, and reducing sugar levels by 9.40% and 8.91%, respectively, compared to the C. Finally, compared with SA, AsA showed greater efficacy in improving growth, antioxidant defense, and ionic balance (K+/Na+ and Ca2+/Na+ ratios), supporting its use as a natural elicitor to enhance salt tolerance in tobacco.