Proline-Mediated Growth Regulation and Antioxidant Defense Enhancement in Lettuce under Salinity Stress

Abstract

This study investigated the mitigating effects of exogenous proline application on lettuce (Lactuca sativa L.) plants subjected to salinity stress induced by NaCl. The experiment evaluated vegetative growth, photosynthetic pigments, chemical constituents, and antioxidant enzyme activity under varying proline concentrations (with 200 mg/l identified as the optimal treatment) and salinity conditions. Salt stress significantly impaired vegetative growth, reducing fresh weight by 6.73%, leaf number by 10.0%, and leaf area by 19.08% compared to control conditions; however, dry matter content increased under salinity. Foliar application of proline effectively alleviated these adverse effects, with 200 mg/l proline achieving the highest plant yield under non-saline conditions (5.00% increase over control) and substantially offsetting growth inhibition under NaCl stress. Photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids) were significantly degraded by salinity, yet proline-treated plants exhibited marked improvement. The 200 mg/l proline treatment enhanced total chlorophyll and carotenoid contents by approximately 9.34% and 9.77%, respectively, relative to untreated controls. Regarding chemical constituents, NaCl stress reduced nitrogen (N), phosphorus (P), potassium (K), and the K/Na ratio while increasing sodium (Na⁺) and chloride (Cl⁻) accumulation. Proline application, particularly at 200 mg/l, counteracted these ionic imbalances by elevating N, P, K, and K/Na levels and suppressing Na⁺ and Cl⁻ uptake. Antioxidant enzyme activities—peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT)—were significantly stimulated under salinity, increasing by 45.87%, 113.1%, and 58.5%, respectively, compared to control plants. Proline spraying further enhanced these enzymatic activities, reinforcing the plant's oxidative stress defense mechanism. In conclusion, foliar application of proline at 200 mg/l represents an effective strategy to ameliorate salinity-induced damage in lettuce by improving growth parameters, photosynthetic efficiency, ionic homeostasis, and antioxidant capacity