Independent and Interactive Effects of Salinity and Proline on Physiological and Biochemical Responses of Agave Plants

Abstract

Salinity is one of the most critical abiotic stresses limiting plant growth and productivity in arid and semi-arid regions. A pot experiment was conducted under greenhouse conditions at the National Research Centre, Giza, Egypt, during two successive seasons (2020–2021) to evaluate the effects of salinity stress and exogenous proline application on growth performance and biochemical attributes of Agave angustifolia var. pacifica. The experiment included four salinity levels (0, 20,000, 25,000, and 30,000 ppm NaCl) combined with three foliar proline concentrations (0, 100, and 200 ppm). Results showed that increasing salinity levels significantly reduced growth parameters, including plant height, leaf number, leaf area index, and root length. Similarly, photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids, and total chlorophyll) were markedly decreased under salt stress. In contrast, salinity stress enhanced osmotic pressure, leaf electrical conductivity, and endogenous proline accumulation, indicating a strong stress response. Exogenous application of proline mitigated the adverse effects of salinity by improving chlorophyll stability, enhancing carbohydrate and protein content, and supporting osmotic adjustment under moderate stress conditions. However, its protective efficiency declined under severe salinity (30,000 ppm NaCl). Interaction analysis revealed that proline supplementation partially alleviated salinity-induced damage, improving the overall physiological performance of agave plants. These findings highlight the potential role of proline as a protective osmolyte and antioxidant in improving salinity tolerance. The study suggests that exogenous proline application can be an effective strategy to enhance the resilience and physiological stability of agave plants under saline environments.