Environmental stress factors, especially salinity, are among the most important abiotic stresses that negatively affect plant production worldwide. High salt levels in irrigation water are a major abiotic stress factor that significantly reduces spinach physiological functioning and production, particularly in irrigated areas. Improving the salt tolerance of spinach is critical for sustainable production, and in this study, we tested the hypothesis that exogenous proline (5 µM), ascorbic acid (1 mM), and salicylic acid (1 mM) applications, applied separately, would reduce salinity stress. These applications were performed at regular 14-day intervals starting from the third true leaf stage. For this purpose, plants were exposed to irrigation water salinities of 0.38, 2.0, 4.0, 7.0, 10.0, and 15.0 dS m−1, and growth, photosynthetic performance, antioxidant enzyme activities, lipid peroxidation, endogenous proline, and mineral contents were assessed. Increasing salinity to 15 dS m−1 decreased leaf area by 53.23% and stomatal conductance by 83.07%, and all these physiological changes were statistically significant. Under salinity conditions, catalase, guaiacol peroxidase, glutathione reductase, glutathione S-transferase, and superoxide dismutase activities increased by 1.13–2.52-fold, while ascorbate peroxidase activity decreased by 59.69%. Malondialdehyde levels increased 6-fold with salinity, indicating enhanced oxidative damage. Consequently, yield decreased by 31% under 15 dS m−1 salinity. Although all exogenous applications alleviated salinity stress, the most significant improvement was observed in proline application. Proline increased yield and chlorophyll content by 9% and 8.5%, respectively, and also increased antioxidant enzyme activities by 24.4–66.7%. Salicylic acid treatment increased the K+/Na+ ratio by 26.6%, and ascorbic acid treatment increased the Ca2+/Na+ ratio by 36.6%. Overall, low-dose proline application was found to improve photosynthetic pigment content and stomatal conductance, antioxidant defenses, and ion homeostasis in spinach against salinity stress, providing a stronger protective effect compared to ascorbic acid and salicylic acid. Furthermore, it can be concluded that proline application could be an effective way to manage salinity-induced limitations to physiological processes and yields, providing practical applications for sustainable production under saline irrigation conditions.
Taş et al. (Wed,) studied this question.