Tolerance to sodium chloride in lettuce enhanced by potassium nitrate

Soil salinization poses a severe threat to modern agriculture and global food security. The salinity issue is exacerbated by warming, water shortage, and drought. Thus, it is critical to find ways to effectively mitigate salt stress. We studied the response of lettuce to salt stress and the application of nitrate as a mitigating agent in a pot experiment. To assess the size of treatment effect on plant health indicators in lettuce leaves, we used malondialdehyde (MDA) as a main stress marker; proline as an osmolyte, an antioxidant, and a protectant of protein-lipid complexes; and pigments. Soil salinization led to a decrease in lettuce leaves' weight by 21.4–34.3%, the Chl a content by 8.3–35.6%, the Chl b content by 32–51%, and the carotenoid content by 7.6–27%. Meanwhile, the MDA decreased to the control level for both 100 mM and 200 mM salinity levels as a result of 40 mM NO₃⁻ treatment. The MDA content increased by 40 and 104% compared to the control when plants were exposed to low-salt (100 mM) and double-salt (200 mM) stress, while increases in proline were 776% and 956%, respectively. A gradual increase in proline concentration was found depending on single (water, nitrate, salt) and mixed (salt and nitrate) treatments. Treatment of lettuce with potassium nitrate resulted in the return of chlorophyll a and b content to the control level, the concentration of carotenoids even exceeded the control level. We conclude that amendment of saline soil with potassium nitrate can alleviate NaCl stress via reducing oxidative stress and promoting the accumulation of protective, compatible solutes such as proline.