Exogenous proline improve the growth and yield of lettuce with low potassium content

The purpose of this study was to evaluate the role of proline in relieving the growth inhibition of lettuce grown under low potassium conditions. Lettuce was hydroponically grown in a half-strength Enshi formula nutrient solution containing 2 mM K+ and treated with exogenous prolines (0.5, 1.0, 1.5, and 2.0 mM). The results revealed that exogenous proline applications with 1.0 and 1.5 mM alleviated oxidative damage in lettuce leaves, as demonstrated by lower lipid peroxidation and hydrogen peroxide concentrations, together with an increased activity of antioxidant enzymes. Additionally, the application of 1.0 or 1.5 mM proline enhanced the levels of proline, soluble sugars, and ascorbic acid in leaves compared with lettuce grown under low potassium condition. The growth analysis results suggest that addition of 1.0 and 1.5 mM exogenous proline increased the relative growth rate, mainly by increasing the leaf area ratio. To a lesser extent, this growth was caused by an increase in the net assimilation rate. We proposed that the application of proline in certain concentrations could increase the yield and quality of lettuce through regulating the biochemical process of plants under low potassium conditions.