Protective Effect of Proline Against Boron Deficiency on Photosynthesis and Proline Metabolites in Cotton (Gossypium hirsutum L.) at Different Growth Stages

Boron (B) is the most common trace element deficiency of cotton (Gossypium hirsutum L.) plantation in China. Whereas the narrow range of the deficiency and toxicity restrict the application of B, proline (Pro) mitigates many kinds of stress damage in plants, including B deficiency. How the pro-regulation of pro-metabolites affects plant growth, photosynthesis and leaf structure under boron (B) deficiency condition remains poorly understood. Here, this study investigates the effects of exogenous proline on the growth of Pro metabolites in cotton (E Kang 10) under B deficiency at different growth stages. We applied three Pro concentrations (0, 1.5 and 3 mg/L) with two B concentrations (0.1, 0.5 mg/L), with a total of six treatments. The addition of exogenous Pro at 1.5 mg/L significantly promoted the accumulation of B in leaves and increased the dry and fresh weight of various parts of the plant under low B stress. Additionally, the content of MDA was reduced, while the content of soluble protein increased in cotton plants under B_0.1 treatment after the addition of Pro. Both 1.5 and 3 mg/L of Pro increased the ratio of chlorophyll a/b under B stress. In the late stage, the synthesis of Pro is mainly promoted by regulating the activities of Arg, P5CS, P5CR and ProDH. Under two B levels, the application of Pro significantly reduced the Pro content in roots at both growth stages, which was related to the decrease in Arg, Glu, GSA and P5CS activities. Additionally, exogenous applicable Pro has a better-alleviating effect on low B stress of cotton seedlings.