Exogenous application of 5-aminolevulinic acid Improves Chilling Tolerance in Rapeseed (Brassica napus L.) Seedlings

Abstract Rapeseed is an important oil crop in China. Low winter temperatures (LT) often limit plant growth and cause seed yield losses in rapeseed. Although exogenous application of 5-aminolevulinic acid (ALA) has been shown to enhance plant tolerance to various abiotic stressors, its physiological mechanisms for improving cold tolerance in rapeseed are not yet fully understood. In this study we investigated the physiological responses to exogenous ALA using the chilling-tolerant cultivar Zayou15 (ZY15) and the chilling-sensitive cultivar Huiyou 49 (HY49) under low-temperature stress. The results showed that low-[A1] temperature stress caused a considerable decrease the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as in the contents of endogenous hormones GA, IAA, and CTK. These resulted in the accumulation of reactive oxygen species, with both the content of H 2 O 2 and O 2 - showing considerable increases, as well as in membrane lipid peroxidation with considerable increases in both malondialdehyde content (MDA) and relative electrical conductivity (REC). Moreover, the contents of chlorophyll（Chl）b, Chl a/Chl b, transpiration rate ( Tr ), net photosynthesis ( Pn ), stomatal conductance ( Gs ), and intercellular CO 2 concentration ( Ci ) were considerably decreased by low-temperature stress, resulting in remarkable growth inhibition (with the aboveground fresh weight significantly decreased). Although, 20 mgL -1 exogenous ALA significantly reduced O 2 - , H 2 O 2 , and MDA content, and REC in both cultivars; it did not increase POD, SOD, CAT, or ascorbic acid peroxidase (APX) under short-term low-temperature stress (12–48 h). However, ALA substantially increased the activities of GA, Pn , Tr , Gs and Ci, resulting in a considerable increase in aboveground fresh weight. Post-treatment spraying of rapeseed plants with 5 mL of 20 mg·L-1 ALA was more effective than pre-treatment spraying. Both cultivars sprayed with ALA post-treatment showed greater reduction in MDA content. The optimum ALA dosage and concentration were 5 mL and 20 mg·L-1, respectively, leading to considerable decrease in MDA, and REC content and increase in Chl content in leaves. In conclusion, our results demonstrate that exogenous application of ALA is an appropriate strategy for rapeseed to resist winter low-temperature stress.