Assessment of exogenous application of proline on antioxidant compounds in three Citrus species under low temperature stress

Major antioxidants were studied in three Citrus (C. reticulata, C. sinensis, and C. paradisi) under low-temperature stress (1, −1, −3°C). Regulatory effects of exogenous proline (0, 15, 20 mM) were studied as well. The results indicated that treatment of exogenous proline reduced both hydrogen peroxide and lipid peroxidation in three Citrus species. The high-performance liquid chromatography chromatograms at −3°C showed that exogenous proline increases chlorogenic, gallic, p-coumaric, and ferulic acid from phenolic acids as well as naringin, quercetin, and rutin from flavonoids. The amount of oxalic, citric, and ascorbic acid increased with increasing proline concentration but tartaric acid decreased. Exogenous proline increased Gamma-aminobutyric acid although its content showed no significant difference between 15 and 20 mM. Both low-temperature stress and treatment of proline induced the activity of ascorbate peroxidase (APX) and catalase. In contrast, lipoxygenase activity reduced with temperature decrease as well as increasing exogenous proline levels. Also, treatments of low temperature and proline showed that the levels of endogenous proline, catalase and APX in C. reticulata are higher compared with two other species and their amount increases with the reduction of temperature.