Interaction of polyamine and proline on the activity of enzymatic and non-enzymatic compounds in the peel of three Citrus species under low temperature stress

Plants activate antioxidant defense mechanisms under stress, which help maintain the structural integrity of cell components and possibly reduce oxidative damage. Low- temperature stress led to the production of reactive oxygen species and oxidative damage to plants. In this study, the effect of putrescine and proline on reducing the production of reactive oxygen species and increasing the activity of antioxidant enzymes in the peel of three Citrus species were investigated. The results showed that with decreasing temperature, the production of reactive oxygen species and activity of antioxidants increased in three Citrus species. In both C. reticulata and C. sinensis species, the activities of antioxidant enzymes were higher compared with C. paradisi. However, the production of reactive oxygen species in C. paradisi was higher than the other two species. Treatment of fruits with proline and putrescine led to a decrease in the production of reactive oxygen species. The highest amount of glutathione peroxidase and ascorbate peroxidase was observed at -3°C temperature and in fruits treated with proline 20 Mm. Exogenous application proline and putrescine increased the levels of endogenous proline in Citrus species. The levels of endogenous proline under both low temperature and exogenous proline and putrescine were higher in C. reticulata compared with two other species. Overall, the treatment of putrescine and proline has led to the improvement of defense activities in stressed plants and has significantly increased in the C. reticulata species.