Major antioxidants were studied in three Citrus (C. reticulata, C. sinensis, and C. paradisi) under lowtemperature 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 lowtemperature 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.

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In this study, various fruit compounds including phenolic acid, flavonoid, anthocyanin, tannin, moisture content, carotenoid, soluble, and reducing sugar date varieties were investigated. The results showed that there were differences among varieties of date palm in terms of synthesis of chemical compounds. The maximum and minimum SOD, APX, and CAT activity were detected in the Klote and Khasab variety, respectively. The highest amount of phenolic compounds, flavonoids, and anthocyanins were found in a variety of Klote and the lowest amount were observed in the Khenizi variety. The analysis of tannin's content among different date varieties showed that the highest amount of tannin in the Khenizi variety and the lowest amount in the variety of Klote were observed. There is a positive correlation between phenolic compounds and antioxidant capacity in different date palm varieties. The comparison of antioxidant's capacity of different varieties showed that Klote had the highest amount, while Khenizi had the lowest amount of antioxidant. Also, the highest amount of soluble sugar in the Klote and the lowest amount in Khasab were observed. The amount of reducing sugar in the variety of Klote and Gorbani were higher than other varieties and lower level have been seen in Khenizi. The amount of carotenoids was the highest in the Klote variety and the lowest in the Khenazi and Halili varieties. The highest amount of moisture was in the Mazafati and the lowest amount viewed in the Khenizi.

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