Evaluation of phenolic and flavonoid compounds in pollen grains of three Citrus species in response to low temperature

Low temperature during the reproductive stage causes sterility of pollen grains and reduces yield. Phenolic compounds function as stress indicators because they accumulate to high levels in many plant tissues in response to a wide range of biotic and abiotic signals. Branches with unopened flowers were collected from trees of three Citrus species (C. reticulata, C. sinensis and C. paradisi) and exposed to different temperatures (5, 10, 15, 20 and 25 °C) for six hours. The highest amount of polyphenol and flavonoids was observed at 5 °C and then their content reduced with increasing temperature in all species. Total phenols and flavonoids content in control temperature as well as all treatments showed that C. paradisi has lower amounts of these compounds than the two other species. HPLC analysis demonstrated that low temperatures, particularly 5 °C, induce flavonoids accumulation as both peak number and flavonoid levels for retention times in pollen grains of all species. The comparison of the HPLC patterns of the three species showed that flavonoid extracts in pollen of C. paradisi decrease in peak number and flavonoids levels for retention times in both control and treatments. Citrus reticulata showed an increase of eight- and two-fold in peaks of one and four, respectively, at 5 °C. Peaks of one and six in C. sinensis increased about four- and three-fold, respectively, at 5 °C compared to the control. In C. paradisi, the levels of peaks two to four were near to the baseline in control samples. With temperature reduction, peak two showed no significant increase but the levels of peaks one, three and four in samples exposed to 5 °C increased approximately three-, three- and four-fold, respectively. ?