This study was undertaken to compare freezing tolerance of 10 commercial Iranian pistachio cultivars in January and March, and to investigate the relationship between freezing tolerance and changes in proline, phenolic compounds, soluble carbohydrates, total soluble proteins, malondialdehyde (MDA), hydrogen peroxide (H2O2), and water content of buds in both sampling stages. After subject to six different freeze-temperature tests (-9, -13, -17, -21, -25, and -29 °C), bud LT50 values were evaluated using the measurement of electrolyte leakage and bud browning. In January, ‘Chrokeh,’ ‘Shahpasand’ and ‘Rezaee’ showed the highest freezing tolerance, reached at 50% mortality at -15.20, -14.48 and -14.46 °C, respectively, whereas the least hardy cultivar, ‘Kaleh Ghouchi’ indicated 50% mortality at -11.43 °C. Freezing tolerance of all cultivars lessened slightly (1.24 °C on average) in March. In this period, ‘Sefid Pesteh,’ ‘Shahpasand,’ ‘Abbas Ali,’ ‘Kaleh Ghouchi,’ and ‘Fandoghi’ showed the least freezing tolerance, while ‘Rezaee,’ ‘Momtaz,’ and ‘Chrokeh’ were the highest hardy cultivars. There were significant differences in all the characteristics mentioned above among pistachio cultivars in both sampling dates. Negative correlations were observed between LT50 values with proline, phenolic compounds, soluble carbohydrates, and total soluble proteins. Also, there was a positive correlation between LT50 values with the concentration of MDA and H2O2 on both dates and with bud water content in March. These results indicated osmoregulants are valuable indices to screen freezing tolerance in pistachio cultivars.
