The objectives of this study were to compare freezing tolerance (FT) of 12 field-grown grapevine cultivars at four stages: Nov., Jan., Mar. and Apr., and to investigate the relationship between FT and soluble carbohydrates, proline and water content changes of buds during acclimation and deacclimation stages. After exposure to various freeze test temperatures bud LT50 values were estimated using electrolyte leakage and bud browning measurement. Bud FT of all cultivars studied increased with the overall trend of declining temperatures from Nov. through Jan., after which the buds began to deacclimate from Mar. and gradually lost FT in Apr.; overall LT50 means were -13.79, -21.74, -20.38 and -10.06, respectively. However, cultivars did not show alike acclimation and deacclimation rates and, for example, 'Yaquti', an early ripening cultivar, showed relatively higher FT in early autumn, but was one of the least hardy cultivars in later stages. In Jan. (full hardening), cultivars were classified as hardy (‘Red Sultana’, ‘Fakhri’, ‘Shahani’ and White Sultana), moderately hardy (‘Tabarzeh’, ‘Gaznei’, ‘Thompson Seedless’, Laal, ‘Sahebi’ and ‘Rishbaba’) and least hardy (‘Yaquti’ and ‘Ruby Seedless’). The pattern of soluble carbohydrates and proline changes were highly consistent with the LT50 profile. Bud water content, especially in early autumn, had a stronger correlation with FT, compared to the concentration of cytoplasmic osmoregulants. Likewise, soluble carbohydrates proved to be superior indicator of FT, as compared to proline. Our results showed osmoregulants as well as bud water content are useful indices in order to screen FT in a range of grapevine germplasm resources.
