Winter freezing injury is one of the most important factors limiting grapevine production. The main purpose of this study was to investigate the effects of foliar potassium sulphate (K2SO4) fertilisation on the cold-hardiness of ‘Bidaneh Sefid’ grapevine (Vitis viniferaL.). Four concentrations of potassium sulphate [0.0, 0.5, 1.0, or 2.0% (w/v)] were sprayed to run-off on all leaves of each plant on five occasions at 2-week intervals starting on 11 June 2011. Cane samples were collected on 25 December 2011 and 23 February 2012 and analysed for their carbohydrate, proline, and protein concentrations, then exposed to 3 h freezing treatment between 0ºC and –30ºC, at intervals of 3ºC, to assess their level of cold-hardiness. The results showed that increased K2SO4 concentrations resulted in increased carbohydrate, proline, and protein concentrations. The latter increases were greater in cane samples collected in December 2011 than in February 2012, for which the levels of cold-hardiness were also lower. LT50 values (i.e., the lethal temperature at which 50% of tissues died) were estimated by the tetrazolium stain test and by electrolyte leakage measurements. The results indicated that K2SO4 sprays significantly increased cold-hardiness. Significant positive correlations were found between the results of both the tetrazolium stain tests and electrolyte leakage measurements and carbohydrate, proline, and protein concentrations in the cane samples. Compared to the canes, the buds were more vulnerable to cold-injury in December and in February. We conclude that the application of five K2SO4 sprays during the growing season has the capacity to increase the freezing tolerance of grapevine (‘Bidaneh Sefid’) and could be used as a prophylactic tool to reduce freezing injury during the Winter.
