The effects of salicylic acid (SA) (0.01, 0.1 and 1 mM) and cold on freezing tolerance (freezing injury and ice nucleation activity) were investigated in winter wheat (Triticum aestivum cv. Dogu-88) grown under control (20/18 degreesC for 15, 30 and 45-day) and cold (15/10 degreesC for 15-day, 10/5 degreesC for 30-day and 5/3 degreesC for 45-day) conditions. Cold acclimatisation caused a decrease of injury to leaf segments removed from the plants and subjected to freezing conditions. Exogenous SA also decreased freezing injury in the leaves grown under cold (15/10 degreesC) and control (15 and 30-day) conditions. Cold conditions (10/5 and 5/3 degreesC) caused an increase in ice nucleation activity by apoplastic proteins, which were isolated from the leaves. For the first time, it was shown that exogenous SA caused an increase in ice nucleation activity under cold (15/10 and 10/5 degreesC) and control conditions. These results show that salicylic acid can increase freezing tolerance in winter wheat leaves by affecting apoplastic proteins.