HEAT AND CHILL ING/FREEZING STRESS Assessment of Cold and Heat Tolerance of Winter-grown Canola (Brassica napus L.) Cultivars by Pollen-based Parameters

Introduction and cultivation of new crops in a given environment require management practices and trait selection that enable the crop species to perform to its potential. Canola is an important oilseed crop (Downey 1990, Zhang et al. 2003) and its cultivation is expanding, particularly in the western world (FAO 2006), because of its importance as both an oilseed and a bio-diesel crop. Winter-grown canola has attracted attention from both producers and researchers since its introduction in the 1980s (Rife and Zeinali 2003). Canola is cultivated both during winter and spring seasons in United States that expose the crop to winter kill and frost and high temperatures, respectively, during the reproductive period. The temperatures during winter and spring are known to influence all the crucial steps of the reproductive cycle including gametogenesis, pollination, fertilization and embryogenesis (Lardon and Triboi-Blondel 1994, Angadi et al. 2000). Temperature stress events, which are being experienced now, are expected to intensify because of an increase in emission of greenhouse gases and associated changes in climate. Climate models project an increase in the surface temperature of the Earth by 1 to 11 C by 2100 (Houghton et al. 2001, Stainforth et al. 2005). Additionally, short episodes of extreme climatic events including low and high temperatures are predicted to occur more frequently in the near future (Meehl and Tebaldi 2004). Studies have shown that these projected changes in climate drastically