A Simulation Model of Cold Hardiness and Freezing Injury in Alfalfa as a Function of Cultivar Type

Cold hardiness and freezing injury. Even though the mechanisms of dormancy and winter hardiness are not fully understood, studies have shown a strong correlation of cultivar dormancy characteristics to cold hardiness and winter survival (Cunningham et al., 1995; Jung et al., 1967; McCaslin, 1994; McKenzie et al., 1988; Schwab, 1993; Smith et al., 1986). These studies concluded that cultivar types of all dormancy ratings tolerated lower freezing temperatures as cold hardiness increased. However, cold-sensitive cultivars suffered higher rate of plant death at similar freezing temperatures compared to the winter-hardy cultivars. Based on these data, accumulation of cold hardiness was developed as a function of cultivar type which in the model is characterized by fall dormancy rating (Fig. 1). Fall dormancy ratings (FDR) are supplied as user input. Cultivar ratings for dormancy are routinely published by the seed companies or are available from cultivar evaluation trials. Besides cultivar type, the rate of accumulation or break-up of cold hardiness is further modified by temperature and snow cover. Carbohydrate reserves affect cold hardiness accumulation only when the reserves in the root and crown fall below 10%. The process of winter acclimation resulting in cold tolerance is modeled with a simulated cultivar hardiness index. Cold tolerance to freezing injury increases as the hardiness index increases.