DIFFERENTIAL ACCUMULATION OF CARBOHYDRATES IN ALFALFA CULTIVARS OF CONTRASTING WINTERHARDINESS

Soluble sugar composition and starch reserves are significantly altere d during cold hardening of alfalfa (Medicago saliva L.). we characteri zed the relationship between carbohydrate composition of the crown and freezing tolerance in cultivars of contrasting winterhardiness during their acclimation to low temperature under controlled (two cultivars) and natural hardening conditions (three cultivars in 1991-1992 and si r cultivars in 1992-1993). During hardening under environmentally cont rolled conditions, freezing tolerance and crown levels of soluble suga rs increased significantly with a concomitant decrease in starch. Diff erences in freezing tolerance between a cold-tolerant and a cold-sensi tive cultivar were closely associated to the accumulation of the oligo saccharides raffinose and stachyose but were not related to the levels of sucrose. Incubation of prehardened plants at subzero temperatures (-2 degrees C) increased freezing tolerance and promoted higher accumu lations of sucrose, raffinose, and stachyose and further reduction in starch levels. tinder natural hardening conditions, sucrose concentrat ion in crowns was poorly related to the differences in midwinter level s of freezing tolerance between cultivars. Stachyose and raffinose acc umulated later in fall than sucrose and reached higher levels in winte rhardy than in nonhardy cultivars. Our results show that the accumulat ion of sucrose, stachyose, and raffinose and the decrease in glucose, fructose, and starch levels are temporally related to the development of freezing tolerance in alfalfa. However, differences in the maximum level of freezing tolerance between nonhardy and winterhardy cultivars are better related to the capacity of the plants to accumulate stachy ose and raffinose than to accumulate sucrose.