Differential Accumulation of Carbohydrates in Alfalfa Cultivars of Contrasting Winterhardiness

Abstract

Soluble sugar composition and starch reserves are significantly altered during cold hardening of alfalfa (Medicago sativa L.). We characterized 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 six cultivars in 1992–1993). During hardening under environmentally controlled conditions, freezing tolerance and crown levels of soluble sugars increased significantly with a concomitant decrease in starch. Differences in freezing tolerance between a cold‐tolerant and a cold‐sensitive cultivar were closely associated to the accumulation of the oligosaccharides raffinose and stachyose but were not related to the levels of sucrose. Incubation of prehardened plants at subzero temperatures (−2°C) increased freezing tolerance and promoted higher accumulations of sucrose, raffinose, and stachyose and further reduction in starch levels. Under natural hardening conditions, sucrose concentration in crowns was poorly related to the differences in midwinter levels of freezing tolerance between cultivars. Stachyose and raffinose accumulated later in fall than sucrose and reached higher levels in winterhardy than in nonhardy cultivars. Our results show that the accumulation 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 stachyose and raffinose than to accumulate sucrose.