ENZYMATIC CONTROL OF SOLUBLE CARBOHYDRATE ACCUMULATION IN COLD-ACCLIMATED CROWNS OF ALFALFA

Sucrose and the raffinose family of oligosaccharides (RFO), stachyose and raffinose, accumulate in crowns of alfalfa (Medicago sativa L.) du ring Low temperature acclimation. In vitro and in vivo evidence from t he Literature suggests that these sugars might have important adaptive roles in freezing tolerance. To better understand the regulation of c old-induced accumulation of soluble sugars, we measured the activity o f key regulatory enzymes involved in the metabolism of sucrose and RFO in crowns of three alfalfa cultivars of contrasting winterhardiness w hich had been acclimated to winter conditions. Both sucrose-phosphate synthase (SPS) and galactinol synthase (GS) showed markedly higher act ivity at low temperature, There was a close relationship between the a ctivities of SPS and GS and the levels of sucrose and RFO, respectivel y. A delay of approximately 2 wk was observed between the rise in GS a ctivity and RFO accumulation. For both SPS and GS, the increase in act ivity occurred earlier and reached higher levels in two winter hardy c ultivars than in a nonhardy cultivar, Zn contrast, the activity of the RFO degrading enzyme alpha-galactosidase (alpha-gal) was identical in all cultivars. The activity of sucrose synthase (SS), mainly involved in sucrose hydrolysis, decreased during fall acclimation. In cold-acc limated alfalfa, both acid and neutral invertase activities were low a nd comparable in all cultivars. Our results indicate that differential accumulation of soluble sugars between alfalfa cultivars of contrasti ng winterhardiness is the result of differences in synthetic capacity rather than differences in hydrolytic activities.