Solute accumulation in heat seedlings during cold acclimation: contribution to increased freezing tolerance

Accumulation of sugars, amino acids and glycinebetaine in leaf tissues during cold acclimation was simultaneously monitored and compared in three wheat cultivars that have different freezing tolerance. Freezing tolerance was the decreasing order of cv. Norstar (NO) > cv. Chihokukomugi (CH) > or = cv. Haruyutaka (HA). During cold acclimation, there was a significant increase in osmotic concentration in the three cultivars. The increase was largely due to the increase in soluble sugars and the extent of the increase was the greatest in NO and the least in HA. While there was a considerable increase in glucose, fructose and sucrose during the first week of cold acclimation, the increase in raffinose occurred only after the second week. The total sugar content was the order of NO > CH > HA after 4 weeks of cold acclimation. Proline increased in all cultivars after 1 week of cold acclimation but a prolonged cold acclimation resulted in different profiles: no further increase occurred in HA while an additional increase occurred in other two cultivars. In all three cultivars, a noticeable increase of glycinebetaine occurred only after the second week of cold acclimation with the amount being the order of NO > CH > HA. It is concluded that a substantial part of the increase in osmotic concentration during cold acclimation was due to the increase in sugars, but the extent of the contribution of each compatible solute is cultivar-specific and can be associated with the degree of the maximum freezing tolerance attainable.