Abstract:

This research studied the transformation and transportation of non-structural carbohydrates in the roots and leaves of Medicago sativa and their relationship with cold resistance during cold hardening in two alfalfa varieties： Zhaodong， a strongly cold resistant variety， and was Sardi 10， a weakly cold resistant variety. Experimental plants were subjected to a cooling treatment to simulate early cold-hardening （CH₁）， a low-temperature treatment to simulate advanced cold-hardening （CH₂）， or normal growth conditions （CK） over seven days following cutting. Changes in soluble sugar and starch contents in roots and leaves and the difference between the two varieties were analyzed in regrowth after the seven days to assess cold resistance. It was found that： After cold hardening， the semi-lethal low temperature （LT₅₀） of alfalfa roots and leaves decreased with increase in their soluble sugar and starch contents， and there were significant negative correlations between the LT₅₀ and the soluble sugar and starch contents of roots and leaves. After 14 days of regeneration under normal growth conditions， the soluble sugar and starch contents in the roots and leaves decreased. However， soluble sugar content in roots and leaves of alfalfa increased， starch content in leaves decreased， and starch content in roots changed little after CH₁ treatment. The soluble sugar content in the roots and leaves of Sardi 10 was significantly higher than that in Zhaodong， while the content of starch in the leaves of Sardi 10 was significantly lower than Zhaodong. After CH₂ treatment， the soluble sugar content in Zhaodong roots increased and starch content decreased. The changes in soluble sugar and starch contents in Sardi 10 roots were opposite to those in Zhaodong， and soluble sugar content in Sardi 10 roots was lower than Zhaodong. Cold hardening changed the aboveground-belowground direction transfer of soluble sugar. Starch in the roots of normally growing plants was converted into soluble sugar， which was transported to the leaves and used together with the soluble sugar transformed from the starch of leaves to regenerate aboveground organs. In CH₁， the starch in the leaves of the two varieties was degraded into soluble sugar， which was transported to the roots， resulting in an increased soluble sugar content in the roots and leaves. At this stage， more leaf starch was consumed in Sardi 10 than Zhaodong. In CH₂， Sardi 10 and Zhaodong had reverse starch-sugar transformation directions in the roots， with the transformation from starch to soluble sugar in Zhaodong roots and from soluble sugar to starch in Sardi 10 roots. These results indicate that alfalfa consumes less non-structural carbohydrates during early cold hardening， but transforms starch into soluble sugar in the root in the later stages， resulting in increased root cold resistance. This data provides important insight into the reasons for differences in cold resistance among alfalfa varieties.

Key words: alfalfa, cold hardening, soluble sugar, starch, transformation