Non-structural carbohydrates contributed to cold tolerance and regeneration of Medicago sativa L

Soil water content only affected regeneration time, whereas the NSC content was related to the success of alfalfa regeneration. Non-structural carbohydrates (NSCs) are important factors influencing the overwintering and regeneration of alfalfa. In this study, we analyzed eight in-situ samplings at three depths of coarse roots (crown, 20 and 40 cm depths) during the overwintering period and assessed the dynamic change and allocation of root NSCs under three irrigation frequencies (irrigation once every second day/4 days/8 days). Primary results showed that: (i) before cold acclimation, irrigation once every second day was beneficial to the accumulation of soluble sugars and starch in crown tissues, which would be maintained until the following spring and accelerate the regeneration time of alfalfa; (ii) during the overwintering process, the soluble sugars and starch contents in the crown were significantly higher than those in deeper roots, and there was an asynchronous effect caused by the change in soluble sugars and starch among roots at three depths; and (iii) the change trend of soluble sugar and starch contents was consistent with that of semi-lethal temperature, and there was a significant negative correlation between the content of soluble sugar (R² = 0.8046) and starch (R² = 0.6332) and the semi-lethal temperature. This study demonstrated that NSCs are the key driver of cold tolerance and regeneration under the three irrigation frequencies evaluated. Our results provide further insight into the allocation of NSCs in winter. This improved understanding of the mechanism of overwintering will allow for improved water management of alfalfa in high latitude areas.