Histochemical characteristics and differentiation of the belowground buds of Medicago archiducis-nicolai during overwintering

To determine the mechanisms underlying belowground bud overwintering in Medicago archiducis-nicolai, we examined the histochemical characteristics and differentiation of buds during the period of overwintering, based on the histochemical staining of paraffin sections. Observations revealed that the starch content in overwintering buds is stored primarily in the cortex, pith, the peripheral young leaves and basal tissue of bud tips, and in the basal part of the leaf primordium, whereas protein is mainly stored in the growth cone of bud tips, apical leaf primordia, and cambium of the bud body. At the cold-adaptation stage in late autumn, the bud body was observed to undergo rapid elongation and starch in the buds began to undergo degradation, although total starch contents remained at a high level during this stage. At the beginning of winter, there was a retardation in both bud differentiation and elongation, concomitant with a reduction in temperatures. At the freezing-stress stage in winter, there was a cessation of growth in both the bud cone and bud body, and at this stage, the starch content of buds had been almost completely degraded. Thereafter, in response to gradually increasing temperatures, the bud body slowly began to resume growth, although meristematic growth of the bud cone did not recover fully until the growing stage in spring. In conclusion, our findings indicated that in belowground buds of M. archiducis-nicolai, the transition to a dormant state is characterized by growth stagnation and a reduction in the starch contents of buds, and that dormant buds can withstand cold stress during the period of overwintering on the Qinghai-Tibet Plateau. This cold tolerance can be attributed to the degradation of starch in buds, which, by increasing the concentration of soluble sugars in cells, contributes to reducing their freezing point.