Changes in starch granules in the storage tissues of the underground organs of Medicago archiducis-nicolai

Medicago archiducis-nicolai is distributed in the high, cold area of the eastern edge of the Qinghai-Xizang Plateau, and has strong cold resistance. In order to explore the cold resistance mechanism of its underground organs, the conventional paraffin sectioning method was used to observe how starch granules changed in the root, crown, and rhizome storage tissues during natural wintering. We found that starch was mainly stored in the parenchyma cells of the pith and phelloderm of underground organs during the wintering process, and a small concentration existed in the xylem parenchymal cells. In contrast, the distribution of starch granules in the crown and roots was denser. The trend of changing starch quantities in different underground organs before and after wintering was basically the same: there was an overall pattern of first decreasing and then increasing. Starch accumulated in the underground organs in large quantities during the early stage of wintering, gradually disappeared in the middle stage of wintering, and accumulated in the later stage. Among the underground organs, the crown responded the fastest to temperature changes, whereas the rhizome was the most sensitive to low temperatures. This study found that the rhizome of M. archiducis-nicolai had the functions of expansion, reproduction, storage, and cold resistance. The unique underground organ of this plant differs from all other Medicago species. The asynchronicity of the formation of cold resistance in the underground organs of M. archiducis-nicolai during the wintering period is its adaptive strategy to cope with the cold alpine environment.