Mitochondrial structure and respiratory metabolism in cold resistance of alfalfa seedling root

Roots, the main organ of alfalfa (Medicago sativa L.) for overwintering, are critical for the cold resistance of alfalfa, while the effect of root respiration characteristics on its cold resistance is still unclear. The effect of low temperature on the mitochondrial ultrastructure and respiratory metabolism of alfalfa roots with different fall dormancy rating and different cold resistance was examined. Low temperature induced mitochondrial swelling, directly inhibiting the activities of mETC complex II, III and IV, decreasing ATP synthesis and aggravating H2O2 accumulation, which inhibited the root growth. Compared to Gannong No. 5, the damage of low temperature to mitochondrial structure and mETC complex II and III activities was lower, meanwhile, the ATP lack and H2O2 accumulation were lighter in Xinmu No. 4 roots. The alternative pathway (AP) respiratory rate was restricted, but the AP capacity, the percentage of AP to the total respiration, was increased to bypass excess electrons and reduce the production of H2O2, which was higher in Xinmu No. 4 roots. Thus, the low sensitivity of the mitochondrial structural stability and mETC complexes II and III to low temperature and the increase of AP capacity contributed to the root cold resistance of alfalfa with low fall dormancy rating.