Hypoxia tolerance and adaptation of anaerobic respiration to hypoxia stress in two Malus species

Seedlings of two Malus species (M. hupehensis and M. toringoides) were hydroponically grown in normoxic and hypoxic nutrient solutions to determine their hypoxia tolerance and adaptation of root anaerobic respiration to hypoxia stress. The growth of both species was inhibited under hypoxia stress. During hypoxia stress, the generation of superoxide anion radical (O2−) and content of hydrogen peroxide (H2O2) in roots were significantly increased in both species with similar trends, which led to an increase of malondialdehyde (MDA) content and relative membrane permeability (RMP). The degree of growth inhibition and the levels of O2−, H2O2, MDA and RMP were greater in M. toringoides than in M. hupehensis. Under hypoxia stress, pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH) activities in roots were increased during the first 12 days of hypoxia stress and then gradually decreased in both species. The contents of acetaldehyde, alcohol and lactate in roots were also increased and showed similar trends as the activities of anaerobic respiration enzymes. The increases in PDC and ADH activities and lactate content in M. hupehensis under hypoxia conditions were greater than those of M. toringoides, but alcohol and acetaldehyde contents showed opposite trends. These data suggest that M. hupehensis is more tolerant of hypoxia and had less damage from oxidative stress than M. toringoides under hypoxia stress. The capability for anaerobic respiration is up-regulated in roots of Malus in response to hypoxia stress to minimize damage, and the higher hypoxia tolerance of M. hupehensis may be partly due to the higher enzyme activity of PDC, ADH and LDH and lower accumulation of acetaldehyde and alcohol.