Identification Of Rootzone Hypoxia Tolerance In Apple Rootstocks And Study On Their Mechanism

Molecular oxygen is the terminal electron acceptor in the mitochondrial electron transport chain and is required by several enzymes, higher plants need oxygen for growth and metabolism. But frequently they experience oxygen deficiency in the root-zone mainly due to soil waterlogging,soil compaction and submergence. Apple is one of the most important fruit crops in the world; hypoxia stress in the root-zone is thought to be one of those important factors which restrict the development of apple. 12 apple rootstocks were treated with hydroponics hypoxia and waterlogging to study their root-zone hypoxia tolerance, hypoxia-tolerance apple rootstock Malus hupehensis Rehd and hypoxia-sensitive apple rootstock M.toringoides Hughes were selected as experimental materials to study physiological responses difference, and M.robusta Rehd was used to induce hypoxia-tolerance under hypoxia stress. The physiology and biochemical mechanism of hypoxia stress on apple rootstocks and salicylic acid (SA) was also discussed under hypoxia stress, all above provided the theoretic base and technical guideline for apple tree cultivation and soil management. The results as follows:1. Growth of all apple rootstocks was inhibited to various degrees under root-zone hypoxia stress. After 15 days hypoxia treatment,the number of new leaves, root length, plant height, plant biomass and content of soluble protein in roots under root-zone hypoxia stress obviously declined comparing with control, fresh weight decreased 9%～41% and dry weight decreased 14%～40%. The sensitivities of 12 apple rootstocks to hypoxia stress were obviously different,The content of soluble protein in roots under root-zone hypoxia stress obviously declined comparing with control, while proline(Pro) increased and variation percentage of the was significantly different.2. Based on subordinate function, adversity resistance coefficient, waterlogging index and cluster analysis of 9 physiological and 6 growing indexes, the hypoxia-tolerance of 12 apple rootstocks were comprehensively evaluated and revealed M.hupehensis Rehd, M. robusta Rehd and M. sieboldii Rehd held higher tolerance to root-zone hypoxia stress, while M. sieverii Roem and M.toringoides Hughes were sensitive to root-zone hypoxia stress.3. The reactive oxygen species (ROS) in roots of apple rootstock under hypoxia stress accumulated, the generation rate of superoxide radical (O2 ) and the content of hydrogen peroxide (H2O2) increased, the membrane lipid peroxidation was more serious, which leaded to more content of malonaldehyde (MDA), so the membrane was damaged. The oxidative damages to membrane in roots of M. toringoides Rehd were more serious than M. hupenensis Rehd.4. The anti-oxidative system could reduce the damages to membrane in roots of apple rootstock under hypoxia stress at some extent. The activities of peroxidse (POD), superoxide dismutase (SOD), catalase (CAT) increased during the stress; the activities of ascorbate peroxidase (APX) and glutathione reductase (GR) increased in early period of stress, but decreased in later period until less than control, while the anti-oxidative enzyme activities of M. hupehensis Rehd increased rapidly. The contents of antioxidants such as ascorbic acid (AsA) and glutathione (GSH), the ratio of AsA and oxidative ascorbic acid (AsA/DHA), the ratio of GSH and oxidative glutathione (GSH/GSSG) also increased in early period, but gradually decreased later and were less than control in media and latter period of stress. The results suggested the effects of hypoxia stress on the plant were different from genotype; the damages to M.toringoides Hughes were more serious than M.hupehensis Rehd.and M. hupehensis Rehd could enhance their tolerant ability through increasing the activities of anti-oxidation enzymes.5. Exogenous 0.5 mmol?L-1 SA application could relieve the harm caused by hypoxia stress and result in improved growth of M. robusta Rehd, increased fresh and dry weight of M. robusta Rehd The results suggested treatment with 0.5 mmol?L-1SA mitigated the growth inhibitions in M. robusta Rehd.under root-zone hypoxia stress.6. Under root-zone hypoxia stress, the contents of MDA, O2 , H2O2 and GSH, and activities of antioxidant enzymes were higher than the control, however, the level of AsA were decreased. Exogenous SA significantly decreased the content of O2 , MDA and GSH, increased the activities of SOD, POD, APX and GR under hypoxia stress. It was proposed that exogenous SA as a chemical activator could induce resistance, reduce production of reactive oxygen species, raise the activities of anti-oxidative enzymes, and decrease the membrane lipid peroxidation; therefore it alleviated the damage degree of hypoxia stress of M. robusta Rehd.