Effect Of Root-zone Hypoxia Stress On Physiological Metabolism In Muskmelon Seedlings And Ca～(2+),GABA Physiological Regulation Function

Hypoxia stress in the root-zone is thought be a major determinant in the adverse effect of O₂ deficiency in hydroponic culture and root-cushion formation in substrate culture on plant species. Now hypoxia stress has become a limited factor of soilless culture application to large area production. Therefore, improving the hypoxia tolerance of crops plays a vital role in soilless culture. Ca²+ is the necessary major element of plant growth and a second messenger of coupling extracellular signal and intracellular physiological reaction. It has been known that [Ca²+]_cyt increase rapidly under environmental stress and Ca ²+.CaM compound formation after Ca ²+ combined with target enzyme CaM, which played an important role in stabling the cell wall, cell membrane and membrane protein and relieving the harm of stress to the crops. In plant, nitrogen metabolism can be divided in to inorganic nitrogen metabolism (including N0₃ --N, NH₄⁺-N, NR,) and organic nitrogen metabolism (including amino acid, protein, PAs). Environment stress change the ROS metabolism which cause overproduction of ROS and plants have developed sophisticated mechanism of scavenging ROS. But little is known about the effect of hypoxia stress and exogenous compound such as Ca²+, Ca²+ and CaM inhibitor, GABA on nitrogen and ROS metabolism and whether Ca²+, CaM, GABA are involved in the regulation hypoxia tolerance of crops.Different cultivars were sorted according to the different hypoxia tolerance from 23 muskmelon cultivars. And hypoxia-tolerance muskmelon cultivars 'Dongfang gangqiang' and 'Dongfang xingguang' and hypoxia-sensitivie cultivars 'Xiyu No.l' were choose as experimental materials to investigate the effect of hypoxia stress on growth, nitrogen metabolism and ROS metabolism in muskmelon seedlings. Under hypoxia stress, combined with treatment of exogenous application Ca²+, exogenous foliar spraying La ³+, EGTA, TFP, W7 and GABA to investigated effect of Ca²+, CaM, GABA on intrinsic hypoxia resistance of muskmelon seedlings and elucidate the relationship between Ca²+ , CaM, GABA enhance hypoxia resistance and the nitrogen and ROS metabolism.The result showed that NO₃^--N and NH₄^--N, Pro, PAs contents and NR activities in roots and leaves all increased under root-zone hypoxia stress, but the soluble protein contents decreased, the heat-stable protein contents in roots increased while heat-stable protein in leaves decreased. The increase degrees of NO₃^--N and NH₄⁺-N, NR, Pro, and PAs were root>leaves. And the nitrogenous substance contents of 'Dongfang gangqiang' were more than those of 'Xiyu No.1'. It indicated that the nitrogen metabolism pathway changed under hypoxia stress, NO₃^--N rapidly reduced to NH₄⁺-N and heat-stable protein, Pro, PAs synthesis rapidly played an important role in enhancing hypoxia resistance in muskmelon seedlings.Root-zone hypoxia stress caused the increase antioxidant enzyme (SOD, POD, CAT) activities which could scavenge the ROS, while H₂O₂, MDA contents and O₂^- productive rate also obviously increased. The speed of ROS accumulation was more rapid than that of ROS scavenge and ROS scavenge pathway is different between leaves and roots. The activities of antioxidant enzyme in 'Dongfang xingguang ' were higher than those in 'Xiyu No.1' while the H₂O₂, MDA contents and O₂^- productive rate were lower. It suggested that accumulation of ROS and peroxidation of membrane lipid aggravated under hypoxia stress and ROS metabolism existed difference of different site of muskmelon seedlings and different cultivars.Exogenous application Ca²+ in nutrient solution could relieve the harm caused by hypoxia stress and result in the increase of NO₃^--N and NH₄⁺-N, amine acid, heat-stable protein, ion bond conjugated protein and covalent bond conjugated protein in cell wall, PAs (free, conjugated, bound) and antioxidant enzyme activities as well as the decreased of PAO activities in cell cytoplasm and cell wall and ROS contents in roots and leaves of muskmelon seedlings as compared with the hypoxia stre...