Investigation On Damage Physiology Of Muskmelon Seedling Upon Root-Zone Hypoxia Stress

Hypoxia stress in root-zone is one kind of antibiotic stress, which influences the normal growth and development of muskmelon seedling. Now hypoxia stress has become a bottle neck of soilless culture application to large area production. Although soilless culture of muskmelon is potentially profitable, the problems of O2 deficiency and O2 consumed rapidly limited its large area application. Two muskmelon cultivars differing in hypoxia resistance were chosen as experimental material namely,'Dongfang Xingguang' and 'Xiyu No. 1' . In this thesis, the effects of hypoxia stress on muskmelon seedling growth, photosynthesis, respiration metabolism, ROS metabolism were studied, the changes of respiration metabolism pathways in muskmelon under hypoxia stress in root-zone, and the physiological fundamentals of muskmelon injury caused by hypoxia stress in root-zone were preliminarily elucidated. The main results are as follows:Compared to the control, hypoxia stress caused the inhibition of muskmelon seedling growth. These suggested that the length of root, the height, dry weight, fresh weight of muskmelon seedlings all decreased and the degree of growth inhibition caused by hypoxia stress in 'Xiyu No.l' was obviously higher than that in'Dongfang Xingguang' . Moreover, hypoxia stress significantly increased root activity in a great degree in 'XiyuNo.l' while in 'Dongfang Xingguang' that was another case. Under hypoxia stress, the plasma membrane leakage increased, especially 'Xiyu No.l' which indicate that hypoxia stress causes root injury of'Xiyu No. 1' in a great degree.Under hypoxia stress Pn, Gs, Ci, Tr of muskmelon seedlings all decreased. In'Dongfang Xingguang' which had a strong hypoxia resistance, Pn and Gs could renew the CK level in the anaphase of treatment while 'Xiyu No.l' could not. Hypoxia stress also caused the decrease of Fv/Fm, but in the anaphase of treatement Fv/Fm could renew CK level. Moreover, hypoxia stress obviously caused the decrease of chlorophyll in ' Xiyu No. 1' but in ' Dongfang Xingguang' that was not obvious. Under hypoxia stress, the aerobic respiration of root in two cultivars both decreasd. The decrease range was ' Dongfang Xingguang' >' Xiyu No. 1'. And PDC activity significantly increased and was higher than CK during treatments, but there was no significant difference between the two cultivars. The activity of ADH andLDH both significantly increased, however, ADH activity in ' Dongfang Xingguang' was higher than that in 'Xiyu No.l' ,contrary to that of LDH. It indicated that'Dongfang Xingguang' could keep a relative high level of aerobic respiration and the pathway of anaerobic respiration mainly was alcoholic fermentation while in'Xiyu No.l' mainly lactic acid fermentation. These might explain hypoxia resistance. In 'Xiyu No.l' lactic acid fermentation strengthened and aldehyde accumulated which caused cell injury under hypoxia stress.Under hypoxia stress, starch content decreased in seedling root whereas it accumulated in stem and leaves, especially in leaves of 'Dongfang Xingguang' . Hypoxia stress caused the increase of soluble sugar, sucrose and fructose , especially in strong hypoxia stress cultivars, while the glucose content decreased in both cultivars.Hypoxia stress caused the increase of the antioxidant enzyme (SOD, POD, CAT) activity which can scavenge ROS with the accumulation of H2O2, MDA and the higher productive rate of OJin leaves and roots. Furthermore, POD activity was higher in root than in leaves while that of CAT was activity was different. Providing enough oxygen after hypoxia stress, CAT activity significantly decreased accompied by decreases of SOD and POD activity and a little increase of H2O2 , especially productive rate of O₂^- increased in the root and leaves of ' Dongfang Xingguang' . It indicated that providing enough oxygen as soon as hypoxia stress resulted in injury by the decrease of antioxidant activity and the accumulation of ROS.