| Cucumber (Cucumis sativus L. ) is an important vegetable for greenhouseproduction in China. Oxygen deficiency in the root-zone and root-cushion formationhave become a limiting factor in soilless culture, which influences the growth anddevelopment of cucumber, therefore it is essential to develop some simple,economical and practical methods to alleviate injurous effects of hypoxia stress,which would be helpful in improving soilless culture of cucumber.Calcium is an essential element in plant growth and a second messenger ofcoupling extracellular signal with intracellular physiological reaction. It has beenknown that [Ca2+]cyt increases rapidly under environmental stress, and formsCa2+.CaM compound, which plays an important role in stabilizing cell wall, cellmembrane and membrane protein; Ca2+ activates ATPase and facilitates transport ofassimilates, regulates stoma movement and oxygen-evolving activity, therefore, Ca2+can alleviate damage caused by various environmental stresses. Plants can producesome physiological metabolism changes in response to the hypoxia stress, the level ofreactive oxygen species, antioxidative enzyme activities, the content of polyaminesare under dynamic change. All these are involved in the regulation of plant adaption toenvironmental stress. However, there has been no systematic work on the effect ofexogenous Ca2+ on reactive oxygen species and polyamines metabolism in cucumberseedlings under hypoxia stress.Using two cucumber cultivars with different resistance to hypoxia stress,'Zhongnong No. 8' (a hypoxia-sensitive cultivar) and 'Lvbachun No.4' (ahypoxia-resistant) as materials and precise control of dissolved oxygen in nutrientsolution through dissolved oxygen controller, we selected suitable Ca2+ concentrationsfor foliar spray or by adding to the nutrient solution, we investigated the effects ofexogenous calcium on reactive oxygen species and polyamines metabolism, effects of calmodulin antagonist W7 on reactive oxygen species, polyamines metabolism andphotosynthesis, and explored physiological function of calcium in cucumber seedlingsunder hypoxia stress. Main research findings were as follow:1. Foliar spraying 10 mmol·L-1 CaCI2 or maintaining 8mmol·L-1 Ca2+concentration in the nutrient solution increased the fresh and dry weight of cucumberseedlings, improved the growth of shoot and root, alleviated the growth inhibition ofcucumber seedlings under hypoxia stress. Exogenous Ca2+ increased Pn, Gs, Ci, Tr ofleaves. But foliar spray of higher concentration CaCI2 (more than 20 mmol·L-1)aggravated leaf damage and growth inhibition. Higher concentrations Ca2+(more than16 mmol·Ll) in the nutritient solution also resulted in growth inhibition. These resultsindicated that appropriate concentration of Ca2+ could alleviate the damagements ofhypoxia stress to cucumber.2. Under hypoxia stress, the contents of free, conjugated, bound polyamine inleaves and roots of the two cucumber cultivars increased to different degrees. Afterexogenous Ca2+ treatment, the contents of Put, Spm, Spd were higher than thosewithout Ca2+ treatment, PAO activity was lower than those without Ca2+ treatment,but the activities of polyamine synthetases such as ADC, ODC, SAMDC were higherthan those of treatments without Ca2+. These observation indicated that exogenousCa2+ adjusted polyamines synthesis and degradation, promoted the increase ofpolyamines content, calcium is involved in polyamines metabolism in cucumberseedlings under hypoxia stress.3. Under hypoxia stress, the activities of antioxidative enzymes in roots andleaves of cucumber seedlings increased significantly, the contents of ROS and MDAalso obviously increased, the increased range of antioxidative enzymes activities werehigher in 'Zhongnong No. 8' than those of 'Lvbachun No.4', but the increased rangeof the contents of ROS and MDA range were much lower in 'Zhongnong No. 8' thanthose of 'Lvbachun No.4'. After the exogenous Ca2+ treatment, the MDA, H3O2contents and O2.- production rate decreased in cucumber seedlings, the activities ofSOD, POD, CAT, APX, and GR increased, the level of membrane lipid peroxidationdecreased, indicating that exogenous Ca2+ treatment alleviated injury due to hypoxiastress, and increased the adaptability of cucumber seedlings to hypoxia stress.4. Compared with the hypoxia treatment, the growth of cucumber leaves androots of pretreated cucumber seedlings with calmodulin antagonist W7 weresignificantly inhibited. The activities of SOD, POD, CAT, APX, GR, DHAR, and MDAR decreased and the contents of AsA, DAsA, GSH, GSSG decreased, but O2.-production rate, the contents of H2O2 and MDA in leaves and roots of cucumberseedling were higher than those without W7 treatment. Hypoxia stress andpretreatment with W7 decreased the contents of free, conjugated, bound polyaminesand Pn, Gs, Ci, and Tr, leading to the decrease of the adaptability of seedlings tohypoxia stress. These results indicated that Ca2+ modified the adaptability of seedlingsto hypoxia stress by the formation of Ca2+. CaM, and therefore regulating polyaminesmetabolism, reactive oxygen species metabolism and photosynthesis of cucumberseedlings.5. Under hypoxia stress, there were significant difference in all observationdetermined between 'Zhongnong No.8' and 'Lvbachun No.4' in all treatmentsbecause of the their different genetic backgrounds of the two cucumber cultivars.Hypoxia-resistant cultivar 'Lvbachun No.4' exhibited better adaptability thanhypoxia-sensitive cultivar 'Zhongnong No.8'. in growth, antioxidative enzymes,polyamines metabolism and photosynthesis. Physiological metabolism paths in rootsand leaves of the same cultivar in adaptation to hypoxia stress were different.In addition, Foliar spraying of CaCl2 or addition of CaCl2 to nutrient solutioncould alleviate the harm of hypoxia stress to cucumber. Two experiments usingexogenous Ca2+ (positive aspect) and calmodulin antagonist W7 (negative aspect) incucumber seedlings under hypoxia stress were carried out. Results indicated that Ca2+alleviated the injury due to hypoxia stress to the cucumber seedlings. by adjusting theantioxidant systems and polyamines metabolism.
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