Studies On Effects Of Exogenous Spermidine On Photosynthesis And Carbohydrate Metabolism In Cucumber Seedlings Under Hypoxia Stress

Hypoxia stress in the root-zone is thought to be a major determinant in the adverse effect of O₂ deficiency in hydroponics and root-cushion formation in substrate culture on plant species. Now hypoxia stress has become a limited factor of soilless culture to large scale application. Therefore, improving the hypoxia tolerance of crops plays a vital role in soilless culture. Polyamines, mainly including putrescine (Put), spermidine (Spd), spermine (Spm), are ubiquitous low-molecular-mass aliphatic amines generated in organism metabolism. Many evidences have showed that polyamines are associated with plant growth, differentiation, senescence and responses to stress. Under many kinds of stress conditions, polyamine the content of in plants change rapidly. However, little is known about their physiological relevancy in plant subjected to hypoxia stress.The hypoxia-tolerant cucumber cultivar 'L(u|¨)bachun No.4' and hypoxia-sensitive cultivar 'Zhongnong No.8' were chosen as experimental materials to investigate the effects of exogenous foliar by spraying Spd on growth, photosynthesis and carbohydrate metabolism in hypoxia-stressed cucumber seedlings cultivated in nutrient solution whose dissolved oxygen concentration was precisely controlled by dissolved oxygen controller. The physiological role of polyamines in alleviating damages caused by hypoxia stress was discussed. Main research results were as follows:Compared with the control, hypoxia stress inhibited growth, and reduced significantly fresh weight, dry weight, shoot height, stem diameter, leaf area, root length, root area, root volume and total root tips amount of cucumber seedling. The exogenous Spd treatment decreased the difference in fresh weight, dry weight, shoot height, stem diameter, leave area, root length, root area, root volume and total root tips amount relative to the control. For the hypoxia-sensitive cultivar 'Zhongnong No.8', exogenous Spd was more effective than that for hypoxia-tolerant cultivar 'L(u|¨)bachun No.4'. Hypoxia stress caused a significant decrease in net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) in cucumber seedling leaves compared with the controls, and intracellular CO₂ concentration (Ci) was exhibited higher significantly than the control during the later period of hypoxia stress, which suggested that the decrease in Pn in cucumber seedlings leaves mainly resulted from the stomatal limitation during the earlier period of stress, however, non-stomatal limitation during the later period. Compared with the hypoxia treatment, exogenous Spd increased Pn, Gs and Tr in hypoxia-stressed cucumber seedling leaves, and decreased Ci during the later period under hypoxia stress. These results indicated that exogenous Spd could improve Pn of cucumber seedlings by preventing stoma closure and stimulating CO₂ uptake.In every treatment, both Pn and Gs rose in the morning and fell in the afternoon, and Pn reached to the apex at noon. Cucumber seedlings treated with hypoxia stress only exhibited the lowest Pn maximum of all the treatments. The Gs maximum appeared at different time due to treatment. Under hypoxia stress, the decrease of Pn exhibited higher in the morning significantly for both cultivars. Compared with the hypoxia treatment, exogenous Spd increased Pn in two cultivars cucumber seedlings, and raised WUE at noon (12:00 and 14:00) for both cultivars.Under hypoxia treatment, the contents of starch and glucose decreased in cucumber roots while increased in leaves, and the contents of sucrose and fructose increased in leaves and roots. The activities of SS and SPS increased in leaves and roots, and the activity of AI increased in leaves while decreased in roots under hypoxia stress. Compared with the hypoxia treatment, exogenous foliar spraying Spd increased the contents of sucrose, glucose and fructose in leaves and roots, and decreased the content of starch in leaves while increased the content of starch in roots. In addition, exogenous Spd increased the activities of SS and SPS in leaves and roots, decreased the activity of AI in leaves while increased that in roots under hypoxia stress. It indicated that regulation of carbohydrate metabolism with exogenous Spd was an important mechanism of alleviating the harm of hypoxia stress to cucumber seedlings.