Effects Of Exogenous24-Epibrassinolide On Reactive Oxygen Species And Carbon Metabolism In Cucumber Seedlings Under Hypoxia Stress

Plant hypoxia frequently occurs when internal oxygen levels fall below a critical value. Spring floods, winter ice encasement, excess rainfall, irrigation followed by poor drainage, soil compaction and bad management in soilless cultivation are common conditions that lead to root-zone hypoxia or anoxia. These deleterious effects are responsible for the slower growth and reduced yield observed in many agricultural crops after flooding and other hypoxia-induced conditions. Brassinosteroids (BRs) are a class of plant polyhydroxysteroids similar to animal and insect steroid hormones that have been recognized as a new kind of phytohormones and play an essential role in plant development. Intensive research conducted on BRs reveals that they elicit a broad spectrum of physiological responses in plants, including germination and breaking of seed dormancy, organ differentiation, vascular tissue development, flowering and senescence, and tropisms. Besides that, BRs can protect plants from various biotic and abiotic stresses. However, the relationship between brassinolide and hypoxia tolerance of vegetables and the physiological regulation function of brassinolide is still not well known.In the present study, two cultivars of cucumber (Cucumis sativus L.), cvs. Lubachun No.4and Zhongnong No.8, were used. It has been reported that cv. Lubachun No.4is hypoxia-resistant as in most cucumber cultivars, whereas cv. Zhongnong No.8is comparatively sensitive to root-zone hypoxia. Cucumber seedlings were hydroponically grown for8days in normoxic and hypoxic nutrient solutions with and without addition of24-epibrassinolide (EBR) at1μg.L-1The objectives of the present study were to investigate the effects of root-zone hypoxia with dissolved oxygen (DO) at1mg.L-1on plant growth, carbon and ROS metabolism, to elucidate the relationship between the changes of carbon and ROS metabolism in cucumber plants and hypoxia tolerance, and the physiology regulation function of exogenous EBR in improving hypoxia tolerance of cucumber plants. Main research results were as follows:1. Root-zone hypoxia caused severe inhibition of shoots and roots growth of cucumber (expressed by shoot fresh weight, shoot dry weight, total root length, total root surface, total root volume and total root tip numbers). The inhibition was more marked in cv. Zhongnong No.8than in cv. Lubachun No.4.2. Root-zone hypoxia significantly inhibited TCA cycle but accelerated anaerobic respiration of cucumber roots. Under root-zone hypoxia stress, compared with Zhongnong No.8seedling roots, the extent of decrease in SDH, IDH activities and increase in LDH activity, lactate and pyruvate contents were less in Lubachun No.4seedling roots, conversely, the extent of increase in PDC and ADH activities and alcohol, alanine contents in Lubachun No.4seedling roots was greater than that in Zhongnong No.8; all these response of respiratory metabolism to root-zone hypoxia were the reasons for two cucumber cultivars of different hypoxia tolerance; there was no difference about AlaAT activity and acetaldehyde content between two cultivars.3. Two genotypes cucumber accumulated different types of carbohydrates under hypoxia and showed significantly diverse changes on contents of starch and glucose in roots; stable starch contents and increased glucose contents were observed in Lubachun No.4roots; on the contrary, these two types of sugars were significantly reduced in Zhongnong No.8roots. Furthermore, changes of glycolytic enzymes activities showed obviously different between two genotypes cucumber in response to root-zone hypoxia; AI activity slightly decreased during hypoxia, and PFK activities were higher than control level throughout the entire experiment, but they increased during the first4days of hypoxic treatment, and then declined in subsequent days in Lubachun No.4roots; on the contrary, AI activity rapidly reduced during hypoxia, and slightly decreased PFK activities were observed in Zhongnong No.8roots. These results suggest that changes of contents of starch and glucose and activities of AI and PFK are some of primary characteristic for cultivar difference.4. Under hypoxia stress, the contents of ROS, MDA and antioxidants, and the activities of SOD, CAT, APX, and DHAR were enhanced in cucumber seedlings roots of both two cultivars. The extent of increase in ROS and MDA contents in Lubachun No.4roots was less than those in Zhongnong No.8roots, however, the extent of increase in activities of SOD, CAT, APX, DHAR and AsA and GSH contents in Lubachun No.4roots was greater than those in Zhongnong No.8roots.5. Application of EBR to hypoxic nutrient solution alleviated the inhibition of plant growth. For example, EBR treatment increased the total root length by23%and37%, and the total root area by26%and29%, respectively, in cv. Lubachun No.4and cv. Zhongnong No.8seedling roots; the EBR application significantly increased the number of the total root tips of cv. Zhongnong No.8seedling, but had no effect on cv. Lubachun No.4seedling roots. Greater promotion by EBR treatment was observed in cv. Zhongnong No.8seedling roots than in cv. Lubachun No.4.6. EBR takes part in the regulation of respiratory metabolism of cucumber plants under hypoxia. EBR treatment significantly enhanced activities of SDH, IDH and ADH, decreased activities of LDH and AlaAT, contents of lactate in roots of both two cultivars and PDC activity in cv. Zhongnong No.8seedling roots. However, EBR treatment had no effect on PDC activity in cv. Lubachun No.4roots and contents of acetaldehyde, alcohol and alcohol in roots of two cultivars. These results suggest that EBR added to hypoxic nutrient solution may partly resume aerobic respiration, stimulate the shift from lactate fermentation to alcohol fermentation in hypoxic roots, and eventually enhanced tolerance of cucumber plants to root-zone hypoxia.7. EBR takes part in the regulation of carbohydrate metabolism of cucumber plants under hypoxia. EBR added to hypoxic nutrient solution caused an increase in the concentrations of fructose and sucrose and activities of major glycolytic enzymes in the roots but not in the leaves. These results suggest that EBR added to hypoxic nutrient solution may stimulate the photosynthate allocation down to roots and activities of major glycolytic enzymes, and eventually enhanced tolerance of cucumber plants to root-zone hypoxia.8. Under hypoxia, EBR takes part in the regulation of ROS metabolism of cucumber plants. Application of EBR to hypoxic nutrient solution significantly increased activities of SOD, APX, GR, and contents of AsA and GSH, however, the chemical rapidly reduced the accumulation of O2·-,H2O2and MDA. From the results obtained in this work, it can be concluded that oxidative damage on cucumber plants caused by hypoxia stress can be considerably alleviated by adding EBR to nutrient solution and the tolerance of plants was elevated.