Effects Of Exogenous Proline On Physiology Metabolism Of Cucumber Seedlings Under High Temperature Stress

Cucumber is a worldwide important vegetable crop, which occupies an important position in the facility cultivation, but the high temperature in summer and autumn is one of the main factors to limit the facilities for year-round cultivation of cucumber. In most parts of our facilities, midday temperatures above40℃often seriously affected the yield and quality of cucumber. Proline is a small molecular osmotic substance and the largest water-soluble amino acid, and many plants are able to accumulate a high level of proline under stress conditions. The synthesis, accumulation and metabolism of proline in plants is a physiological and biochemical process that subject to the regulation of abiotic stress and cellular proline concentration, therefore, proline accumulation may be a signal when plants under stress. But so far, the mechanism of proline in the physiological processes of high temperature stress is not known much about yet, especially seldom reported in the study on horticultural crops.In this experiment, cucumber (Cucumis sativus L.) cultivar ’Xintai Mici’(sensitive to high temperature) was chosen as experimental material. By using artificial intelligence climate chamber cultivation and foliar spraying proline, we studied the relationship between exogenous proline and plants growth, antioxidant system, photosynthetic fluorescence characteristics and nitrogen metabolism of cucumber seedlings under high temperature stress. And also, the physiological regulatory function of exogenous proline to high temperature in cucumber seedling plants was discussed. The main results are as follows:1. Under high temperature stress, as compared with the water treatment, exogenous proline promoted the growth of cucumber seedlings in varying degrees, especially the proline concentration of3mmol-L-1effected most obviously, making shoot dry weight, root dry weight and whole plant dry weight increase18.8%,11.8%and17.5%respectively.2. Under high temperature stress, the superoxide anion (O2-) production rate, malondialdehyde (MDA) content and electrolyte leakage (EL) in cucumber seedling leaves were increased significantly, and the cell membrane was subjected to the preoxidation injury. Proline pretreatment improved scavenging capacity of reactive oxygen species pairs (ROS) by increasing superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity, thus effectively easing membrane lipid peroxidation injury induced by the high temperature stress.3. In high temperature stress4h,8h, as compared with the water treatment, glutathione reductase (GR) activity, dehydroascorbate reductase (DHAR) activity, GSH/GSSG ratio and the reduced ascorbic acid (AsA), glutathione (GSH) contents were significantly increased in proline pretreatment, indicating that exogenous proline can increase scavenging reactive oxygen species ability on cucumber seedlings ascorbate-glutathione cycle to alleviate the damage on antioxidant system of cucumber under high temperature stress.4. Compared with the water treatment, the contents of chlorophyll a, chlorophyll b and total chlorophyll were increased in proline pretreatment, especially in high temperature stress12h the difference most significantly. When the high temperature stress in8h, net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE) and the maximal photochemical efficiency (Fv/Fm), actual photochemical efficiency (ΦPS Ⅱ), effective photochemical efficiency (Fv’/Fm’), photochemical quenching coefficient (qP), electron transport rate (ETR) of PS Ⅱ were significantly increased, while the transpiration rate (Tr) and non-photochemical quenching coefficient (NPQ) lower.5. Under high temperature stress, as compared with the water treatment, dry and fresh weight of cucumber plants, N03--N content and nitrate reductase (NR) activity were significantly increased in proline pretreatment, while NH4+-N concentration decreased, indicating that exogenous proline can significantly reduce the impact of high temperature stress on nitrogen nutrition metabolism of cucumber seedlings.6. Under high temperature stress, proline pretreatment improved free proline, soluble sugar and soluble protein contents and enhanced the osmotic adjustment ability in order to maintain the body’s water balance of the plants. The results suggested that exogenous proline improving plant osmotic adjustment ability by increasing osmotic substance content in cucumber seedling leaves in order to alleviate high temperature stress on cucumber leaves membrane lipid peroxidation injury.