| Cucumber (Cucumis sativus L.) which originated from the warm and humid tropical rainforests in the Himalaya Range and the south of the north India likes warm but could not resist high temperature.The continuous high temperature in summer leads to high temperature in horticultural facilities. Usually pad and fan cooling system is used for greenhouse cooling. However the cooling effect is not significant and the greenhouse is still maintain a high temperature.The aerial part of the plants could alleviate the medium-high temperature stress by respiration, but the underground root system are forced to tolerate. Therefore, high rhizosphere temperature will seriously affect the root function. The cucumber cultivar’Chunqiuwang 2’was used as test material. Firstly, the effects of different concentration of salicylic acid (SA), Proline and 6-Benzylaminopurine were investigated on seedling leaves to find out the optimal concentration of exogenous substances at high rhizosphere temperature on physiological characteristics of the cucumber in hydroponic system. Secondly, the effects of three kinds of the optimal concentration of exogenous substances on the cucumber seedling roots physiological characteristics were investigated to screen out optimal exogenous substances at high rhizosphere temperature with hydroponic cultivation. Finally, the effects of optimal exogenous on nitrogen metabolism of cucumber seedlings leaves or roots under high rhizosphere temperature stress were investigated. The major results were as follows:1. Different concentration of salicylic acid solution can significantly reduce superoxide anion (O2·-) production rate, malondialdehyde (MDA) content and electrolyte leakage (EL); significantly increased the superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) activity and soluble protein and proline (Pro) content except the 0.20 mmol/L treatment which will reduce the proline (Pro) content. Effects of exogenous salicylic acid ease seedling rhizosphere temperature stress has dose effect,0.1 mmol/L SA is the best concentration to alleviate the damage caused by membrane lipid peroxidation of high rhizosphere temperature.2.3mmol/LPro treatment could reduce the production rate of superoxide anion (O2·-) especially in the 6th and 8th day the treatment group were significantly lower than others. At the same time the treatment increased the peroxidase (POD), catalase (CAT) activity and proline (Pro) content, while the treatment groups were significantly higher than others in the 6th and 8th day. The results showed that,3mmol/L Pro treatment could reduce the level of ROS in plant, improve the regulating permeability ability of the plant in order to ease the stress of high temperature membrane lipid peroxidation injury of cucumber leaves by increasing antioxidant enzyme activity and osmotic adjustment substances in cucumber seedling leaves under the high temperature stress.3.5μmol/L 6-BA treatment to alleviate the damage of high rhizosphere temperature effect is the best which significantly decreased superoxide anion generation rate (O2·-), malondialdehyde (MDA) content, and increased content of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity, free proline and soluble protein of cucumber seedlings.4. Superoxide anion production rate (O2·-), malondialdehyde (MDA) content and electrolyte leakage (EL) significantly increased in roots of cucumber seedlings under high rhizosphere temperature stress. The cell membrane was damaged by the peroxidation obviously.0.1mmol/L SA enhanced the reactive oxygen species (ROS) scavenging ability by increasing superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity which effectively alleviated the membrane lipid peroxidation injury induced by root temperature stress; at the same time, SA treatment increased the contents of free proline and soluble protein which enhance the plant osmotic adjustment ability, so as to maintain water balance in the plant.5.0.1 mmol/L salicylic acid (SA) can significantly increase the activity of nitrate reductase (NR), glutamine synthetase (GS), glutamate dehydrogenase (GDH), glutamate synthase (GOGAT) and nitrate nitrogen in root and leaf, but decrease the ammonium nitrogen content significantly. The results show that:salicylic acid pretreatment can prevent excessive accumulation of ammonium nitrogen, enhance the nitrogen metabolism ability of cucumber seedlings under high temperature stress, relieve the damage of high rhizosphere temperature stress on cucumber seedling and enhance the ability of cucumber seedling to resist high temperature stress in.Above all, the best was observed in the treatment of 0.1 mmol/L SA among three kinds of exogenous substances to alleviate high rhizosphere temperature.0.1 mmol/L SA significantly decreased ROS levels, ammonium content and significantly increased the activity of antioxidant enzymes, key enzyme activities of nitrogen metabolism, the content of osmotic adjustment substances and nitrate were increased.0.1 mmol/L SA could alleviate high rhizosphere temperature stress and improve stress resistance effectively. The above results can provide some theoretical reference to improve root temperature for cucumber cultivation in summer. |
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