| The phenomenon of soil secondary salinization happens frequently in the greenhouse vegetable industry in China, which has become the primary obstacle of sustainable utilization of greenhouse soil in lots of areas.The change of soil environment could affect the organization, quantity and distribution of soil microorganisms.While,the varieties, quantity and its change of soil microorganisms,to some extent,reflect the speed of mineralization of soil organic matter and the status of soil fertilizer supplying.Therefore, in recent years more and more scholars believe that sufficient adjustment of soil microorganisms and the improvement of soil secondary salinization in production are the key points to overcome continuous cropping obstacles.This research took cucumber which has the problem of serious continuous cropping obstacle as the raw material, taking salt-tolerant species Jinchun 5 and salt-sensitive species Jinyou 1 as the test metirals and NaCl as salt stress conditions, imitating the concentration of salt stress for 0 mmol·L-1, 100 mmol·L-1 and 200 mmol·L-1, the concentration of root exudates for 0 mL·strain -1,30 mL·strain -1 and 20 mL·strain -1, using PCR-DGGE to study Salt-stressed affction on rhizosphere community structure of soil microbial diversity from the different concentrations of cucumber root exudates. At the same time ,measuring cucumber seedling plant height, stem diameter, root activity, chlorophyll content, soil microbial rhizosphere quantity, soil nutrient content, soil enzyme activity and yield of one plant by conventional methods. we established the theoretical foundation for relieving salt stress and a comprehensive understanding of the relationship among the salt stress in continuous cropping obstacle, root exudates and the soil environment. Through experiments conclusions as follows:1. Salt-stressed reduced the cucumber plant height, stem diameter growth significantly .The plant height, stem diameter growth had been promoted in low concentrations of cucumber root exudates ,while in high concentration they had been inhibited.2. Salt-stressed reduced the chlorophyll content of cucumber seedlings, under the condition of NaCl to 100 mmol·L-1, the low concentration of root exudates has a significant role in promoting the leaf chlorophyll content, with the plants to the forth-leaf period such promoting role is not significant, under the conditions of NaCl to 200 mmol·L-1, root exudates played a significant " inhibit in low concentration and promote in high concentrations" role in the leaf chlorophyll content. During the second-leaf period ,root exudates showed a significant " inhibit in low concentration and promote in high concentrations"effect on cucumber root activity , this role was reduced with the plant growing.3 Salt-stressed reduced the quantity of soil bacteria and soil fungi, in the second-leaf period and third-leaf period, root exudates of cucumber increased the quantity of soil bacteria in low concentration and decreased in high concentration , from the forth-leaf period, the root exudates inhibited the increasing of soil bacteria; root exudates increased the quantity of soil fungi in low concentration and decreased in high concentration;in the forth-leaf period, the root exudates played a significant promoting role to soil fungi of the salt-sensitive species Jinyou 1, but played a significant inhabiting to the salt-tolerant species Jinchun 5; under the condition of NaCl to 100 mmol·L-1 , root exudates inhibited the quantity of the soil Fusarium oxysporum, under the conditions of NaCl to 200 mmol·L-1, root exudates played a promoting role on soil Fusarium oxysporum.4 Salt-stressed reduced the soil urease, invertase, catalase activities, low concentrations of root exudates promoted the three soil enzymes'activities, and the high concentration of root exudates on the three soil enzymes, there is inhibition of enzyme activities. Salt-stressed improve soil polyphenol oxidase activity. Urease activity of Variety Jinchun 5 and the Jinyou 1 are approaching and the invertase, catalase activities of Jinchun 5 are more than Jinyou 1 but Polyphenol oxidase activity is less than Jinyou 1, the Jinchun 5 variety showed some salt-tolerant characteristics.5 Salt-stressed reduced the content of the soil available nitrogen, phosphorus, organic, low concentrations of root exudates on soil available nitrogen and phosphorus content played a promoting role but high concentrations of root exudates showed inhabitation especial for the content of available phosphorus, and was not significant to the content of organic, salt-stressed increased the content of Available K.. With the increasing of salinity, soil PH and EC values increased, root exudates decreased the EC values. The content of soil nitrogen, available potassium of salt-tolerant variety-Jinchun 5 are higher than salt-sensitive Jinyou 1. organic matter content of salt-tolerant varieties Jinchun 5 of pre-plant growth was low untill the forth-leaf period, then organic matter content of salt-tolerant varieties are higher than the salt-sensitive varieties.6 The combination of traditional dilution-plate method and molecule methods, the results of its study is in substantial agreement, salt-stressed reduce the number of bacterial groups, low concentrations of root exudates stimulated the soil bacteria activities and would have some new breeding group of bacteria.From the study, we know that salt-stressed forced the soil enzyme activities and soil available nutrients to reduced, which reduced microbial populations and abundance, deteriorated the soil micro-environment, inhibited the growth of cucumber, after exerted a certain extent low concentration of root exudates, which would improve the soil micro-environment and relieved salt-Stress condition; but after imposed high concentration of root exudates in soil micro-environment could deteriorated the role of the salt-stressed. Salt-tolerant varieties by improving the soil micro-environment to enhance the salt-tolerant characteristics of its own.
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