Effects Of Salt Stress On Cucumber Root Exudates And Soil Microorganisms

With the development of facilities horticulture, areas of protected vegetables were increased year by year. Cucumber (Cucumis sativus L.), one kind of main vegetables in protected horticulture, had a severe continuous cropping problem, which caused the obstacle of continuous cropping to be getting obviously day by day. And with the increase of cultivating years, root exudates of crops and pathogenic bacteria accumulated constantly, caused secondary salinization, nutrient disbalance, deterioration of soil micro ecological environment and other problems that affected the yield and quality of vegetables seriously.This experiment took two kinds of cucumbers with different saline tolerance as experimental materials, designing four NaCl concentration gradients (0,100,200,300 mmol·L-1), studied growth and physiological indexes of cucumbers with different saline tolerance in entire growth period, changes of soil physicochemical properties and microbial quantities, and types and contents of root exudates, and studied effects of salt stress on the diversity of the structure of soil microbial community using the technology of PCR-DGGE. Provide the theory bases of the improvement and regulation of soil secondary salinization and the sustainable development of protected agriculture. Reach the following conclusions by the experiment.1. Cucumber seedlings suffered different salt stress after field planting. Jinchun 5 showed higher saline tolerance than that of Jinyou 1. Plant heights, stem diameters, leaf numbers and numbers of pistillate flower of cucumbers were reduced with the increasing of salt stress. Low salt treatments could increase the leaf area, and high salt treatments could reduce the leaf area.2. With the migration of cucumber growth period, root activities and chlorophyll contents of cucumber leaves of all treatments increased first and then went down. Jinchun 5 (salt-tolerant) in all treatments was better than Jinyou 1(salt-sensitive).3. Cucumber yield per plant reduced with the increasing of salt concentration. Yield per plant of every treatment of salt-tolerant cucumbers were higher than those of salt–sensitive ones4. With the migration of cucumber growth period, soil EC value, quick results potassium contents almost displayed the downtrend. Effects of salt stress on pH value diminish gradually. Contents of soil organic matter of all treatments almost increased first and then went down. Contents of soil alkali-hydrolyzable nitrogen of salt-tolerant cultivar increased first and then went down. Contents of soil alkali-hydrolyzable nitrogen of control and low salt treatment of salt-sensitive cultivar increased first and then went down, and those of high salt treatments always increased. Contents of soil available phosphorus of salt-tolerant cultivar were lower than those of salt-sensitive cultivar under low salt-stress. Contents of soil available phosphorus of salt-tolerant cultivar were higher than those of salt-sensitive cultivar under high salt-stress.5. Degrees of salt stress and different growth periods of cucumber affected the soluble sugar content of cucumber root exudates. Contents of soluble sugar of salt-sensitive cultivar were relatively higher. 16 kinds of free amino acids were detected from root exudates. Praline was not found in any sample. Salt stress could increase species and contents of amino acids. 65 kinds of compounds were detected in cucumber root exudates using GC-MS. With the increase of salt concentration and the migration of cucumber growth period, types of detected compounds of every treatment almost displayed the downtrend.6. Low salt stress promoted the increase of quantities of soil bacteria and actinomyces. High salt stress reduced quantities of soil bacteria and actinomyces. Compared with two cultivars, quantities of soil bacteria of salt-sensitive ones were more under low salt stress, and it was opposite to high salt stress. Quantities of soil actinomyces of salt-sensitive cultivar were larger than those of salt-tolerant one at the end stage of cucumber growth. With the growth of cucumber, quantities of soil fungi increased. There were more fungi in the soil of salt-sensitive cultivar. Quantities of soil Fusarium oxysporum increased. Quantities of soil Fusarium oxysporum of salt-tolerant cultivar were lower than those of salt-sensitive one before the exuberant fruit stage. Quantities of soil Fusarium oxysporum of salt-tolerant cultivar were more than those of salt-sensitive one at the uprooting stage.7. Salt stress could reduce the richness of soil bacterial community. The richness of salt-sensitive cultivar was high under low salt treatment, and the richness of salt-tolerant cultivar was high under high salt treatment. It could also to be found out by analyzing the cluster analysis between lanes in DGGE electrophoretograms that high alt stress affected the soil bacterial community structure of salt-sensitive cultivar and reduced the richness of soil bacterial community.In summary, salt stress caused the soil fast-acting nutrient to reduce, changed soil micro ecology environment, made quantities of soil microorganisms lower, caused the cucumber growth to receive inhibition and the descend of physiological indexes. Salt stress made species and contents of amino acids in root exudates increase and those of compounds reduction. Jinchun 5 had stronger adaptability to the high salt stress, which could strengthen its saline tolerance by improving soil micro environment.