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Title: DETERMINING RATE OF CHANGE IN CUCUMBER RHIZOSPHERE MICROBIAL COMMUNITY COMPOSITION IN RESPONSE TO SOIL PH, SALINITY, AND BORON

Author
item Ibekwe, Abasiofiok - Mark
item Grieve, Catherine
item Poss, James
item GRATTAN, STEPHEN - U.C. DAVIS, CA
item Suarez, Donald

Submitted to: Proceedings of the International Salinity Forum
Publication Type: Proceedings
Publication Acceptance Date: 4/11/2005
Publication Date: 4/25/2005
Citation: Ibekwe, A.M., Grieve, C.M., Poss, J.A., Grattan, S., Suarez, D.L. 2005. Determining rate of change in cucumber rhizosphere microbial community composition in response to soil pH, salinity, and boron. In: Proceedings of the International Salinity Forum, Managing Saline Soils and Water: Science, Technology, and Soil Issues. April 25-27, 2005. Riverside, CA pp:77-80.

Interpretive Summary: The rhizosphere (i. e. the volume of soil adjacent to and influenced by plant roots) supports a diverse microbial community. The structure and richness of the community depend on several factors including plant species, plant developmental stage, mineral nutrient uptake by plants, excretion of specific organic compounds by plants, microbial competition for nutrients, solid attachment sites, and environmental conditions. The purpose of this study was to determine the interactive effects of salinity (3 and 8 dS/m), boron (0.7, 5 and 8 mg/l), and pH (6.5 and 8) on distribution, diversity, and composition of the microbial communities associated with cucumber plants grown in sand cultures. The effects of treatment on rhizosphere microbial development dynamics were characterized by extracting community DNA from plant roots, then amplifying and separating gene fragments by PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) analysis. Results of the study showed that microbial diversity increased with plant age. Microbial diversity was more severely affected by pH than any other experimental variable, especially during the early stages of plant growth. This finding suggests that, in the sand tank system, pH must be properly controlled for several weeks after planting in order to avoid adverse effects on nutrient availability to plants.

Technical Abstract: The salinity of the soil plays a prominent role in the microbial selection process as environmental stress can reduce bacterial diversity. In the San Joaquin Valley (SJV) of California, the problem of increasing salinity and consequently, decreasing crop productivity, are major concerns. An experiment was conducted in a closed, recirculating volumetric lysimeter system (VLS) consisting of 24 experimental plant growth units at determine the interactive effects of salinity, boron and pH on the rhizosphere microbial composition of cucumber (Cucumis sativus L. cv. Seminis Turbo hybrid). Plants in the VLS were irrigated from individual reservoirs containing a modified half-strength Hoagland’s nutrient solution combined with various salinity, boron, and pH treatments. . Community DNA was extracted from plant samples and a 236-bp DNA fragment in the V3 region of the small subunit ribosomal RNA genes of eubacteria was amplified and the 16S rRNA gene was separated in a DGGE gel. The results indicated that salinity and pH were the most influential factors determining the diversity of bacteria in the rhizophere of plants. The study showed that the impact of low pH on microbial community was more severe during the first week than week seven of the study. Therefore, soil pH must be properly managed during the early stages of plant development as this may have severe impact on nutrient availability to plants.