Author
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Staley, Thomas |
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Submitted to: Soil Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/8/2003 Publication Date: 8/1/2003 Citation: STALEY, T.E. INITIAL WHITE CLOVER NODULATION UNDER SATURATION LEVELS OF RHIZOBIA RELATIVE TO LOW-LEVEL LIMING OF AN ACIDIC SOIL. SOIL SCIENCE. 2003. V. 168. P. 540-551. Interpretive Summary: Forage production is often limited by the lack of beneficial populations and activities of microorganisms in the rhizosphere. This is particularly true for soils that impose stresses on the establishment, growth and persistence of forage legumes, such as the acidic, infertile and shallow soils of the Appalachian Region. Utilizing white clover and rhizobia in nonsterile soil as a model system, the causes of the poor association of the symbiotic partners in an acidic soil and the effect of low-level liming were investigated. It was determined that impaired root metabolism was not the cause of poor nodulation under the more acidic conditions, but rather that symbiosis establishment, likely at the root hair level, was responsible. These results suggest that improving the establishment of forage legume symbioses in the soils of the Region will necessarily involve the selection of acid-tolerant legume cultivars. Technical Abstract: Improved symbiosis establishment between effective rhizobia and forage legumes is often important for maximizing sward yields, particularly when seeding is done on acidic, low-fertility soils. New information on the earliest events of this process could provide insights into the development of germplasm, both plant and bacterial, better adapted to such soils. Using a recently developed plate model system (Staley. 2002. Soil Sci. 167:211-221), consisting of a narrow range (pH 4.71-4.99) of limed soils planted with white clover (Trifolium repens L.) seedlings, we further evaluated nodulation assessment methodology (staining and nodule stage), endeavored to distinguish between soil pH-induced plant and rhizobia limitations on nodulation by employing saturation (daily) inoculation, and attempted to determine if previously observed nodulation reduction by low pH soil was a consequence of inhibition of symbiosis establishment or nodule maturation by utilizing a gusA-marked mutant of Rhizobium leguminosarum bv. trifolii. Over the course of all experiments in all soils, rhizobial MPN populations never declined below 1.3 x 104 CFU g-1 dry soil. Significant (P < 0.05) nodulation responses to soil pH increase were found for both the wildtype and mutant stains, whether assessed using unstained (fresh) or stained (methylene blue or GUS) roots. The pattern of gusA nodule ontogeny under saturation levels of rhizobia, relative to soil pH, suggests that inhibition of symbiosis establishment (cellular effects), rather than differences in nodule maturation (tissue effects), is the explanation for our observed soil pH effects on nodulation. Re-investigation of root responses revealed small, but significant (P < 0.05), differences due to soil pH, primarily for lateral roots. Taken together, these results corroborate our previous, wildtype rhizobia findings of a positive effect of soil low-level liming (only 0.25 pH increase) on nodulation and extend them to earlier times (4-6 DAP) in symbiosis establishment, but refute our previous finding of no root growth and developmental effects. More importantly, they strongly suggest that nodulation reduction in the lower pH soils of our model system was due to disruption of rhizobia function (but not growth and viability) or root hair growth and/or function. |
