A genotypic comparison of plasticity of root system development during soil drying in soybean (Glycine max (L.) merrill)
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Water deficit is responsible for significant losses in soybean (Glycine max (L.) Merrill) yield under dryland conditions. Under drought, increases in root depth and density, i.e., developmental plasticity, enable plants to sustain high rates of water extraction and help to maintain yield (O'Toole and Bland, 1987; Sponchiado et al., 1989; White and Castillo, 1989). The objective of this project was to screen and identify soybean lines exhibiting diversity in root system characteristics that are associated with drought tolerance, in order to enable physiological and genetic analyses of the regulatory mechanisms involved. A genetically diverse collection of soybean genotypes was selected for screening. To evaluate root plasticity in response to soil drying, an experimental system that allowed spatial and temporal monitoring of root proliferation was developed. Plants were grown in soil columns 1.5 meters in depth, and subjected to soil drying for three weeks. The vertical distribution of root development was monitored photographically at weekly intervals, and plants were harvested at the end of the experiments for taproot length and shoot biomass. In nine genotypes tested under greenhouse conditions, final taproot length ranged from 98% to 150% of well-watered controls. Genetic variation in the response of rooting density to drought was also demonstrated. However, comparison between genotypes was complicated by variation in leaf area development and, as a result, in rates of soil drying and plant stress development, as well as by seasonal variation in the greenhouse environmental conditions. Therefore, growth chamber studies which allowed equivalent rates of soil drying between two selected genotypes were conducted. These studies demonstrated that under equivalent rates of soil drying, insignificant differences in the response of rooting density to drought between the two genotypes were observed.
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