Plasticity of Soybean Root System Development Under Water Deficits.

Under drought conditions, plant root systems may exhibit developmental plasticity including increases in rooting depth and proliferation of lateral roots. We have utilized a seedling system with controlled water deficit imposition to select for rooting characteristics favorable to performance under drought, and have identified substantial genetic diversity in the capacity for increased lateral root development in response to mild water deficits. To determine whether genotypic differences are consistent between seedlings and more mature plants, selected genotypes with contrasting responses in the seedling system are being evaluated during four-week soil drying treatments of plants growing in 1.5 m-deep soil columns in controlled-environment chambers. Ideally, comparison of responses between genotypes requires comparable rates of soil drying and plant water stress development. To achieve this objective, pairs of lines with similar leaf area development (and, therefore, whole plant transpiration rates) are being studied using a whole tube weighing system to monitor soil water depletion. Visible root length distribution is monitored photographically on the face of the tubes at intervals during the experiment, and the total root length distribution is measured by harvesting the root system at the end of the experiment. Studies of one genotype pair have been completed: Magellan (superior seedling response) and Pana (inferior seedling response). Under well-watered conditions, the two lines exhibited similar root development profiles, whereas in the soil drying treatment, Magellan exhibited much greater root length at depth compared to Pana, consistent with expectations from the seedling system results. Studies of additional genotype pairs are ongoing. Supported by the Missouri Soybean Merchandising Council.