Plasticity of Soybean Root System Development Under Water Deficits.

Under drought conditions, plant root systems may exhibit developmental plasticity resulting in morphological changes that extend the water-absorbing surface area, including increases in rooting depth and proliferation of lateral roots. The objective of this study is to identify soybean genotypes which exhibit genetic diversity in root system developmental plasticity in response to water deficits, in order to enable physiological and genetic analyses of the regulatory mechanisms involved. The long-term goal is to incorporate desirable rooting traits into the breeding program to facilitate development of cultivars that are more drought tolerant. Because of the difficulties in studying root system development, a key objective is to establish the validity of using a seedling system to select for rooting characteristics which favor performance under drought. Accordingly, studies are being conducted at the seedling stage under precisely-controlled water deficits, during three-week soil drying treatments of plants growing in 1.5 m-deep soil columns in controlled-environment chambers, and in the field. Using the seedling system, 36 Plant Introduction and adapted soybean lines were studied. The results showed substantial genetic diversity in the capacity for increased lateral root development (number and total length of roots produced) under water deficit conditions. Studies of the spatial and temporal patterns of lateral root development showed that the promotion of lateral root length under water deficits was due to increased root production rate than earlier root initiation or promotion of root elongation. Genotypes with either superior or inferior root plasticity responses at the seedling stage were chosen for more detailed studies using the soil column system. The results showed consistent genotypic differences in lateral root developmental plasticity under water deficit conditions between seedlings and more mature plants. Field studies of selected genotypes are in progress. Supported by the Missouri Soybean Merchandising Council.