Drought Tolerance Mechanisms in Cultivated and Wild Soybean Species

SEVERSIKE, THOMAS MATTHEW. Drought Tolerance Mechanisms in Cultivated and Wild Soybean Species. (Under the direction of Dr. Thomas Rufty). Soybean [Glycine max (L.) Merr. ] is an important source of protein and oil in the current world market. Soybean yields can drastically decrease when droughts occur, thus subjecting farmers to significant financial risk. Considering that rainfall and other climate factors are projected to change in the future, drought tolerant soybean varieties are needed to help mitigate this risk. Soybean breeders are making progress with drought tolerant varieties by using exotic soybean germplasm to increase genetic diversity. The wild soybean species, Glycine soja (Sieb. and Zucc.), is another potential source of genetic diversity for drought traits. The present research was conducted to explore drought tolerance mechanisms in cultivated and wild soybean genotypes, with additional attention to temperature effects that could interact with physiological mechanisms. In the first series of experiments, a ‘shoot-based’ mechanism was examined in wellwatered plants to determine how plants adjusted their water use patterns in response to a stressful aerial environment. Second, ‘below-ground’ experiments were conducted to investigate water use in water-replete and water-deficit soil environments, as well as the root morphology of plants grown in hydroponics. Last, wild soybean was evaluated in closedcanopy field experiments for agronomic traits in comparison with cultivated soybean. Drought tolerant genotypes in soybean appear to possess a suite of traits that allow them to control water use in response to atmospheric and soil stresses, and now the same is being discovered in wild soybean genotypes. Wild soybean made physiological adjustments in its water use in response to both aboveand below-ground drought stresses. A layer of complexity was discovered when wild and cultivated soybean exhibited their drought tolerance mechanisms exclusively near their optimum growth temperatures of 25 and 30 °C, respectively. A similar trend was evident in root growth and morphology. Wild soybean had almost twice as much root length as the cultivated type when grown at 25 °C. However as temperature increased to 35 °C, cultivated soybean root length was higher than wild soybean. As expected, cultivated soybean also surpassed the wild type in the field. Domestication of soybean has lead to higher yields, improved seed quality, and other improved agronomic characteristics. The results of this work suggest that wild soybean has potential drought tolerance traits that could be useful in cultivated soybean genotypes. Our study has revealed challenges with temperature interactions in integrating traits from wild to domesticated soybean, but it also offers quantification of the agronomic productivity of wild soybean, which may be higher than previously suspected by many soybean breeders. Drought Tolerance Mechanisms in Cultivated and Wild Soybean Species by Thomas M. Seversike A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy