2007 Annual Report 1a.Objectives (from AD-416) To determine the plant physiological mechanisms that are responsible for the slow-wilting phenotype that is segregating in three recombinant inbred populations and to establish a congruence between the slow-wilting phenotype and the physiological phenotype. 1b.Approach (from AD-416) The first step will be to focus on the extreme soybean breeding lines based on past phenotyping for expression of slow-wilting. The critical issue is to determine if there is congruence between the slow-wilting morphological phenotype and the underlying physiological mechanism in three QTL populations. The establishment of a congruence between the slow-wilting phenotype and the physiological phenotype would be a major accomplishment providing options for screening for drought tolerance in breeding populations. Phenotyping at the physiological level would also allow the marker information to be revisited for associations with this trait. Two of the QTL populations have been mapped with DNA markers. The third QTL population is in process. These three populations will be grown at the Sandhills Research Station in North Carolina for observation under drought conditions. Data will be collected and analyzed in each year. 3.Progress Report This report serves to document research conducted under a specific cooperative agreement between ARS and the University of Florida. Additional details of research can be found in the report for the parent project 6645-21220-008-00D, Increasing The Competitive Position Of US Soybeans In Global Markets Through Genetic Diversity And Plant Breeding. The mechanism of slow wilting under drought is only partially understood. Preliminary investigation has shown that slow-wilting types respond to drying air differently than normal soybean cultivars. It appears that as the air dries prior to the onset of drought, the stomata partially close, reducing transpiration, and causing the plant to save soil moisture. As drought develops, the slow wilting types have more soil water available for use than standard cultivars, giving rise to the slow-wilting phenotype. If correct, this unusual response should be reflected in canopy temperature. The effect would be that the canopy temperatures of slow wilters are higher than that of normal types prior to drought, because of reduced transpiration. During drought, slow wilting types have lower-than-normal canopy temperature because these plants have water to transpire while others have exhausted their water supply and, as a result, close stomata. The application to breeding is that slow wilting types might be identified via canopy temperature without the need for a drought environment. To investigate this idea, we employed a random inbred population (RIL) population derived from Benning x slow-wilting PI 416937. This population has been previously mapped with 200 polymorphic SSR markers by Dr. Boerma. The 150 F4-derived breeding lines were irrigated until the end of July and allowed to experience drought, thereafter, as it occurred. Canopy temperatures were taken with hand held infrared thermometers prior to drought. In addition, canopy wilting ratings will be taken during stress and seed yield will be taken at maturity. Canopy temperature, the slow-wilting trait, and yield will be related to each other. Activities were monitored through quarterly progress reports, site visits, frequent emails and phone calls.