Physiological Characterization of the SoyNAM Parental Lines under Field Conditions

The narrow genetic pool of soybean (Glycine max L. Merr.) in North America can limit its future yield gains. Among the worldwide germplasm collection of 45,000 unique landraces, only 80 contribute 99% to the collective parentage of North American soybean cultivars. Among these 80 landraces, just 17 contribute to 86% of the collective parentage of the modern cultivars. The Soybean Nested Association Mapping population (SoyNAM) was therefore developed with the objective of diversifying the soybean gene pool. Forty diverse soybean genotypes from maturity groups (MG) 1 through 5 were crossed with a common MG 3 parent to develop 40 recombinant inbred populations. Each of these populations has 140 recombinant inbred lines (RILs) and have been genotyped with molecular markers and characterized for few important traits. This experiment was conducted during three consecutive summers, in Fayetteville, Arkansas with the objective to phenotype the SoyNAM parental lines for yield and drought-related traits. And, then identify the extreme genotypes among these parental genotypes, which have either not been mapped previously or if mapped have not been mapped very extensively. Canopy coverage was estimated through aerial digital images taken 3 to 4 times until canopy closure. After canopy closure, during late vegetative or early R1 stage, shoot samples were taken that were used to determine N2 derived from the atmosphere (NDFA), shoot nitrogen and ureide concentrations, and δ13C (an indirect measure of water use efficiency). Two harvests were made at mid-R5 and two weeks later, to calculate seed growth rate and effective filling period. Wilting measurements were taken towards the end of irrigation cycles when drought symptoms started appearing. Yield and harvest index (HI) were determined from a bordered section of each plot at maturity. Statistical analysis indicated that several parents differed statistically from the hub parent. Some genotypes were also identified as common extreme parents for more than one trait. Identification of such divergent parental lines will aid in selecting recombinant inbred populations for future quantitative trait loci mapping studies