Identification of QTL for Root Traits Associated with Flooding Tolerance of Soybean (Glycine max).

Soybeans (Glycine max) are known to be sensitive to flooding stress. Flooding at the reproductive stage can reduce soybean yield by 50 to 56%. However, screening efforts have resulted in the identification of genotypes differing in susceptibility to flooding. The flooding tolerant genotype PI408105A showed only a 32.1% reduction in yield compared to 81.2% reduction in the flooding sensitive breeding line S99-2281.  An F_7:8 recombinant inbred population (RIP) that segregates for flooding tolerance was developed from a cross of PI408105A x S99-2281. To investigate the segregation of the RIP for root traits we grew 200 recombinant inbred lines (RILs) in the greenhouse in polyethylene cones (8 cm in diameter x 30 cm length) containing (25% Metro-Mix 350, 75% Promix PGX) at one plant per cone. Flooding treatment was applied at the early vegetative stage by placing the cones in 50 L plastic tubs and adding water to the tubs up to 3 cm above the soil surface. At 10-d after the onset of flooding, the water was drained and plants of each RIL were rated for adventitious formation using a scale from 1 to 5 with 1 = less than 3 adventitious roots; 3 = 3-6 adventitious roots; and 5 = more than 6 adventitious roots. The roots were transferred to nail boards to preserve the root architect during root washing. Root photos were taken and the roots and shoots were dried for biomass determination. Maximum and total root length and root distribution were determined using the WinRhizo software. The experiment was repeated four times.  PI408105A plants produced 32% more adventitious roots and 74% root biomass than S99-2281 plants. Transgressive segregation was detected for both root traits. QTL mapping of the root traits and their correlation with seed yield reduction due to flooding stress will be presented.