Top-Soil Root Architecture Characteristics of Obsolete and Modern Soybean Cultivars and Relationships with Shoot and Seed Nutrient Contents

Breeding of soybean (Glycine max) in the US has significantly increased seed yields over the course of the last century. While the impact of breeding on aboveground plant characteristics has received considerable attention, little is known about whether, and if so, how root system characteristics changed as a result of breeding for increased yield. Therefore, 24 maturity group IV soybean genotypes released over the course of 75 years (1930-2005) were grown at two locations in two years to examine root system characteristics and nutrient accumulation in shoot tissue and seeds. Top-soil root architecture was assessed following excavation during the seed filling stage. At the same time, aboveground biomass was harvested, dried and later analyzed for nutrient content. Seed yield was determined at harvest maturity and subsamples were analyzed for nutrient concentration. Significant genotype and environment as well as genotype by environment were observed for a many of the examined root system characteristics as well as content and/or concentrations of nutrients in shoot tissue and seeds. Assessment of a range of root architectural traits revealed significant genotypic differences that also were associated with Year of Release (YOR). In particular, overall root system complexity was positively correlated with yield, nodule size and density, phosphorus concentration in shoot tissue, and with seed macro- and micronutrient contents. However, negative correlations between YOR and some seed macro- and micronutrient concentrations were found in both years. Overall, analyses revealed that breeding for high yields also affected top-soil root architecture and nutrient uptake of soybean. Additional research is needed to elucidate the extent to which the changes in root system architecture underlie greater seed yield of modern cultivars.