Phenotypic evaluation of root traits in soybean and applicability to plant breeding

The southeastern USA is vulnerable to drought during critical times in the development of soybean [Glycine mcrr (L.) Merr.]. A Japanese plant introduction, PI 416937, exhibits drought tolerance and may be an important breeding line for the region. PI 416937 has an extensive fibrous root system that confers drought tolerance. The objective of this study was to relate visual root scores to other measurable root characteristics in order to provide breeders with a means for rapid phenotypic evaluation of soybean roots. Root score, root surface, and root dry weight were measured for 'Lee 74' and PI 416937 at three soybean stages of development in 1992 and 1993. Nodule number and nodule dry weight were also measured during the second year. PI 416937 had higher root score, root surface, nodule number, and nodule dry weight than Lee 74. Root score was positively correlated with root surface, nodule number, and nodule dry weight (r = 0.74, 0.83, and 0.80, respectively). Phenotypic root scores could be utilized effectively in selection programs to rapidly evaluate large numbers of progeny in order to identify those with extensive fibrous root systems.