Two Genes Affecting Stem Termination in Soybeans1

Inheritance of the common determinate (abruptly terminated) stem type (dt 1 ) vs indeterminate (tapered) stem (Dt 1 ) in soybeans [ Glycine max (L.) Merrill] was confirmed as monogenic. The heterozygote Dt 1 dt 1 showed a distinct intermediate or semi‐determinate phenotype in the genetic backgrounds studied. A stem type resembling this Dt 1 dt 1 heterozygote was found in a few true‐breeding varieties and was shown to be controlled by a single dominant gene designated Dt 2 . Crosses between the two types gave F 2 ratios of 1 indeterminate:11 semi‐determinate:4 determinate, the expected ratio for independent segregation with dt 1 epistatic to Dt 2 ‐dt 2 . The primary effect of both dt 1 and Dt 2 is to hasten the termination of apical stem growth, which decreases both plant height and number of nodes, but dt 1 has a much greater effect. In tests at Urbana of dt 1 and Dt 2 BC 5 isolines of the indeterminate (Dt 1 dt 2 ) commercial varieties ‘Harosoy’ and ‘Clark,’ plant height was reduced by 45 to 60% by dt 1 and 12 to 15% by Dt 2 . Seed yield was only slightly reduced by dt 1 and was unaffected by Dt 2 . Lodging resistance was consistently improved by dt 1 and in some instances by Dt 2 . Time of maturity and seed size and composition were slightly affected in some cases.