Two Genes Affecting Stem Termination in Soybeans1

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Abstract
Inheritance of the common determinate (abruptly terminated) stem type (dt1) vs indeterminate (tapered) stem (Dt1) in soybeans [Glycine max (L.) Merrill] was confirmed as monogenic. The heterozygote Dt1dt1 showed a distinct intermediate or semi‐determinate phenotype in the genetic backgrounds studied. A stem type resembling this Dt1dt1 heterozygote was found in a few true‐breeding varieties and was shown to be controlled by a single dominant gene designated Dt2. Crosses between the two types gave F2 ratios of 1 indeterminate:11 semi‐determinate:4 determinate, the expected ratio for independent segregation with dt1 epistatic to Dt2‐dt2.The primary effect of both dt1 and Dt2 is to hasten the termination of apical stem growth, which decreases both plant height and number of nodes, but dt1 has a much greater effect. In tests at Urbana of dt1 and Dt2 BC5 isolines of the indeterminate (Dt1 dt2) commercial varieties ‘Harosoy’ and ‘Clark,’ plant height was reduced by 45 to 60% by dt1 and 12 to 15% by Dt2. Seed yield was only slightly reduced by dt1 and was unaffected by Dt2. Lodging resistance was consistently improved by dt1 and in some instances by Dt2. Time of maturity and seed size and composition were slightly affected in some cases.

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