MORPHOLOGICAL STUDY OF THE MECHANISM CAUSING BRACHYTIC STEM IN SOYBEANS
A comparative morphological study of stem arrangement in soybean, Glycine max (L.) Merr., was performed to determine the mechanisms responsible for the usual stem morphology in PI 227224, a line characterized by geniculate (zigzag) stem and shortened internodes. Stem morphology was compared in three lines (1) PI 227224; (2) Davis, a normal-stemmed cultivar; and (3) Ga 76-1T, an F3 brachytic line from a cross between PI 227224 and Davis. Two independent morphological parameters were found to cause the conspicuous geniculation of the stem. Precocious expansion of the axillary buds first results in lateral displacement of the main shoot apex. Secondly, development of shortened internodes is necessary for pronounced phenotypic expression of the geniculation because more elongated internodes undergo significant geotropic straightening which reorients the tilted shoot apex to result in a plant with minimal evidence of a zigzag stem.