Developmental anatomy and morphology of fasciation in the soybean (Glycine max)

The anatomy and morphology of two isogenic lines of `Harosoy' soybean, one fasciated and the other not, were studied over the growing season. Fasciation results from a gradual elaboration of the shoot apical meristem along one transverse axis; thus, a conical apex becomes a generative ridge. It does not result from a fusion of preexisting apical meristems. Two weeks after planting, apices of fasciated plants become wider than apices of nonfasciated ones. When measured at two-thirds height, both stem fasciation and a 30%-40% decrease in pith cell size become detectable at 6 wk. At 7 wk, leaf number is doubled and phyllotactic pattern is altered in fasciated plants compared with normal ones. At 8 wk, fasciated stem circumference is twice that of the normal stem. Cell number across the longest diameter of the fasciated stem is about one order of magnitude greater than the across the normal stem, based on measurements of pith cell dimensions. Pith cell shape is the same in both genotypes. The increased leaf number of fasciated plants is associated in time and position with an increase in total number of stem vascular bundles and in number of those exhibiting characteristics of sympodia. At seed maturity, that portion of the fasciated main stem that had increased in breadth as it had grown in length bears few or no fruits. The portion above, which had decreased in breadth as it had grown in length, is the most fruit-laden portion of the fasciated stem. Fasciated stems branch less than nonfasciated ones. Of the three kinds of branches observed on fasciated stems, only one, the rarest, has an anomalous ontogeny. It derives from a bifurcation of the ridgelike apical meristem; the others arise as axillary buds.