Root morphology of young Glycine max, Senna obtusifolia, and Amaranthus palmeri

Root development and the associated acquisition of water and nutrients are an important part of weed competitiveness. Characterization of root morphol ogical development, however, is inherently problematic because of the compl exities of soil-plant interactions. In this study, we used hydroponically g rown plants and digital imaging to examine root characteristics of Glycine max and the competing weeds Senna obtusifolia and Amaranthus palmeri. The p urpose was to define inherent differences in root length and surface area t hat would contribute to growth responses during the establishment phase in the field. The methodology involved growing plants for 16 to 22 d, dissecti ng and staining root segments, mounting subsamples on slides, and imaging u sing a stereomicroscope and digital camera. Microscopy was required because of the small diameters of a significant proportion of the the weed roots. With plants of similar root fresh weights (4.5 to 5.0 g), counting of indiv idual roots revealed that S. obtusifolia and A. palmeri had 2 and 3.7 times more roots than G. max (4,616 and 7,781 vs. 2,120, respectively). The imag ing analyses indicated that roots of S. obtusifolia and A. palmeri had 2.9 and 5 times more length than G. max (10,042 and 17,192 cm vs. 3,418 cm, res pectively). Furthermore, the analysis of length in different root diameter classes indicated that weed roots were noticeably finer then those of G. ma x. Approximately 84% of S. obtusifolia root length was contributed by roots in the 0.1- to 0.25-mm range, whereas 45% of the G. max roots were in the 0.1- to 0.25-mm range and 48% were in the 0.25- to 0.75-mm range. In contra st, 68% of A. palmeri length was contributed by roots smaller than 0.1 mm i n diameter with 26% in the 0.1- to 0.25-mm range, Based on the expression o f root characteristics observed here, root systems of these weed species wo uld have finer roots with much greater length that would occupy a much larg er volume of soil than those of G. max. Presumably, this would result in a competitive advantage in the acquisition of water and nutrients, especially when availability is limited.