never been rated for Al tolerance (Palmer et al., 1996). The few soybean germplasm evaluations to date indicate Screening methodology remains a practical barrier in the breeding that soybean can be screened for tolerance to Al-rich of Al-tolerant soybean [Glycine max (L.) Merr.]. Our objectives were to (i) develop a repeatable sand-media culture method for Al toler-acid soil with some degree of success (Sartain and Kamance screening of plants, (ii) compare Al response of genotypes in prath, 1978;Hanson and Kamprath, 1979; Campbell and sand culture to a standard hydroponics-based seedling culture, and Horst and Klotz, 1990;Foy et al., 1992, (iii) establish a practical guide for the use of hydroponics and sand-1993b; Spehar, 1994). However, genotypic rankings for culture screening methods in the selection of Al-tolerant soybean. We Al tolerance often vary among soil types. developed a sand-media culture method and imposed 0 and 450 M Aluminum saturation, the percentage of cation-ex-Al 3؉ activity treatments upon 10 diverse soybean genotypes. The exchange capacity occupied by Al, has been employed in periment employed a randomized complete block design with nine recent decades to classify the potential for Al toxicity replications. Root weight and relative root surface area (RRSA) were in soils, but it has not helped soybean breeders to predict determined at 18 d after transplanting (DAT). In hydroponics, the changes in genotypic rankings for Al tolerance that may genotypes were compared for taproot elongation after 3 d of exposure to 0, 2, and 5 M Al 3؉ activity treatments in a split plot design with occur from one soil type to the next (Kamprath, 1984; six replications. Aluminum stress was imposed successfully (approxi-Fageria et al., 1988). Researchers have attributed dismately 57% of the growth in control) in hydroponics and sand culture, crepancies in genotype rankings to the different concenbut discrepancies between methods were apparent. The hydroponicstrations of Al, P, Ca, Mg, organic acids, and other soil based seedling screen produced an inflated range of genotypic response components which greatly affect and potentially mask and altered Al tolerance rankings in comparison with sand culture.

Root development and the associated acquisition of water and nutrients are an important part of weed competitiveness. Characterization of root morphological development, however, is inherently problematic because of the complexities of soil–plant interactions. In this study, we used hydroponically grown plants and digital imaging to examine root characteristics ofGlycine maxand the competing weedsSenna obtusifoliaandAmaranthus palmeri. The purpose was to define inherent differences in root length and surface area that would contribute to growth responses during the establishment phase in the field. The methodology involved growing plants for 16 to 22 d, dissecting and staining root segments, mounting subsamples on slides, and imaging using a stereomicroscope and digital camera. Microscopy was required because of the small diameters of a significant proportion of the weed roots. With plants of similar root fresh weights (4.5 to 5.0 g), counting of individual roots revealed that 5.obtusifoliaandA. palmerihad 2 and 3.7 times more roots thanG. max(4,616 and 7,781 vs. 2,120, respectively). The imaging analyses indicated that roots ofS. obtusifoliaandA. palmerihad 2.9 and 5 times more length thanG. max(10,042 and 17,192 cm vs. 3,418 cm, respectively). Furthermore, the analysis of length in different root diameter classes indicated that weed roots were noticeably finer then those ofG. max. Approximately 84% ofS. obtusifoliaroot length was contributed by roots in the 0.1- to 0.25-mm range, whereas 45% of theG. maxroots were in the 0.1- to 0.25-mm range and 48% were in the 0.25- to 0.75-mm range. In contrast, 68% ofA. palmerilength was contributed by roots smaller than 0.1 mm in diameter with 26% in the 0.1- to 0.25-mm range. Based on the expression of root characteristics observed here, root systems of these weed species would have finer roots with much greater length that would occupy a much larger volume of soil than those ofG. max. Presumably, this would result in a competitive advantage in the acquisition of water and nutrients, especially when availability is limited.

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