Greenhouse experiments and bioassays

Petri dish bioassays of residues in soil—(1) segments of ground dried shoots cut from the soil surface 35 days after planting, (2) 2-cm segments of roots rinsed free of soil, and (3) aqueous extracts of shoot segments, 50 g extracted with 1 L distilled H2O—confirmed phytotoxicity. Petri dishes were filled with oven-dried sterile soil or nonsterile soils, and dried residues were uniformly spread and watered with distilled water, or in the case of extracts, added directly. Dishes were incubated for 72 h at 27°C in a growth chamber. 

Radicle elongation was a more sensitive measure than germination. Phytotoxicity decreased as the distance from rye shoot material to assay seeds increased. Generally, cress and lettuce were more sensitive indicators than the grass species tested. Phytotoxic compounds from shoots were water extractable. To evaluate the effect of plant part and quantity of residue, roots, shoots, or both were assayed at 0.0, 0.25, 0.5, 1.0, and 2.0 g/dish and placed 5 mm away from the assay seed. Similar amounts of paper towels were used as controls. Shoot tissue was about twice as toxic as root tissue. The data suggested that under field conditions, a 

DIBOA was quantified and purified. Phytotoxicity was compared in bioassays for germination and seedling growth inhibition against weeds and tomato with P-phenyl-lactic acid (PLA) and P-hydroxybutyric acid (HBA), two previously identified and known phytotoxins from rye. Both DIBOA and BOA were consistently more toxic than PLA and HBA (Barnes Putnam 1987). 

2 -Oxo-1,1 -azobenzene (AZOB), a compound with strong herbicidal activity, was isolated from a soil to which BOA had been added. A parallel experiment 

The papers summarized above represent the efforts of several workers over many years. They well demonstrate the Koch s postulates approach to allelopathy. The identification of the source of the chemicals, their structural characterization and specific testing individually and in mixtures in bioassays, elimination of obvious physical factors accounting for some of the inhibition, demonstration of activity in soil in both the field as well as in complementary laboratory bioassays, and demonstration of a role for microorganisms have all been convincingly shown. While information on flux rates and soil concentrations is still needed. 

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