Bioassay techniques are often used to study the effects of allelochemicals on plant processes, and it is generally observed that the processes are stimulated at low allelochemical concentrations and inhibited as the concentrations increase. A simple empirical model is presented to analyze this type of response. The stimulation-inhibition properties of allelochemical-dose responses can be described by the parameters in the model. The indices, p% reductions, are calculated to assess the allelochemical effects. The model is compared with experimental data for the response of lettuce seedling growth to Centaurepensin, the olfactory response of weevil larvae to alpha-terpineol, and the responses of annual ryegrass (Lolium multiflorum Lam.), creeping red fescue (Festuca rubra L., cv. Ensylva), Kentucky bluegrass (Poa pratensis L., cv. Kenblue), perennial ryegrass (L. perenne L., cv. Manhattan), and Rebel tall fescue (F. arundinacea Schreb) seedling growth to leachates of Rebel and Kentucky 31 tall fescue. The results show that the model gives a good description to observations and can be used to fit a wide range of dose responses. Assessments of the effects of leachates of Rebel and Kentucky 31 tall fescue clearly differentiate the properties of the allelopathic sources and the relative sensitivities of indicators such as the length of root and leaf.

Allelochemicals are assumed to possess specific biological properties and responses of an organism are external expressions of such properties. Based on this assumption, a mathematical model has been constructed to interpret the characteristic responses of an organism to allelochemicals. Several sets of experimental data have been compared with the model predictions; good agreement between the model and data is observed.

The release of alkaloids by barley was quantified by HPLC. Hordenine was released from the roots of barley in a hydroponic system for up to 60 days. The amount reached a maximum, 2μg/plant/day, at 36 days, then declined. Effects on white mustard by hordenine and gramine included reduction of radicle length and apparent reduction in health and vigor of radicle tips. Transmission electron microscopic examination of white mustard radicle tips exposed to hordenine and gramine showed damage to cell walls, increase in both size and number of vacuoles, autophagy, and disorganization of organelles. The evidence of the morphological and primary effects of barley allelochemicals at the levels released by living plants indicates that the biologically active secondary metabolites of barley may lead to a significant role in selfdefense by the crop.

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Allelopathy is identified particularly with chemical activity between plants; entomologists refer to allelochemicals in a broader context. Recent work shows that several groups of compounds associated with allelopathy also play a part in communication between plants and other organisms. It is argued that such communication is part of the similarities in plant and animal responses to stress and may contribute to plant defense.

Allelopathic effects of barley (Hordeum vulgare L.) on white mustard (Sinapis alba L.) were assessed using modified bioassays that reduced other environmental influences. In a Petri dish bioassay, germination of white mustard was delayed and the radicle lengths were significantly inhibited at a density of 0.5 barley seed/cm(2). In a 'siphoning' bioassay apparatus, when the two species were sown together, radicle elongation of white mustard was not inhibited one day after sowing but became increasingly inhibited as bioassay time increased. Barley allelochemicals were released from the roots in a hydroponic system for at least 70 days after commencement of barley germination. Solutions removed from the hydroponic system of growing barley delayed germination and inhibited growth of white mustard. The allelopathic activity of barley was further confirmed at a density of 0.3 barley seed/cm(2) in a modified stairstep apparatus.

Maku big trefoil and Dewey birdsfoot trefoil were grown at 14/10, 20/16, 26/22 and 32/28�C (14/10 h) in growth cabinets. Growth rates and condensed tannin (CT) concentrations were measured as soil moisture was changed from field capacity (FC) to 20% FC to FC again in three regrowth cycles. Seasonal dry matter accumulation and CT concentrations were also measured on these cultivars together with Sharnee big trefoil, Monarch cicer milkvetch, Chemung crownvetch and WL605 lucerne at two sites in the field. Condensed tannin concentrations were not greatly affected by temperature alone. However, moisture stress induced proportionately larger reductions in growth rates of big trefoil as temperature increased, and correspondingly larger rises in CT concentrations. Condensed tannin concentrations in birdsfoot trefoil were largely unaffected by these factors. In big trefoil, the responses in CT levels and growth rates continued even when soil moisture was returned to FC. Low temperature did not appear to affect CT concentrations in big trefoil, but caused some slight elevation in concentrations in birdsfoot trefoil. In the field, CT levels in the big trefoil cultivars, and in birdsfoot trefoil, were elevated in summer and autumn, when temperatures were high and periods of soil moisture stress occurred. The potential for elevated CT levels in big trefoil appears to be enhanced when growth rates are reduced by moisture stress and temperaturs are high. Big trefoil should be carefully screened for CT concentrations in the process of cultivar development. High CT levels are not a problem in birdsfoot trefoils such as Dewey.

Summary Residues from mature, harvested crops of sorghum (Sorghum bicolor Moench.), sunflower (Helianthus annuus L.), oilseed rape (Brassica napus L.), wheat (Triticum aestivum L.) and field pea (Pisum sativum L.), exhibited selective effects on weed germination and growth under field conditions. Weed species in the study included Avena fatua L., Avena sterilis ssp. ludoviciana (Durieu) Nyman, Echinochloa crus‐galli (L.) Beauv., Phalaris aquatica L., Phalaris paradoxa L., Lolium perenne L., Vulpia myuros (L.) Gmel., Hibiscus trionum L., Polygonum aviculare L., Bilderdykia convolvulus (L.) Dumort. and Lamium amplexicaule L. All crop residues tested, and in particular wheat and pea residues, promoted the germination and growth of A. fatua and A sterilis ssp. ludoviciana. Other grass weeds, however, were inhibited by the presence of crop residue, the extent of inhibition being dependent on residue type. Germination response of dicotyledonous weed species was also a function of residue type, H. trionum numbers were significantly higher in plots where oilseed rape, sorghum or sunflower residue had been incorporated, while L. amplexicaule was inhibited by these residue types and stimulated by pea and wheat residues.