Allelochemicals toxicity

The chemistry, distribution and biocidal action of these toxic allelochemicals Is discussed. 

In contrast to the leaf surfaces much more is known about the chemistry of the leaf interior, which is probably considerably more complex. These compounds representing nutrients, toxic allelochemicals and allelochemicals with a host plant sign character, influence feeding behavior (see Scriber, Chapter 7). As can be seen from Table 1.1, contact chemoreceptors for all types of compounds have been identified in a variety of insect species. With the exception of the two beetle larvae and the locust, all of the examples are lepidopteran larvae mainly because the two pairs of sensilla styloconica on the galea (maxilla) of these larvae can be investigated easily (Schoonhoven and Dethier, 1966). The relatively small number of contact chemoreceptors (four sensilla with four neurons) and their importance for food plant discrimination (Hanson and Dethier, 1973) make them ideal for the study of contact chemoreception in relation to host plant selection. 

Relating the effects caused by specific allelochemicals to those caused by an allelopathic plant is complicated because the inhibitory substances released from a plant are often unknown, and generally several different compounds are involved. However, the actions of the weeds studied in our investigations have certain parallels to those found from pCA and FA. The allelopathic nature of Kochi a, Jerusalem artichoke, and cocklebur was established, since both aque-ous extracts and weed residues reduced sorghum growth. The data show a concentration dependency characteristic of allelopathy, and some difference in toxicity among the three weeds was observed with cockle-bur the most toxic. 

The bulk of the allelopathy literature has dealt with direct toxic effects on other plants. However, as developed in this review, it is obvious that allelochemicals may have a major impact on plant root-microbial interactions. Such interactions could lead to growth inhibition in the microorganisms (or in roots) and affect other factors of the root-microbe association resulting in effects... 

Laboratory algal assays for monitoring the toxicity of an allelochemical are very useful, due to the ease and rapidity with which the organism can be cultured and their responses measured. They also require less equipment, space and time of the researcher. 

With liquid media bioassays, it is possible to obtain a lot of information about the activity of the allelochemical (toxicity, type of inhibition on the growth curve, morphological modification on the test alga, etc.). But the time of response can be long (about 15 d) and for disk bioassay more equipment, space etc. are necessary. 

SCGE technique to assess genetic toxicity of allelochemicals... 

To produce QSAR models, a data set containing chemicals within a specified well-defined end-point is necessary. Since our knowledge about the properties of the natural compounds that surround us is very poor, especially for allelochemicals and toxicological evaluation of synthetic pesticides is well documented (regulators oblige the chemical industry to produce experimental data for synthetic chemicals, before they can be marketed), when allelochemicals toxicity values are not available, pesticides with similar structure can be used in the analysis. Therefore suitable data sets can be defined with pesticides and their activities, to predict the toxicity (activity) of the allelochemicals. 

Examples of statistical analysis for QSAR developing using different techniques is given in Table 2 that shows the prediction of the Daphnia magna toxicity for allelochemicals (BOA, DIMBOA and MBOA) using QSARs models obtained with different statistical techniques (PLS, MLR, and Neural Networks). 

Lo Piparo E., Fratev, F., Mazzatorta, P., Smiesko, M., Fritz, J.I., Benfenati, E. (2006). QSAR Models for Daphnia magna toxicity prediction of Benzoxazinone allelochemicals and their transformation products. Journal of Agricultural and Food Chemistry 54 1111-1115. 

Phenolic acids can be allelopathic but their presence in soil is ephemeral due to rapid degradation and/or sorption by soil particles (Inderjit 2004). Sorption of benzoic acid onto soil particles increased with concentration and it may explain the reason for the limited allelopathic effect of benzoic acid at concentrations often recorded in natural soil (Inderjit 2004). Microorganisms help to generate allelochemicals, but they may also modify toxic compounds into nontoxic compounds (Khanh et al. 2005). Allelochemicals are changed in composition and quantity during the residue decomposition. Allelopathy plays an important function in nutrient recycling (Rice 1984). 

Chase WR, Nair MG, Putnam AR (1991) 2,2 -Oxo-l,l -azobenzene Selective toxicity of rye (Secale cereale L.) allelochemicals to weed and crop species II. J Chem Ecol 17(1) 9—19... 

One role of cyanobacterial allelochemicals may be to alter the motility and distribution of competing photoautotrophs. In a recent study, Kearns and Hunter (2001) examined the effects of toxic metabolites from the filamentous cyanobacterium A. flos-aquae on a unicellular phytoplankton species, Chlamydomonas rein-hardtii. A. flos-aquae synthesizes both microcystins as well as anatoxins, providing the authors with an ecologically relevant opportunity to assess the individual and combinatorial effects of these toxins on an alga. 

Certain tree species such as Betula pendula and Picea abies fail to develop in association with heather, Calluna vulgaris (40. 1). This apparently results from the production by heather of an allelochemical toxic to growth of mycorrhizae of Betula and Picea. Fruticose soil lichens are often allelopathic to the growth of mycorrhizae and forest tree seedlings also (42). Removal of reindeer moss (a lichen) in field tests resulted in accelerated growth of pine and spruce. 

---