Potential allelochemicals

D. Vaughan and B. G. Ord, Extraction of potential allelochemicals and their effects on root morphology and nutrient contents. Plant Root Growth an Ecological Perspective (D. Atkinson, ed.), Blackwell, Oxford, 1991, p. 399. 

Of the compounds listed in Table II, chlorsulfuron and acifluorfen (Figure 2, VI and VII, respectively) look particularly interesting with regard to manipulation of allelochemical production. Both of these compounds can cause large increases in the phenolic titre of plant tissues (e.g. Figures 6 7). Suttle et al. (75) suggested that chlorsulfuron would be an excellent chemical for manipulation of the quality and quantity of secondary phenolic compounds in studies of the role of these potential allelochemics in ecological Interactions. 

By no means have all the potential allelochemicals produced by plants been isolated and identified. There are certainly unknown compounds that are active at very low concentrations. When these compounds are purified, physiological bases for their action will need to be determined. To say that inhibition of mineral absorption is the sole, primary mechanism of action of allelochemicals is unjustified. However, undoubtedly some allelochemicals can inhibit mineral transport and alter other membrane phenomena. 

Experiment The isolation and identification of bioactive molecules is a major goal of chemical ecology and can be achieved by guided bioassays on suitable sensitive systems. Here a bioassay based on the growth and morphogenesis of protonemal system of bryophytes grown in vitro is described to test potential allelochemicals from lichen Cladonia foliacea. 

Table 1 Experiment 2. Guided isolation and identification of potential allelochemicals from Cladonia foliacea. O, fraction O P, fraction P UA, usnic acid ca, caulonema ch, chloronema sb, side branches. Protonemal system from shoots after 21 days in culture tested against three fractions from C. foliacea thallus and in control Mohr medium. The observations represent means calculated from about 300 protonemata for each species. P. squarrosa is the species least affected, while 7. flavovirens is the most affected by lichen fractions. 

Spore germination and protonemal growth and morphogenesis are therefore, useful systems to test in vitro, with good reproducibility, potential allelochemicals both through direct co-existence test in vitro or using the described bioassays to monitor/guide isolation, purification, characterization of chemical structure of bioactive compounds. 

Fig. 4 Experiment 2 Guided isolation and identification of potential allelochemicals from Cladonia foliacea. A, Regenerating protonema from Pleurochaete squarrosa leaves after 14 days in control sample B, regenerating protonema from P. squarrosa after 14 days in the inhibiting fraction P from lichen Cladonia foliacea . C, regenerating protonema from P. squarrosa after 14 days in the stimulating fraction O from lichen Cladonia foliacea. Light microscope (a and c x 25, b x 50).

Giordano, S., Basile, A., Lanzetta, R., Corsaro, M. M., Spagnuolo, V. and Castaldo Cobianchi, R. (1997). Potential allelochemicals from the lichen Cladonia foliacea and their in vitro effects on the development of mosses. Allelopathy Journal 4 89-100. 

Aliotta, G., Cafiero, G., De Feo, V. and Sacchi, R. (1994). Potential allelochemicals from Ruta graveolens L. and their action on radish seeds. Journal of Chemical Ecology 20 2761-2775. 

Tannins have have been widely considered to reduce the digestibility and thus nutritional value of plant imterial to herbivores. Evidence from vivo studies exploring the potential allelochemical effects of tannins is reviewed together with infonnation regarding the probable irechanisms leading to observed effects. 

Aliotta, G., Della Greca, M., Monaco, P., Pinto, G., Pollio, A., and Previtera, L. 1996. Potential allelochemicals from aquatic weeds their action on microalgae. In Narwal, S. S. and Tauro, P. (Eds.), Allelopathy Field Observations and Methodology. Scientific Publishers, Jodhpur, India, 243-254. 

While virtually all plants contain chemical constituents which have allelopathic potential, only those chemical constituents that are released to the surrounding environment can be considered potential allelochemicals. A potential allelochemical can only be designated as an allelopathic agent after it has undergone a rigorous proof of demonstrated phytotoxic activity. The chemical nature of recognized allelochemicals is diverse, and almost every class of secondary plant metabolite has been implicated in allelopathy. 

---