Allelochemical effects

Other kinds of bloassays have been used to detect the presence of specific allelochemical effects (8), effects on N2 fIxatlon (9), the presence of volatile compounds (10) and of Inhibitory substances produced by marine microalgae (11). Putnam and Duke (12) have summarized the extraction techniques and bioassay methods used In allelopathy research. Recent developments In high performance liquid chromatography (HPLC) separation of allelochemlcals from plant extracts dictates the need for bloassays with sensitivity to low concentrations of compounds contained In small volumes of eluent. Einhellig at al. (13) described a bloassay using Lemna minor L. growing In tissue culture cluster dish wells that maximizes sensitivity and minimizes sample requirements. 

Allelochemic effects of aquatic macrophytes on algae are discussed. Bloassays of chromatographic fractions from Eleocharls mlcrocarpa Torr. Indicate that oxygenated fatty acids are the causative agents. Methods of Isolation of these materials from aquatic macrophytes and from natural waters are described. Purification and structure determinations show that prominent components of the fraction are C q tri-hydroxycydopentyl and hydroxycydopentenone... 

To show the dimensions of the allelochemical effect with all three above-mentioned paradigms before the era of mineral fertilizers, production of a crop required application of 20 tons of manure per hectare later, with mineral fertilizers only several centner per hectare are required, with regulators such as herbicides, 2 to 20 kg, and regulators such as Agrostemin only a few grams per hectare. 

This does not mean that analysis of allelopathy in an arid environment should be done differently from that which is customary in a humid environment yet it is important to estimate the extent to which inorganic salts (excreted by the plant or released from its litter) are involved in the allelopathic effect. So far as secondary metabolites are concerned, it should be of interest to compare their production under humid and stressed conditions. It is suggested that for the evaluation of the allelochemical effect, species suppressed in their natural habitat should be preferred over any other standard seeds commonly used for evaluating germination inhibitors. Also, efforts to isolate allelochemicals from soils will assist in the establishment of allelopathy on a more concrete basis than is available at present. 

This lack of development in aquatic allelochemistry appears an anachronism. It is not. Specifically reflecting the pervasive and peculiar effects of water, analysis of allelochemical events in aquatic systems has presented unique problems. Among the most significant are (a) the widespread and unpredictable occurrence of secondary activity, and (b) the difficulties of distinguishing between ultra-trace nutrient requirements and allelochemical effects. Both generate a need for unusually rigid experimental control. 

Other work indicates that some cases of higher plant allelopathy are mediated by a combination of direct allelochemical effects operating in conjunction with enhancement of the severity of infection from root rot organisms. These observations make it clear that the biological activity of allelochemicals is closely tied with other facets of the environment. 

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. 

Evidence for the Allelochemic Effect of Tannins from Feeding Studies... 

The information in Table I reveals differences between sane of the inportant types of tannin. Tannic acid is unique for its occurrence in all reports of diet-induced gut lesion and gastrointestinal damage (vertebrate or invertebrate) it is also the only tannin for which metabolism and excretion are reported. By contrast condensed tannins are not thought to leave the gut lumen. Thus,in comparison with tannic acid condensed tannins would not be espected to drain 1-carbon metalx>lism, notably methionine resources. However, the reports by Elkin (] ) and Ford ( ) of corrective methionine treatment for chicks fed a condensed-tannin diet, indicate that sane condensed tannin may enter the body or that methionine can react with tannin in the gut, so perhaps the position is once again not clear cut on tannin structure and its impact on methionine levels. The conclusion can, however, be made that despite its hydrolyzability, tannic acid does exert an allelochemical effect which is not abolished by hydrolysis. 

Chiapusio, G., Sanchez, A. M., Reigosa, M. J., Gonzalez, L. and Pellissier, F. 1997. Do germination indices adequately reflect allelochemical effects on the germination process J. Chem. Ecol. 23, 2445-2453... 

Another problem or consideration that may interfere with some aspects of allelopathy (especially in field experiments) has arisen over the past fifty years with the introduction and use of a multitude of xenobiotic compounds as insecticides, fungicides, plant growth regulators, harvest aids, and herbicides. A variety of such compounds have been and continue to be used on a world-wide scale. Some of these chemicals and/or their transformation products are persistent in soils and water. Other such chemicals are routinely applied at various times during a year. These xenobiotics may interfere or interact with naturally occurring allelochemics and thus alter or even mask certain natural allelochemical effects. Researchers will have a more difficult time to find natural areas that do not contain xenobiotic... 

Secondary Metabolites from Plants and Their Allelochemic Effects... 

Insofar as the allelochemic effects on higher plants are concerned the most dramatic effects were obtained with intact week-old corn plants (Z.ea ma.vs L.) treated with concentrations of carboxyatractyloside at 10", 10 3 and 10- M (100 ul placed inside the uppermost leaf sheath) (35). Responses were visible at 72 hours and consisted of necrotic lesions on leaves at 10 2 M and chlorosis within leaf sheaths at 10 3 and 10 M. One week following treatment there was massive necrosis of the leaves at 10 2 M and a 50% inhibition of growth, while 10 3 m treated plants were inhibited 25%... 

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