Allelochemical effects plants

EINHELLIG ET al. Effects of Allelochemicals on Plant- Water Relationships 181... 

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. 

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. 

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. 

Einhellig, F. A., Stille Muth, M., and Schon, M. K. 1985. Effects of allelochemicals on plant-water relationships. ACS Symposium Series 268, 179-195... 

Pehuelas, J., Ribas-Carbo, and M., Giles, L. 1996. Effects of allelochemicals on plant respiration and oxygen isotope fractionation by... 

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%... 

Phytotoxic compounds are produced by many crop species, and some of these compounds may play a role as allelochemicals in plant interference. This has bear hard to prove, much less taken advantage of in weed management. Many crops have been screened for allelopathic potential [28]. However, in all but a few cases, the effects of crop allelopathy are quite subtle compared to the effects of chemical herbicides. Efforts to produce allelopathic crop varieties through conventional breeding have not generated commercial varieties sold with this trait. Molecular biology approaches to this problem might be helpful. 

Biochemical basis of susceptibility or resistance of rice varieties to C. suppressalis has been studied in detail at IRRI recently (18). Allelochemics, mainly plant volatiles, were obtained from the ground leaf sheath tissue by steam distillation. The distillate was extracted with diethyl ether and after vacuum evaporation, a yellow oily residue was recovered which had the characteristic odor of each respective variety. The oily extracts of Rexoro (susceptible) and VTKM6V (resistant) varieties were tested as such, or as acid, basic, and neutral fractions for eliciting SSB moth s orientational and ovipositional responses. The whole extracts were also assayed for their effects on SSB eggs, larvae, and pupae. 

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. 

The term allelopathy was coined by Molisch in 1937 Q). Presently, the term generally refers to the detrimental effects of higher plants of one species (the donor) on the germination, growth, or development of plants of another species (the recipient). Allelopathy can be separated from other mechanisms of plant interference because the detrimental effect is exerted through release of chemical inhibitors (allelochemicals) by the donor species. Microbes associated with the higher plants may also play a role in production or release of the inhibitors (2). 

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