Allelopathy growth inhibition

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... 

On the basis of results to date, it is concluded, as shown in Figure 7, that allelopathy could be a source of crop yield reduction throughout the time crop yields are reduced by weed presence, but it is most apt to be a factor from the midpoint of the curve on. Thus, the contribution of allelopathy to crop yield reduction likely is less than that of competition for light, water, and nitrogen. The reduction in crop yield could be the result of inhibition in growth, inhibition in reproduction, or some combination of the two. 

Scopoletin is purported as the most widely distributed coumarin in higher plants, and scopoletin, umbelliferone, and esculetin are the ones most frequently linked to allelopathy. Given their phenylpropane origin, it is not surprising that these simple coumarins have many actions in common with the cinnamic acids. One variance is that coumarin and scopoletin have been reported to decrease mitosis,2 whereas at least at concentrations that correlate with growth inhibition, the phenolic acids do not appear to affect cell division. 

Chemical analysis of sweetgum seedlings treated with fescue leachates showed that growth inhibition was associated with an impaired absorption of phosphorus an nitrogen. Obviously, conditions of decomposition, allelochemical enhancement of disease, the nature of the secondary products from microbial activity, and interactions among allelochemicals are all significant variables in intercrop allelopathy. The uniqueness of the chemical environment for each crop sequence and situation will continue to confound precise analyses of effects on yield. 

Colicins are pore-forming proteins, produced by certain strains of E. coli, that kill or inhibit the growth of other, competing bacteria and even other strains of E. coli (a process known as allelopathy). Channel-forming colicins are released as soluble monomers. Upon encountering a host cell, the colicin molecule traverses the bacterial outer membrane and periplasm, then inserts itself... 

The beKavior of P. hysterophorus in the field would appear to fall under TKe classic definition of allelopathy as defined by Rice (4), i.e., this plant adversely affects other nearby vegetation by extruding chemicals which inhibit the growth of these other plants (5-10). 

Although many physiological and biochemical processes In plants are affected by various allelochemicals, In most Instances the details of the mechanism of action of a particular allelochemical have not been elucidated. Because soil mediates the transfer of most allelochemicals (except perhaps volatile compounds) from a donor to a receiver, plant roots are often the first tissues to contact an allelochemical. Thus, It Is not surprising that root growth and development are Inhibited In many Instances of allelopathy (1.-3) One of the primary physiological functions of plant roots Is the absorption of mineral nutrients. Therefore, It Is logical that the Influence of allelopathic Interactions on mineral absorption by plant roots has been Investigated. 

Although the definition of allelopathy Includes stimulation as well as Inhibition of growth by allelochemicals (1., 4), allelochemicals that definitively affect mineral absorption by plant roots have been found to primarily Inhibit, rather than stimulate, the process. The first part of this review presents evidence that alteration of mineral absorption Is a physiological mechanism of allelopathy. Possible physiological and biochemical bases for the Inhibition of mineral absorption by allelochemicals are then discussed. 

Our research on allelopathy in tall fescue (Festuca arundinacea Schreb.) was begun to determine if growth inhibitors were present, if tall fescue had an inhibitory effect on plants growing in association with it, and finally to identify the chemicals responsible for inhibition. 

There are numerous reports of allelopathy in the literature, but often the identity of the allelochemical(s) is unknown. There are, however, many cases where specific compounds or groups of compounds have been implicated as allelopathic agents. Table 1 summarizes some examples of sources and identities of allelochemicals that directly inhibit plant growth. These secondary compounds have been implicated as a driving force in ecological succession ( 1 ). 

Allelopathy arises because growth stimulating, or inhibiting plant and microbial produced biochemicals which are released into the environment. The stress conditions such as moisture, temperature, fertilizer, soil, pests,... 

A characteristic feature of allelopathy is that the inhibitory effects of allelopathic compounds are concentration dependent. Dose-response curves with known compounds show an inhibition threshold. Below this level either no measurable effect occurs, or stimulation may result. Although the concentration of a compound required to exceed the inhibition threshold varies extensively according to different sensitivities among species and also among phases of the growth cycle for higher plants, the concept of an inhibition threshold seems consistent. Thus, it is reasonable to evaluate how, and if, a subthreshold concentration of an allelochemical may contribute to allelopathic interference. Also in need of evaluation is how environmental conditions may influence the deleterious action of an allelochemical and the concentration required for an effect. Such interactions are especially pertinent for those environmental situations that place some degree of stress on plant functions. 

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