Allelopathic response

Nostoc was attributed to a putative antibiotic released during the stationary phase of colony growth, suggesting that this may be an inducible allelopathic response to nutrient limitation (Schagerl et al. 2002). 

In many instances, the materials or plant substances that prove to be allelopathic in laboratory or pot culture experiments may not elucidate similar magnitude of allelopathic response on aquatic weeds in aquatic environments, watersheds, or wetlands. Hence, it is imperative to confirm plant products for their allelopathic potential on weeds in their own natural habitat. A search was made on allelopathic plant products for use in water hyacinth control programs at Department of Agronomy, Annamalai University. Ten of 55 different plant products that showed allelopathic suppression of water hyacinth within 48 h of treatment were selected and tested for their efficacy in natural habitats. The field testing was done in a two tier model. First, the ten plant products were tested in microponds (simulated natural habitat). Second, the plant products that confirmed to be allelopathic in microponds were further evaluated in natural watersheds. 

There is some confusion in the literature as to when it is appropriate to apply the term allelochemical to phenolic acids. Since phenolic acids and their derivatives are found essentially in all terrestrial soils, it should be understood that the presence of phenolic acids in soil does not automatically imply that these phenolic acids are functionally allelochemicals. In theory, phenolic acids in soils, depending on their chemical state, concentrations, and the organisms involved, can have no effect, a stimulatory effect, or an inhibitory effect on any given plant or microbial process. For phenolic acids in the soil to be classified as allelochemicals requires that a) the phenolic acids are in an active form (e.g., free and protonated), b) they are involved in chemically mediated plant, microbe, or plant/microbial interactions and c) the concentrations of the active forms in the soil solution are sufficient to modify plant or microbial behavior, either in a positive or negative manner.8,49 However, changes in microbial behaviour associated with the utilization of phenolic acids as a carbon or energy source would not qualify as an allelopathic response. 

Caffeine (Fig. 11.3), widely used by humans as a stimulatory drug, has so far been detected only in a few plant species. The biological roles of caffeine are believed to be in defense against herbivory or as an allelopathic response to potential competitors.83 Caffeine is derived from the purine alkaloid xanthosine. From xanthosine, three methylations are necessary to produce caffeine. First, xanthosine is methylated on N7 by 7-methylxanthosine synthase (MXS or 7NMT) to produce 7-methylxanthosine, which is enzymatically hydrolyzed to produce 7-methylxanthine and ribose.85,86 The methylations of N1 and N3 of 7-methylxanthine to produce 1,3,7-trimethylxanthine (caffeine) occur in young leaves of tea, and the same enzyme, caffeine synthase, apparently catalyzes both reactions.55 In coffee plants, caffeine is mainly found in the beans but also occurs in the leaves. Caffeine is stored in the vacuoles of coffee leaves as a complex with polyphenols such as chlorogenic acid.87 In contrast to tea, coffee plants appear to have separate enzymes for each step of N-methylation.57... 

Chemicals with allelopathic potential are present in virtually all plant tissues, including leaves, stems, roots, rhizomes, flowers, fruits, and seeds. Whether these compounds are released from the plant to the environment in quantities sufficient to elicit a response, remains the critical question in field studies of allelopathy. Allelochemics may be released from plant tissues in a variety of ways, including volatilization, root exudation, leaching, and decomposition of the plant residues. 

Table I provides general information on the identified allelopathic weeds and the crops they affect. More detailed information on two of these weeds, johnsongrass and purple nutsedge, will be presented to show the tenuous nature of the evidence for allelopathy. These two species are undoubtedly allelopathic, at least under certain conditions. Rigorous proof that allelopathy is the agent responsible for even a specific portion of the interference exerted by them is not easily attained, however, even though these are the weeds with the best research data available of those weeds that occur in the Mid-South.

It has been documented (5J that allelopathic agents are released into the environment by (a) exudation of volatile chemicals from living plant parts, (b) leaching of watersoluble chemicals from above-ground parts in response to the action of rain, fog or dew,... 

Plant physiologists and other biological scientists also have their important role to play in allelopathy. They must devise suitable bioassays to detect the suspected allelopathic compounds, follow the biological activity of the individual and associated chemicals, develop activity profiles for identified chemicals, and determine the conditions (dose/response) for chemicals to arrive at the threshold levels. They must also determine which chemicals contribute... 

Several general characteristics of the results compiled in Table I are worthy of mention. Compared to the variety of chemicals postulated to be involved in allelopathy (1), few specific compounds have been tested for inhibition of mineral absorption. The most extensively studied compounds are the phenolic acids, probably because of their being ubiquitously found in nature (1). Also, several flavonoids are inhibitory to mineral absorption (Table I). Both of these groups of compounds are often cited as being responsible for allelopathic interactions between plants. 

To date, no clear evidence of inducible defenses among freshwater macroalgae has been reported, in contrast to their marine algal counterparts. For example, certain species of marine brown algae increase phlorotannin production in response to damage by mesograzers (Amsler and Fairhead 2006 see Chaps. 3 and 7). Whether Chara and Cladophora, two species of freshwater chlorophytes with putative allelopathic activity, increase allelochemical concentration in response to competitors remains to be seen (see Sect. 5.7.3). 

Many allelochemicals are decomposed in soil, either abiotically (37) or by microorganisms (95-100). Obviously, the attainment of active concentrations of allelochemicals in soil depends on the relative rates of addition and inactivation. It is important to understand also that microbial decomposition of allelochemicals does not necessarily result in a decrease in allelopathic activity. In fact, the reverse may be true. Hydrojuglone is oxidized in soil to juglone, a quinone that is inhibitory to some species at a 10 ° M concentration (101). Isoflavonoids produced by red clover are decomposed to even more toxic phenolic compounds (95) and to repeat, amygdalin from peach roots is changed to hydrogen cyanide and benzaldehyde which cause the peach replant problem (88), and phlorizin from apple roots is decomposed to several phenolic compounds that appear to be responsible for the apple replant problem (100). 

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. 

Although indirect and probably quite rare, another route has been reported for allelochemical interference with plant-water relationships. Lovett and Duffield (47) identified benzylamine as an allelochemical in the leaf washings from the cruciferous weed Cametina sativa (L.) Crantz. Subsequent work showed benzylamine induced hydrophobic conditions in the soil, and these conditions could reduce water availability for plant growth (48). Thus, indirect action through changes in soil structure could be partially responsible for adverse effects on linseed (Linseed usitatissimm L.) and could enhance more direct allelopathic effects. 

Cuticular diterpenes-duvanes and labdanes. Cutler have found that the cuticular diterpenes of green tobacco have both allelopathic and insect-deterrent effects (38). Present in the cuticle are duvane and/or labdane diterpenes (Figure 3) The levels of these specific cuticular components are believed to be responsible for the observed resistance of some types of tobacco to green peach aphids Myzus persicae (Sulzer), tobacco budworm Heliothis virescens (F.), and tobacco hornworm Manduca sexta (L.) (39). 

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