Alleochemical and Attractants

I. Alleochemicials are compounds produced by individuals to inhibit the growth of other individuals of their own species or other species.

Examples of water soluble alleochemical:

Where the right humid conditions exist for regular leaching of organic material from plant leaves.

Example 1. In the subhumid deciduous forests of South Carolina, for example, two trees, Quercus falcata (Fagaceae) and Liquidambar styraciflua, inhibit undergrowth within the dripline of the leafy crowns.

Where there is:

  • high rainfall,
  • an abundance of mineral nutrients and,
  • the lack of shading phenomena

there is the likelihood of allelopathic effects operating according to Muller and Chou (1972).

In the case of Quercus falcata, salicyclic acid has been isolated from the leaf leachate and shown to be toxic in bioassays.

Example 2. Bracken fern ( Pteridium aquilinum) is a highly successful weed worldwide and it is so dominant that herbs are rarely ever found in bracken stands.

As studied in Southern California (Gliessman and Muller, 1978), it has been found that phytotoxins leached by rain from dead, standing bracken fronds are largely responsible for this herb suppression. Caffeic and ferulic acids are probably major components.

Example 3. Inhibition of seed germination of grasses and other herbs (hydroxybenzoic and hydroxycinnamic acids) by Larrea and Franseria.

In desert shrub communities a number of shrubs release a number of toxic phenolic molecules to the soil through rainwater creating bare zones between the shrubs and grassy areas. Annual herbs would grow but for 2 factors - small animals that live in the shrubs consume them and secondly, the shrubs themselves give off phenols which inhibit the herbs and grasses.

These compounds are found in the soils as well as in the leachates.

Mixtures of related phenolic acids are present may be important since these acids can act synergistically on plant growth. It has been shown, for example, that vanillic and p-hydroxybenzoic acids are more effective inhibitors of seed germination when applied together than when applied separately (Einhellig and Rasmussen, 1978).

Phenolic substances are normally present in leaf tissue mainly in bound form but they are known to be turned over within the plant tissue so that the fact that the leachate contains significant amounts of free as well as bound phenolics

Some acidic phenols could arise in the soil by the microbial decomposition of leaf-litter of some age. Turnover rate of these compounds in the soil is also not completely known, so that the effectiveness of these phenolics as germination inhibitors is in some question.

 

Nair, M.G.; Whitenack, C.J.; Putnam, A.R. Journal of chemical ecology v. 16 (2): p. 353-364; 1990 Feb.

Abstract: 2,2'-Oxo- 1,1'-azobenzene (AZOB), a compound with strong herbicidal activity, was isolated and characterized from a soil supplemented with 2,3-benzoxazolinone (BOA). A parallel experiment with6-methoxy-2,3-benzoxazolinone (MBOA) yielded AZOB as well as its mono- (MAZOB) and dimethoxy- (DIMAZOB) derivatives. These compounds were produced only in the presence of soil microorganisms, via possible intermediates, I and II, which may dimerize or react with the parent molecule to form the final products. In the case of MBOA, it was shown that demethoxylation precedes the oxidation step. Although BOA and 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one (DIBOA) were leached out of rye residues, there were no detectable amounts of the biotransformation products in the soil. When BOA was mixed with soil and rye residue, either under field conditions or in vitro, AZOB was detected. Levels of free BOA in the soil were greatly reduced by incubation with rye residue. AZOB was more toxic to curly cress (Lepidium sativum L.) and barnyardgrass (Echinochloa crusgalli L.) than either DIBOA or BOA.

What happens in dry climates?

Some desert shrubs (Artemsia and Salvia) which invade desert grasslands release aromatic terpenes such as camphor shown below. From the air they deposited onto the soil and can accumulate if no rain occurs - they inhibit the growth of seedlings of other species.

Volatile Alleochemical. A number of examples are given below:

You may be familiar with the smell of some of these compounds.

Biochemicals used by plants to attract attention:

Selection of food by man and other mammals is inevitably a complex matter involving flavor, taste, palatability, color and odor.

It is often difficult to determine the controlling factor in situations where the detection system is so highly sophisticated.

Although human tastes are classified within four groups salty, sweet, bitter, sour&emdash;the palate can respond to and recognize a variety of shades within these groups and similarly the human nose can differentiate between many scents and odors .

A major attractant in mammals is undoubtedly to sweetness. Indeed, if we are to believe Yudkin (1972), man's insatiable sweet tooth is a dangerous feature in his makeup, leading him to overconsume sugar and then develop a variety of disease symptoms as a consequence.

To help enhance the sensation of sweetness, some plants produce the compound miracularin: it is a glycoprotien that occurs in Miracle Fruit - it eliminates sourness or acidity without disturbing the sweet response. With this molecule a sour lemon will taste sweet.

Another flavor enhancer or potentiator you may have had experience with is MSG.

Repellents are perceived as sharpness, bitterness and astringency. In man, whose taste responses are highly developed, a balance between sweetness and acidity or astringency is often necessary and both tastes are commonly present in foods and drink

 

Some characteristic odors and chemicals that can represent them

Almond-Nitrobenzene

Onion-like -Dimethylsulphide

Rancid -Valeric acid

Fruity - Benzyl acetate

Minty - menthol

Disinfectant -Phenol

Soapy Stearic acid

Sweaty - Iso-Valeric acid

Musk-like Exaltolide

Petrol-like Benzene

Sour Acetic -acid

Malty - Isobutyraldehyde

Cool -Camphor

Faecal -Skatole

Fragrant f-Ionone

Sweet - Vanillin

Heavy - Coumarin

Spermous 1-Pyrroline

Floral - Hydroxycitronellal

Fishy - Trimethylamine


One question one might ask of one self, is why are these plants producing these mimicy odors? fecal makes sense for a plant like the arums which are trying to attract flies, but some of the others?

' Based on the opinion of seven odor experts (Harper, 1975). Assessment of the odor of a particular chemical is subjective and the same compound can represent different odors to different people. In addition, with most odors, more than one individual chemical may be chosen to reprecent it.

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