http://science.sciencemag.org/content/sci/171/3973/757.full.pdf

Allelochemics: Chemical Interactions between Species

INTRODUCTION ........ ...... ...... ....... ... ....... ......... 479 THE NATURE OF SECONDARY COMPOUNDS .. ... ..... 480 Introduction 480 Distribution of Secondary Compounds in Space and Time 481 Function of Secondary Compounds 483 Secondary Compounds as Waste Products 483 Secondary Compounds as Protective Agents 484 SECONDARY COMPOUNDS AND PLANT DISEASES .... ..... ... .. ........ .. 485 Introduction 485 Defenses on the Surface of Plants 485 Defensive Function of the Cell Wall ........ ...... ...... ........ 486 Che"1ical Defenses within the Cell .... ...... ...... ........ ........ 486 The Role of Tannins 487 The Role of Phytoalexins 488 SECONDARY COMPOUNDS AND ALLELOPATHY ......... ... ..... ....... ..... ..... ......... 489 Introduction 489 The Complexity of Allelopathy 489 Allelopathic Activity of Secondary Compounds 490 Nonprotein Amino Acids and Allelopathy 490 SECONDARY COMPOUNDS AS INSECT DETERRENTS .. ... .. ... ....... 490 Introduction 490 Insect Chemoreceptors 491 Insect-deterrent Properties of Secondary Compounds 492

Secondary compounds as protective agents

The latest previous comprehensive review of allelopathy (Rice, 1974) covered research done primarily prior to 1973. There have been numerous reviews published in recent years on specific phases of allelopathy (see III), but most of them covered research results published primarily through 1972 also. This review has been restricted, therefore, largely to research conducted subsequent to 1972, except for background information and significant papers overlooked in previous reviews. There has been almost an exponential increase in the rate of publication of papers concerned with allelopathy. Research has been particularly active in relation to the roles of allelopathy in agriculture, forestry, phytopathology, patterning of vegetation, algal succession, and old-field succession. Our increasing knowledge of the conditions under which certain crop residues cause allelopathic effects to subsequent crops should enable us soon to guard against such effects. We are on the threshold of breeding crop plants that will inhibit the chief weeds in a given area through allelopathic action, and thus decrease the need for synthetic weed killers. Our understanding of allelopathic interactions of various plant species has already been used advantageously in reforestation, and future developments are very encouraging. Our increasing knowledge of allelopathy is aiding greatly in our understanding of many ecological phenomena, such as succession and patterning of vegetation. Evidence is mounting that inhibition of nitrification increases as succession progresses toward the climax vegetation, at least in many vegetation types. This leads to a decrease in the loss of nitrogen. Addition of inhibitors to arable lands to prevent nitrification has proved to be valuable in preventing loss of nitrogen and in increasing crop yields. There has been a rapid advance in our knowledge of mechanisms of action of known allelopathic compounds, at increasingly more fundamental levels. We have known for several years, for example, that some of these compounds inhibit uptake of various minerals, but have not known the mechanism of the action. Recent research indicates this may be due, at least in part, to inhibition of a plasma membrane ATPase that is involved with ion absorption. A thorough knowledge of mechanisms of action of allelochemics is essential to our overall understanding and appreciation of the field of allelopathy.

Allelopathy—An update

Allelopathy in Agroecosystems

The present review deals with the phenomenon of autotoxicity - a type of intraspecific allelopathy, where a plant species inhibits the growth of its own kind through the release of toxic chemicals into the environment. This phenomenon has been reported to occur in a number of weeds and crop plants in agroecosystem and wastelands causing the soil sickness. Besides, it plays a significant role in the orchards (of apple, pear, grapes, etc.) where it is the major reason of the replant problem, natural forests and coffee and tea plantations causing the regeneration problems. Not only the higher plants, but even some ferns and algae are also reported to show this phenomenon. Some plants have even developed extensive mechanisms to overcome this phenomenon, whereas the others have adapted to it by making structural and ecological changes providing to them a competitive ecological advantage over the others. Although autotoxicity is a natural phenomena providing selective benefit to the plant, yet the chemicals responsible for this have good potential for weed and pest management.

Autotoxicity: Concept, Organisms, and Ecological Significance

Substances potentially involved in allelopathy are liberated from plants by (a) leaching of foliage by rain, (b) abscission and litter fall, (c) volatilization from foliage, and (d) root exudation. Substances, including metabolites such as mineral nutrients, carbohydrates, amino and organic acids, and growth regulators, can be leached from a wide variety of plants by rain and dew, and the quantity and quality of losses are affected by a great number of both external and internal factors. Materials leached from one plant may have an influence upon the development of the same or other adjacent plants. Plant/plant chemical interactions have been well recognized in commercial agriculture and, in fact, form the basis of many common agricultural practices. They are currently being utilized in modern plant science in the development of bioassay systems for detecting growth regulators, the use of rootstocks to influence the growth and development of scions, in detection and eradication of diseases, and in fruit storage and ripening.

H. B. Tukey

Papers

Implications of allelopathy in agricultural plant science