LITERATURE REVIEW 



In recent years, natural plant chemicals have been shoim to influence the eco- 

 logical relationships of plant-plant, plant-animal, and plant-insect interactions 

 (Sondheimer and Simeone 1970, Rice 1974) . Plant-plant chemical interaction, or allelo- 

 pathy, is defined by Rice (1974) as "any direct or indirect harmful effect by one plant 

 (including micro-organisms) on another through the production of chemical compounds 

 that escape into the environment." This definition provides for three factors that 

 must be present in order for allelopathy to be functioning in a plant community: 

 (1) a plant must produce chemicals, (2) these chemicals must get out of the plant and 

 into the environment, and (3) once in the environment, they must have a harmful effect 

 on the plants in the area either directly or indirectly. If any one of these three 

 requirements is absent, then allelopathy is not functioning. It is also important to 

 emphasize that allelopathy is independent of competition. Allelopathy involves the 

 placement of natural chemicals into the environment, whereas competition involves 

 the rivalry for environmental factors such as light, water, nutrients, etc. (Rice 1974). 



The mechanisms for releasing phytotoxins into the environment are quite variable 

 and dependent upon environmental characteristics. The mechanisms include the leaching 

 of the aerial portions of the plant by rain or fog drip, exudation from the roots, 

 residue left or produced during the decay of plant material, and volatilization 

 (Whittaker 1970, Rice 1974). 



Whittaker (1970) considers allelopathy, or the use of a chemical defense by plants, 

 as a universal phenomenon; however, he states that the actual significance of allelo- 

 pathy is difficult to judge for three reasons. First, the influence of chemistry on a 

 community is not always conspicuous to the observer of community relationships; second, 

 there are different degrees of chemical interaction; and third, species may have an 

 evolved tolerance to the chemical defense of others. 



Allelopathy or chemical defense in plants is usually thought of as a mechanism 

 by which one species is reduced or eliminated by another species in a community. Occa- 

 sionally, the chemicals released are not species specific and they are toxic to the 

 producer species as well. An excellent example occurs in the old-field succession in 

 central Oklahoma and southeast Kansas. 



Old-field succession in Oklahoma is characterized by a 2- to 3-year pioneer weed 

 stage, 9- to 13-year annual grass stage, a perennial bunch grass stage of 13 to 30+ 

 years, and finally true prairie, which has never been reached in old-field succession. 

 The initial pioneer weed stage is short because these plants have been shown to elimi- 

 nate themselves through the production of phytotoxins. These compounds have little 

 effect on the species of the annual grass stage, which then became abundant (Rice 1974) . 

 A similar phenomenon has been observed in the replant problem of fruit trees including 

 cherries, grapes, apples, peaches, apricots and plums. When seedlings of these species 

 are replanted in orchards of the same species, they frequently exhibit retarded shoot 

 and root growth or occasionally even die. Young trees that produce fruit provide lower 

 yields than trees of comparable age planted in nonorchard sites. Detailed studies with 

 peaches and apples indicate that compounds produced by the trees, particularly in the 

 roots, are broken down or rearranged by micro-organisms to toxic compounds that retard 

 seedling growth (Rice 1974) . 



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