Nutrient-allelochemical interactions

Nutrient-Allelochemical Interactions in Host Plant Resistance... 

Many aspects of nutrient-allelochemical interactions are probably key factors In the suitability of a given plant species as a host for a particular Insect. At best, this may be a less than optimal situation for the insect, since even what appears to be a susceptible plant Is likely to be fairly well defended against Insect and pathogen attack. If the plant contains the essential nutrients for the Insect, but the utilization of these nutrients Is blocked In some way by allelochemlcs or by too much or too little water, then growth may be slowed. If, due to behavioral modifiers, the Insect will not feed on the plant, then plant nutrients are not available to the Insect. 

Reese (37) has studied nutrlent-allelochemical Interactions using diets supplemented by plant extracts. He has concluded that many of the deleterious effects may be due primarily to various Interactions between these allelochemical and essential nutrients. It Is Important to not only consider the presence of nutrients, but also the bloavallability of these nutrients to the phytophagous Insect. He cited findings where neonate larvae were found more sensitive to plant allelochemicals than older larvae, and Interpreted the greater resistance of the older larvae In terms of Increasing capability to utilize available nutrients, probably because of the Induction of enzymatic detoxification systems. 

The interactions between rates and efficiencies are complex, involving homeostatic mechanisms that influence an insect s behavior, physiology, growth and reproduction. These interactions are influenced by starvation, induction of preference, nutrients, allelochemics and various environmental (climatic) factors. Classification of a chemical as a repellent, deterrent, feeding suppressant, toxin or digestibility-reducer may be situation-dependent, dose-dependent, and involve chemicals, behavioral and physiological feedback systems (Blau et al., 1978 Duffey, 1977,1980 Reese, 1979 Grabstein and Scriber, 1981). 

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. 

If mycorrhizae are sites of action for allelochemicals, this is an important indirect aspect of allelopathic interaction among plants. Inhibition of mycorrhizal formation or a reduction in the efficiency of mycorrhizal association would reduce the nutrient level of the mycorrhizal plant and subsequently its competitiveness, stress tolerance or nodulation. Although allelochemicals have been implicated in the reduction of nodulation and nodule size, possible mycorrhizal involvement has not been examined. This is a difficult area of research but one that will provide better understanding of this complex situation. 

Several phenolic acids and many nonspecific allelopathic conditions have been shown to alter the mineral content of plants, and certainly phenolic allelochemicals may perturb cellular functions in a number of ways that are of importance to plant nutrition (34,35). However, raising fertility does not always suppress allelopathic inhibition, and the interrelationships between these two factors are still not clear. Bhowmik and Doll (36) showed that allelopathic inhibition of corn and soybeans by residues of five annual weeds was not alleviated by supplemental nitrogen or phosphorus. Similarly, an increase in fertilizer did not overcome inhibition of corn by quackgrass or circumvent the autotoxicity of berseem clover Trifolivm alexandrium L.) (37,38). Even when raising nutrient levels releases Inhibition, it does not mean that allelopathy was inoperative under the original conditions. These instances simply illustrate the importance of the Interaction of the two stress conditions. 

Slansky, F., Jr., Allelochemical-nutrient interactions in herbivore nutritional ecology, in Herbivores Their Interactions with Secondary Plant Metabolites Ecological and Evolutionary Processes, Vol. 2, Rosenthal, G. A., and Berenbaum, M. R., Eds., Academic Press, San Diego, 1992, chap. 4. 

Reese, J. C. (1979) Interactions of allelochemicals with nutrients in herbivore food. In Herbivores Their Interaction with Secondary Plant Metabolites (Rosenthal, G. A. and Janzen, D. H., eds) pp. 309-30. Academic Press, New York. 

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