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T.A. Ebert (12880 Welton Ln., Poway, CA 92064) discussed the relationship between plant nitrogen and aphid attacks. Nitrogen can be taken up by plants in two different forms, nitrate or ammonium. The soil nitrate to ammonium ratio can significantly affect growth and reproduction in plants. Less clear is the impact of the form of nitrogen on higher trophic levels and a plant's ability to deal with herbivory.
In hydroponic systems with sorghum, Sorghum bicolor, and kikuyu grass, Pennisetum clandestinum mixed nitrogen improved plant growth over solutions containing only one form. Solutions with 100% ammonium retarded plant growth less than did 100% nitrate. The form of available nitrogen influenced plant growth, and had a direct impact on aphid growth. The combined effect of nitrogen and aphid attack altered plant physiology.
The mechanism linking nitrogen treatments and aphid growth and reproduction are not clear. However, there were differences in amino acid composition among plants with different nitrogen treatments; amino acid content and carbohydrate-to-amino acid ratios are linked to changes in aphid development.
"It should be apparent that the form of available nitrogen plays a crucial role in plant physiology, that this effect is important for organisms on other levels of the food chain, and that this is a phenomenon that can occur under field conditions,- says Ebert. "These effects will produce habitats with a mosaic of vigorous and weakened plants. These weakened plants are then more susceptible to herbivores and disease. This could help explain the patchy nature of many diseases and insect outbreaks. Such outbreaks could be the result of an interaction between the 'founder effect' and plants weakened by nutritional status.
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Ecological Agriculture Projects, McGill University (Macdonald
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