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| Carbon allocation in beech trees and developmental success of a herbivorous insect |
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M. Henn
Institute of Applied Zoology; Department of Ecology; TU München
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Like in every organism, phytophagous insects are influenced by the quality of their food. It is well known that carbohydrates, and especially proteins and amino acids are important nutrients to ensure successfully the development. Moreover, it is a fact that herbivorous insects are sensitive to compounds of the plant's defence metabolism, mainly to phenolic constituents.
We proofed in our investigations whether elevated CO2 and/or additional N- fertilization cause a shift in carbon allocation between primary and secondary metabolism and when it occurs, how this may influence the development of a herbivorous insect. The hypothesis was that an increase in primary compounds (carbohydrates, proteins and amino acids) may be advantageous for the growth of a phytophagous insect, whereas an enhancement of the amounts of phenolic compounds may affect the development negatively. The investigated plant - herbivore system was beech (Fagus sylvatica) and gypsy moth (Lymantria dispar).
N - fertilization of 2 years old beech trees resulted in a significant increase in the concentrations of protein bound amino acids in the leaves, whereas CO2 treatment caused a significant increase in the amount of the soluble carbohydrates fructose and glucose. In addition, the concentrations of particular phenolic compounds were increased or decreased under elevated CO2. The most pronounced changes, however, were found in leaves of beech trees kept under high CO2 combined with additional N-fertilization. These leaves contained increased levels of soluble carbohydrates and particular phenolic compounds.
The larvae of the gypsy moth L. dispar showed decreased consumption rates per day (RCR), but increased efficiencies of conversion of ingested food into body substance (ECI), when they fed on beech trees which were kept under elevated CO2 combined with high N - fertilization. This means, they fed less of the leaf materials, but transformed the ingested food highly efficient into larval biomass. The weight gained, the absolute, relative growth and percentage of growth of the larvae remained unaffected. It is discussed that RCR and ECI may function as a mechanism to compensate imbalances of food quality to maintain growth rates.
However, significant differences in the development of the gypsy moth larvae were observed when they fed on leaves of beech trees infected with the root fungus Phytophthora citricola: Larvae on infected beech trees which were kept under elevated CO2 combined with high N - fertilization, showed decreased weights and high mortality rates. The role of a beech pathogen as an elicitor of the tree's defence mechanism is discussed.
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