The effect of carbon supply on allocation to allelochemicals and caterpillar consumption of peppermint.

The carbon supply of peppermint plants was manipulated by growing clonal propagules under three carbon dioxide regimes (350, 500 and 650 μl l-1). Feeding by fourth instar caterpillars of Spodoptera eridania increased with elevated CO2 hostplant regime, as well as with low leaf nitrogen content and by a high proportion of leaf volatile terpenoids. Leaf weight increased significantly with the increased carbon supply, but the amount of nitrogen per leaf did not change. The amount of volatile leaf mono-and sesquiterpenes increased proportionately with total leaf dry weight and hence was not influenced by CO2 supply. These results are consistent with ecological hypotheses which assume that allocation to defense is closely regulated and not sensitive to carbon supply per se.