Plant-insect interactions in elevated carbon dioxide: Effects on genetically-modified plants and ecological implications for plant defensive chemistry

In the 21st Century, atmospheric CO2 level will more than double. Current projections suggest an increase from our present level of 370 mul/l to 700--1100 mul/l. In elevated CO2, plants have a higher photosynthesis rate, which lead to lower nitrogen (N) concentrations. I asked three questions: (1) will changes in N and carbon availability in elevated CO2 affect plant allocation to defense; (2) is plant allocation conserved to the degree that transgenic plants containing genes which did not evolve in the species would respond as predicted by the Carbon Nutrient Balance hypothesis and; (3) will compensatory feeding by herbivorous insects lead to higher ingestion of commercially available insect pathogens?;Plant allocation to some defensive compounds was affected by elevated CO2. I showed that cotton plants increase polyphenols, and condensed tannins. This increased allocation was also mediated by soil N level. The results are consistent with the hypothesis of plants shifting allocation depending on the relative availability of carbon and N. Terpenoid aldehydes (TAs) were affected by N availability. However, only helicoide H4 showed an interaction effect between CO2 and N. Results for H4 closely matched those for polyphenols and condensed tannins, whereas other terpenoid aldehydes were lower in high N availability. This suggests that plants can shift allocation to TAs when soil N is high. The observation that, unlike polyphenols, isoprenoid-derived compounds were not affected by changes in CO2 levels demonstrates the complex biochemical apparatus that regulates synthesis of defensive compounds.;I found that in elevated CO2 plant allocation to N-based compounds was affected strongly enough to have a biologically meaningful effect on insect population parameters. Transgenic plants producing the Cry 1Ac toxin from Bacillus thuringiensis (Bt) produced significantly less toxin when grown in elevated CO2. This allocation was mediated by N availability. The reduced concentrations of the Bt toxin differentially affected development of the herbivore, Spodoptera exigua.;In plants grown in enhanced CO2, lower N concentrations lead to higher levels of ingestion of a commercially available Bt. Growth was slowed and mortality was increased for S. exigua larvae feeding on these plants. This suggests that other compounds requiring ingestion will have an increased effect in elevated CO2.