Plant allocation to defensive compounds of transgenic Bt cotton in response to infestation by cotton bollworm under conditions of elevated CO2

Allocation of defensive compounds of transgenic Bt cotton grown under conditions of elevated CO2 in response to infestation by cotton bollworm, Helicoverpa armigera (Hubner) was studied in two successive years (2004 and 2005). Both CO2 levels and bollworm infestation affected allocation of defensive compounds of non-transgenic cotton and transgenic Bt cotton. Both significantly higher foliar condensed tannin and a higher carbon/nitrogen ratio for non-transgenic cotton and transgenic Bt cotton were recorded under elevated CO2 compared to ambient CO2. This result accords with the Carbon Nutrient Balance hypothesis linking limiting nitrogen and excess carbon in cotton plants in response to elevated CO2. Furthermore, significantly higher levels of catalase (CAT) and superoxide dismutase (SOD) and a significantly lower level of true choline esterase (TChE) were also observed in H. armigera fed on transgenic Bt cotton compared with those fed on non-transgenic cotton. Also, there were significant interactions between cotton variety and bollworm infestation time on CAT and SOD activities. Our results indicate that plant allocation to defensive compounds will be affected sufficiently to affect plant-herbivore populations interactions.