Influences of elevated CO2 and pest damage on the allocation of plant defense compounds in Bt-transgenic cotton and enzymatic activity of cotton aphid

Plant allocation to defensive compounds by elevated CO2-grown non-transgenic and transgenic Bt cotton in response to infestation by cotton aphid, Aphis gossypii (Glover) in open-top chambers under elevated CO2 were studied. The results showed that significantly lower foliar nitrogen concentration and Bt toxin protein occurred in transgenic Bt cotton with and without cotton aphid infestation under elevated CO2. However, significantly higher carbon/nitrogen ratio, condensed tannin and gossypol were observed in transgenic Bt cotton GK-12 and non-transgenic Bt cotton 'Simian-3' under elevated CO2. The CO2 level and cotton variety significantly influenced the foliar nitrogen, condensed tannin and gossypol concentrations in the plant leaves after feeding by A. gossypii. The interaction between CO2 level x infestation time (24 h, 48 h and 72 h) showed a significant increase in cotton condensed tannin concentrations, while the interaction between CO2 level x cotton variety significantly decreased the true choline esterase (TChE) concentration in the body of A. gossypi. This study exemplified the complexities of predicting how transgenic and non-transgenic plants will allocate defensive compounds in response to herbivorous insects under differing climatic conditions. Plant defensive compound allocation patterns and aphid enzyme changes observed in this study appear to be broadly applicable across a range of plant and herbivorous insect interactions as CO2 atmosphere rises.