Background
The authors say that "secondary metabolites present in plants provide protection against invaders because of their antimicrobial activity (Kamra et al., 2006)," and they indicate that "elevated CO2 leads to plants allocating more carbohydrate resources to their secondary metabolism (Agrell et al., 2004; Casteel et al., 2008)," which may thus induce them to "generate higher concentrations of defensive compounds that are toxic [to] herbivorous insects (Coviella and Trumble, 1999)." Preeminent in this group of compounds are condensed tannins, which are naturally-occurring secondary carbon compounds produced in the leaves of virtually all families of plants that comprise up to 50% of the dry weight of their leaves, while another such compound of note is gossypol, which is a natural toxin present in the cotton plant that helps to protect it from insect pests such as the cotton bollworm.

What was done
Wu et al. studied the allocation of the carbohydrate resources of two cotton (Gossypium hirsutum L.) cultivars -- transgenic Bt cotton (cv. GK-12) and non-transgenic Bt cotton (cv. Simian-3) -- to condensed tannins and gossypol both before and after injury inflicted upon the plants by the cotton bollworm (Helicoverpa armigera Hubner) over periods of 1, 3 and 12 hours in controlled-environment chambers maintained at atmospheric CO2 concentrations of either 370 or 750 ppm.

What was learned
Before any bollworm injury to the plants, the extra CO2 led to increases of 12 and 14% in the condensed tannin concentrations in the foliage of the Bt-transgenic and non-transgenic cotton plants, respectively, to increases of 10 and 10% in the gossypol concentrations of the transgenic and non-transgenic plants, respectively, and to a 4% decrease in Bt toxin in the transgenic plants. After bollworm injury for periods of 1, 3 and 12 hours, the non-transgenic plants experienced condensed tannin increases of 14, 9 and 9%, respectively, while transgenic plants experienced increases of 16, 9 and 9%, respectively. Corresponding results for gossypol were increases of 7, 10 and 6% for the non-transgenic cultivar and 7, 7 and 6% for the transgenic cultivar. And the transgenic plants also exhibited Bt toxin decreases of 3, 3 and 5%, respectively.

What it means
The four Chinese scientists state that prior studies have demonstrated that increases in condensed tannins and gossypol typically occur "in response to an increasing CO2 atmosphere, especially in combination with injury caused by herbivorous insects (Druy et al., 1998; Roth and Lindroth, 1994)," and they suggest that these increases "may compensate for the Bt toxin loss in the transgenic Bt cotton," which would appear to have been the case in their study as well. Thus, we may expect cotton to become ever less susceptible to damage by the cotton bollworm as the air's CO2 content continues to rise.

References
Agrell, J., Anderson, P., Oleszek, W., Stochmal, A. and Agrell, C. 2004. Combined effects of elevated CO2 and herbivore damage on alfalfa and cotton. Journal of Chemical Ecology 30: 2309-2324.

Casteel, C.L., O'Neill, B.F., Zavala, J.A., Bilgin, D.D., Berenbaum, M.R. and DeLucia, E.H. 2008. Transcriptional profiling reveals elevated CO2 and elevated O3 alter resistance of soybean (Glycine max) to Japanese beetles (Popillia japonica). Plant, Cell and Environment 31: 419-434.

Coviella, C.E. and Trumble, J.T. 1999. Effects of elevated atmospheric carbon dioxide on insect-plant interactions. Conservation Biology 13: 700-712.

Drury, S.J., Good, J.E.G., Perrins, C.M., Buse, A. and Kaye, T. 1998. The effects of increasing CO2 and temperature on oak leaf palatability and the implications for herbivorous insects. Global Change Biology 4: 55-61.

Kamra, D.N., Agarwal, N. and Chaudhary, L.C. 2006. Inhibition of ruminal methanogenesis by tropical plants containing secondary compounds. International Congress Series 1293: 156-163.

Roth, S.K. and Lindroth, R.L. 1994. Effects of CO2-mediated changes in paper birch and white pine chemistry on gypsy moth performance. Oecologia 98: 133-138.