The effect of elevated carbon dioxide on carbon-based and nitrogen-based chemical defenses in Brassica nigra: physiological and evolutionary responses.

Abstract

Steadily increasing global atmospheric CO2 levels have been shown to affect important plant traits including growth and secondary (defensive) chemistry. This study used 10 maternal families of wild mustard, Brassica nigra, to investigate 1) whether levels of carbon-based defenses (phenolics) increase and nitrogen-based defenses (protease inhibitors) decrease in Brassica nigra under elevated CO2, as predicted by the Carbon/Nutrient Balance Hypothesis; 2) whether B. nigra encounters nitrogen-based trade-offs between investment in chymotrypsin and trypsin inhibitors, or between investment in these chemical defenses and growth; 3) if so, whether trade-offs are more severe under elevated CO2; 4) whether elevated CO2 will affect the evolutionary potential of carbon-based and nitrogen-based chemical defenses. Our results suggest that total biomass of Brassica nigra will increase under elevated CO2. In contrast to the predictions of the CNB Hypothesis, there was no evidence that levels of carbon-based defenses will increase or levels of nitrogen-based defenses will decrease in the future under elevated CO2. Our results also indicated that, in general, this population of Brassica nigra does not encounter trade-offs between nitrogen-intensive traits, and that trade-offs will not become more severe as atmospheric CO2 continues to rise. There was some evidence that elevated CO2 may increase the evolutionary potential of at least two traits within this mustard population, since a) families differed in phenology only when grown under elevated CO2, and b) growth under elevated CO2 also resulted in much more pronounced among-family differences in phenolic content.