SPECIES-SPECIFIC RESPONSE OF GLUCOSINOLATE CONTENT TO ELEVATED ATMOSPHERIC CO2

The carbon/nutrient balance hypothesis has recently been interpreted t o predict that plants grown under elevated CO2 environments will alloc ate excess carbon to defense, resulting in an increase in carbon-based secondary compounds. A related prediction is that, because plant grow th will be increasingly nitrogen-limited under elevated CO2 environmen ts, plants will allocate less nitrogen to defense, resulting in decrea sed levels of nitrogen-containing secondary compounds. We present the first evidence of decreased investment in nitrogen-containing secondar y compounds for a plant grown under elevated CO2. We also present evid ence that plant response is species specific and is not correlated wit h changes in leaf nitrogen content or leaf carbon-nitrogen ratio. When three crucifers were grown at 724 +/- 8 ppm CO2, total foliar glucosi nolate content decreased significantly for mustard, but not for radish or turnip. Glucosinolate content of the second and fourth young est m ustard leaves decreased by 45% and 31%, respectively. In contrast, no significant change in total glucosinolate content was observed in turn ip or radish leaves, despite significant decreases in leaf nitrogen co ntent. Total glucosinolate content differed significantly among leaves of different age; however, the trend differed among species. For both mustard and turnip, glucosinolate content was significantly higher in older leaves, while the opposite was true for radish. No significant CO2 x leaf age interaction was observed, suggesting that intraplant pa tterns of allocation to defense will not change for these species. Cha nges in nitrogen allocation strategy are likely to be species-specific as plants experience increasing atmospheric CO2 levels. The ecologica l consequences of CO2-induced changes in plant defensive investment re main to be investigated.