Phloem phytochemistry and aphid responses to elevated CO ₂ , nitrogen fertilization and endophyte infection

Rising atmospheric CO 2 has been shown to alter plant nitrogen metabolism, growth and secondary chemistry. We hypothesized that altered aphid performance under elevated CO 2 is linked to phloem nitrogen chemistry. Rhopalosiphum padi performance on endophyte‐infected or uninfected tall fescue was examined under three levels of CO 2 (ambient, 800 and 1000 p.p.m.) and high and low nitrogen fertilization. Ethylenediaminetetracetic acid‐facilitated exudation was used to sample phloem sap, followed by quantification of relative amino acid concentrations using reverse‐phase high‐performance liquid chromatography. Aphid abundance was reduced at 800 p.p.m. relative to ambient CO 2 but returned to baseline at 1000 p.p.m. The density of aphids was reduced in both the elevated CO 2 treatments. Aphids were unsuccessful at colonizing endophyte‐infected plants, possibly as a result of the presence of loline alkaloids. Multivariate analysis showed that certain groups of phloem amino acids were altered by nitrogen fertilization and CO 2 . We found that four amino acids (valine, arginine, glutamine and aspartate) were correlated with aphid performance. These findings partially explained the effect of plant nitrogen fertilization and elevated CO 2 on aphids. The present study represents a first step toward providing a mechanistic explanation of the aphid performance changes that may result from rising atmospheric CO 2 .