Can phloem sap amino acid composition explain enhanced performance of the aphid Rhopalosiphum padi feeding on barley (Hordeum vulgare) under elevated CO2?

Background/Question/Methods

Atmospheric CO₂ concentration is projected to rise to between 550 ppm and 1000 ppm from the current ambient level of 390 ppm by the year 2100. Increasing atmospheric CO₂ is expected to alter plant metabolism in ways that will likely be relevant for insect herbivores. Studies have shown that while foliage feeder performance tends to decrease under elevated CO₂, results are much more equivocal for phloem feeding insects. Since phloem tissues are less accessible than whole plant tissues, much less is known about phloem biochemical responses to CO₂ and mechanisms driving changes in aphid performance. In this study we examined the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient and 700 ppm CO₂. We measured aphid population abundance and plant biomass responses with and without herbivory. We used aphid stylectomy to sample pure phloem from plants in ambient and elevated CO₂ conditions and HPLC to analyze phloem samples for amino acid concentrations.

Results/Conclusions

Plant biomass responses were dependent on the presence or absence of herbivores. Plant biomass was lower under elevated CO₂ in trials with herbivores present, due to a CO₂-induced increase in herbivore load. Aphid abundance increased by 154% under elevated CO₂ while density increased by 241% due to the combination of higher aphid abundance and lower biomass. There was a significant decrease in aphid development time under elevated CO₂, but fecundity was unchanged. We found significant increases in essential amino acid concentration and the ratio of essential to non-essential amino acids under elevated CO₂. Though there was a general tendency for most amino acids to increase under elevated CO₂, increases in the total concentration, and the concentration of non-essential amino acids, were not significant. Our results suggest that enhanced aphid performance under elevated CO₂ may be linked to changes in amino acid levels, which are known to be limiting nutrients in the aphid diet.