Response of extrafloral nectar production to elevated atmospheric carbon dioxide

Extrafloral nectar attracts ants, whose presence provides protection for the plant against herbivores. Extrafloral nectar is thus a critical component of many plant-insect mutualisms worldwide, so environmental perturbations that alter extrafloral nectar production or composition could be disruptive. The carbon-nutrient balance hypothesis predicts that under elevated CO₂ the total volume of extrafloral nectar will increase but the proportion of the foliar carbohydrate pool secreted as extrafloral nectar will decrease, without any change in the sugar composition of the extrafloral nectar. We investigated the impact of elevated atmospheric CO₂ on extrafloral nectar in an Australian wild cotton species, Gossypium sturtianum J.H.Willis. Under elevated CO₂ there was an increase in the proportion of leaves actively producing nectar and a decrease in the nectar volume per active leaf. Elevated CO₂ did not affect the total volume or composition of extrafloral nectar, but there was a change in how the nectar was distributed within the leaf canopy, as well as evidence of increased turnover of leaves and earlier onset of flowering. By the end of the study, there was no difference in the total resources allocated to extrafloral nectar under elevated CO₂, which contrasts with the predictions of the carbon-nutrient balance hypothesis. Developmental changes, however, could affect the timing of extrafloral nectar production which could, in turn, alter the foraging patterns of ants and their defence of plants.