Elevated CO2 shifts the focus of tobacco plant defences from cucumber mosaic virus to the green peach aphid

Elevation in CO₂ concentration broadly impacts plant physiological characteristics, which influences herbivores and biotrophic pathogens, which in turn regulate the plant defensive response. In this study, responses of tobacco plants to stress in the form of the green peach aphid, Myzus persicae (Sulzer), or cucumber mosaic virus (CMV), or both aphid and CMV combined were investigated in open‐top chambers under ambient and elevated CO₂ concentrations. We measured aboveground biomass and foliar chlorophyll, nitrogen, non‐structural carbohydrates, soluble protein, total amino acid and nicotine content in tobacco plants and also measured aphid population dynamics, body weight, honeydew production and anti‐oxidative enzyme activities in individual aphids. Plants produced more secondary metabolites for defence in both CO₂ treatments when treated with aphid and CMV combined than with either alone. Aphid density significantly increased on CMV‐infected tobacco plants (relative to uninfected plants) under ambient CO₂ but not under elevated CO₂. This suggests that plant defences against virus and aphid would be more efficient under elevated CO₂. Plant defence appears to shift from plant virus to aphid under increasing CO₂ levels, which highlights the potential influences of multiple biotic stressors on plants under elevated CO₂.