CO₂-mediated changes of plant traits and their effects on herbivores are determined by leaf age

Concentration of atmospheric CO₂ is predicted to double during the 21st century. However, quantitative effects of increased CO₂ levels on natural herbivore-plant interactions are still little understood.In this study, we assess whether increased CO₂ quantitatively affects multiple defensive and nutritive traits in different leaf stages of cyanogenic wildtype lima bean plants (Phaseolus lunatus), and whether plant responses influence performance and choice behaviour of a natural insect herbivore, the Mexican bean beetle (Epilachna varivestis).We cultivated lima bean plants in climate chambers at ambient, 500, 700, and 1000 ppm CO₂ and analysed cyanogenic precursor concentration (nitrogen-based defence), total phenolics (carbon-based defence), leaf mass per area (LMA; physical defence), and soluble proteins (nutritive parameter) of three defined leaf age groups.In young leaves, cyanide concentration was the only parameter that quantitatively decreased in response to CO₂ treatments. In intermediate and mature leaves, cyanide and protein concentrations decreased while total phenolics and LMA increased.Depending on leaf stage, CO₂-mediated changes in leaf traits significantly affected larval performance and choice behaviour of adult beetles. We observed a complete shift from highest herbivore damage in mature leaves under natural CO₂ to highest damage of young leaves under elevated CO_2. Our study shows that leaf stage is an essential factor when considering CO₂-mediated changes of plant defences against herbivores. Since in the long run preferred consumption of young leaves can strongly affect plant fitness, variable effects of elevated CO₂ on different leaf stages should receive highlighted attention in future research.