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

2. 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). 3. 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. 4. 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. 5. 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 CO2. 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.