Soil phosphorus mediates trade-offs between constitutive and induced defences in young pine trees.

Main conclusion: Nutrient availability, namely soil phosphorus, modulates trade-offs between constitutive and induced defences in maritime pine, with high phosphorus weakening these trade-offs and shaping plant allocation to different defensive strategies. Abiotic factors modulate trade-offs between plant functions, but their influence on trade-offs between constitutive and induced defences remains poorly understood. We tested for such trade-offs in maritime pine (Pinus pinaster) and examined whether soil phosphorus availability affected these defensive correlations. We conducted a greenhouse experiment with six-month-old pine seedlings from 33 half-sib families, exposing half of the plants from each family to either low or high soil phosphorus availability, one of the main limiting factors for pine development in the study region. Within each fertilization group, we applied jasmonic acid to induce defences in half of the plants per family. Defensive traits measured included resin production and phenolic compound levels. As predicted, we found significant negative correlations between constitutive and induced defences for both defensive traits under low phosphorus availability. However, these correlations were absent under high phosphorus conditions, indicating that overabundance of this nutrient weakened defensive allocation constraints. These findings highlight the role of nutrient availability in shaping plant defence allocation constraints, potentially shaping the correlated evolution of plant defensive strategies.