Genetic variation and phenotypic plasticity of nutrient re‐allocation and increased fine root production as putative tolerance mechanisms inducible by methyl jasmonate in pine trees

Summary 1.  Plant plastic responses to herbivore damage may include rapid, active reallocation of plant resources to reduce the impact of herbivory on future plant fitness. However, whether these inducible tolerance responses can be extended to pine trees and how these responses could be modulated by genetic and environmental factors remains unclear. 2.  Biomass allocation and phosphorus (P) and nitrogen (N) concentrations in above‐ and below‐ground tissues were measured in Pinus pinaster juveniles belonging to 33 open‐pollinated families grown under two P availabilities (P‐deficient and complete fertilization). Measurements were taken 15 days after half of the plants received a foliar spray treatment of 22 mmol L −1 methyl jasmonate (MJ) to simulate above‐ground herbivore attack. 3.  Simulated above‐ground herbivory promoted a strong preferential allocation of biomass below ground in the form of fine roots, leading to an almost two‐fold increase in fine root biomass in MJ‐treated plants and a significant reduction in above‐ground tissues and coarse roots. In addition, MJ signalling increased P and N concentrations in the shoots while reducing (P) or maintaining (N) concentrations in the roots. These results suggest that induced resource sequestration is not a generalized strategy in this pine species. Fine root biomass and concentration of N and P in plant tissues showed additive genetic variation, but responses to MJ signalling did not vary among families. Allocation of biomass to fine roots was not affected by P availability, whereas allocation of P to the shoot was more intense under complete fertilization. 4.   Synthesis: Two new putative tolerance mechanisms inducible by MJ signalling may help to minimize the impact of above‐ground herbivore damage on the future fitness of young pine trees by (i) allocation of carbon to fine roots, this appeared to be a generalized strategy with weak environmental modulation and (ii) reallocation of P and N from roots to shoots, which was largely affected by P availability, and thus susceptible to greater phenotypic variation in heterogeneous environments. We provide evidence that changes in tolerance‐related traits are rapidly inducible by herbivory cues in this pine species. These results should be integrated with induced resistance responses to fully understand the costs and benefits associated with induced responses to herbivory.