Traumatic resin ducts induced by methyl jasmonate in Pinus spp

Key message: Exogenous MJ does not alter the resin duct structure of pines in the cortex, but increases the number, density and mean size of resin ducts in the secondary xylem, particularly in Pinus sylvestris and P. radiata. Abstract: Methyl jasmonate (MJ) is an organic compound capable of modulating defence responses in plants. Exogenous application of MJ has been shown to modify the structure of the resin canal system in conifers by inducing the formation of dense concentric bands of traumatic resin ducts in the xylem. Because inducibility of resin ducts has been little explored across pine species, 3-year-old Pinus pinaster, P. pinea, P. sylvestris and P. radiata trees were sprayed with 0, 25 and 50 mM solutions of MJ, and plant growth, external symptoms and histology were assessed 60 days after treatment. Exogenous application of MJ diminished primary or secondary growth in all species (61 and 25%, respectively) and caused needle damage in P. sylvestris. Exogenous MJ did not alter the resin duct structure of Pinus spp. in the cortex. In the secondary xylem, however, the effect of MJ differed considerably between species: ducts, duct density and area covered by ducts increased in P. sylvestris and P. radiata, but not in P. pinaster and P. pinea. In MJ 50 mM-treated trees, resin duct abundance peaked at the first half of the ring in P. sylvestris and at one-third distance from the previous ring in P. radiata. In MJ treated P. radiata trees, large traumatic axial resin ducts, 120–160 µm in diameter, were aligned in the secondary xylem. The study helps to elucidate macroevolutionary aspects of inducibility of resin-based defences in the genus Pinus, where varying responses to MJ could reveal different defence strategies among species.