Physiological and biochemical responses in Pinus radiata seedlings associated with methyl jasmonate-induced resistance to Diplodia pinea

Effects of methyl jasmonate (MeJA) concentration and number of applications on the resistance of Pinus radiata to Diplodia pinea infection were investigated. Associated changes in defence-related enzymes and chemicals in stem tissue, shoot water relations, photosynthesis and plant growth are also reported. One foliar application of 1.0 mM or 4.5 mM MeJA, one week before inoculation, reduced D. pinea infection in seedlings by ∼60%. Increasing the number of applications did not improve disease control and, in the case of 4.5 mM MeJA, caused yellowing of needles. One application of 18 mM MeJA did not control disease, whilst three applications caused severe phytotoxicity leading to death in ∼50% of seedlings. Seedling growth rate was stimulated following one application of 1.0 mM MeJA, but otherwise exhibited a negative relationship with MeJA concentration and number of applications. A similar relationship was observed between MeJA and seedling fresh weight, dry weight and chlorophyll content. Activities of defence-related enzymes increased in stems after one application of 4.5 mM MeJA and after wounding. By four days after treatment, polyphenol oxidase activity (PPO) had increased 2.3-fold in MeJA-treated seedlings and 3.5-fold in wounded counterparts compared with untreated controls. Soluble guaiacol peroxidase (POX) activity was not induced by MeJA treatment but increased 4-fold after wounding. Both treatments induced a 2-fold increase in the activity of ionically bound POX after 4 days. There was also an increase of at least 40% in α- and β-pinene in the stems after MeJA application or wounding. It was concluded that the response of P. radiata to MeJA was dose dependent, with single applications of low concentrations (1–4.5 mM) promoting seedling health whilst higher concentrations or more frequent applications appeared to be phytotoxic.