Biochemical responses to ultraviolet-C radiation and methyl jasmonate in Pinus radiata seedlings that accompany induced resistance to Diplodia pinea




Irradiation of Pinus radiata seedlings with ultraviolet-C (UV-C) radiation or spray application of methyl jasmonate (MeJA) each resulted in induced resistance to subsequent wound inoculation with Diplodia pinea. Induced resistance was expressed by lower incidence of disease and by reduced size of stem lesions than in untreated seedlings. UV-C was more effective than MeJA and the induced resistance was greatest in seedlings that were irradiated with UV-C for 60 min, 1 week before pathogen inoculation. Induced resistance in the UV-C treated stems was concomitant with increases in peroxidase (POX) and polyphenol oxidase (PPO) activity but showed no correlation with concentrations of α-pinene and β-pinene or total phenolics. Furthermore, POX induction in stem tissue was cumulative and was greatest after three repeat treatments with UV-C at 6, 3 and 1 week before inoculation. In contrast to UV-C, MeJA induced a significant increase in β-pinene concentration in stem tissue but did not affect PPO activity. POX activity was induced by MeJA in stems, although to a lesser extent than by UV-C, but was not affected in needles. This appears to be the first report demonstrating the use of UV-C radiation to induce resistance to fungal infection in a coniferous species. The implications of the underlying differences between UV-C- and MeJA-mediated resistance to D. pinea are discussed.