Testing the systemic induced resistance hypothesis with Austrian pine and Diplodia sapinea

• Systemic induced resistance (SIR) was examined over time in the main stems and shoots of pines.
• Trees exhibited SIR in the main stems, consistent with the systemic induced resistance hypothesis (SIRH).
• Shoots of stem-induced trees were uniformly more susceptible to subsequent infection and did not validate the SIRH.
• Systemic levels of phenolics generally did not respond to inoculations, and were mostly not correlated with resistance.

Systemic induction of defenses (e.g. phenolic metabolites) is considered vital in conifer resistance to pathogens and insects, and forms the mechanistic basis of the systemic induced resistance hypothesis (SIRH). In this study, the SIRH was tested on juvenile Austrian pine. Main stems expressed SIR in a manner that was consistent with the SIRH, while shoots became uniformly more susceptible to subsequent inoculations, demonstrating clear organ specificity in the tree's response. The majority of phenolic metabolites were poorly correlated with phenotype. Thus, the defensive system of Austrian pine is highly plastic and organ specific, and cannot be predicted by phenolic profiles alone.