Different responses of two carbon-based defences in Scots pine needles to nitrogen fertilization

Abstract

Concentrations of two groups of carbon-based defensive compounds, diterpenoid resin acids and phenolics in pine needles, responded differently to nitrogen fertilization. Resin acid concentrations were higher on fertilized Scots pine trees in needles formed one year after the treatment compared to the corresponding needle year-class on control trees. Phenolic concentrations, on the other hand, tended to be lower in fertilized trees than in control trees. These groups of defensive compounds represent two extremes in the continuum of modes of storage of plant defensive compounds; cell vacuoles (phenolics) and multicellular cavities (diterpenoids). This dichotomy may explain why the two groups differed in their response to fertilization. Regression analyses revealed a positive relationship between resin acid concentration and number and size of resin ducts in relation to needle area. There was a significant, but weak, negative relationship between phenolic concentration and number of resin ducts. These results raise the question whether, in addition to terpenoids, which are stored in complex compartments, phenolics, which are stored in cells in the space between resin ducts, to some extent may depend on the space available for their storage. The relevance of these findings for plant defence theories and for expected risks of increased damage by different types of insect herbivores are discussed.