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

Concentrations of two groups of carbon-based defensive compounds, diterpeno id resin acids and phenolics in pine needles, responded differently to nitr ogen fertilization. Resin acid concentrations were higher on fertilized Sco ts pine trees in needles formed one year after the treatment compared to th e 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 c ontinuum of modes of storage of plant defensive compounds; cell vacuoles (p henolics) and multicellular cavities (diterpenoids). This dichotomy may exp lain why the two groups differed in their response to fertilization. Regres sion analyses revealed a positive relationship between resin acid concentra tion and number and size of resin ducts in relation to needle area. There w as a significant, but weak, negative relationship between phenolic concentr ation and number of resin ducts. These results raise the question whether, in addition to terpenoids, which are stored in complex compartments, phenol ics, which are stored in cells in the space between resin ducts. to some ex tent may depend on the space available for their storage. The relevance of these findings for plant defence theories and for expected risks of increas ed damage by different types of insect herbivores are discussed.