Altered constitutive and inducible phloem monoterpenes following natural defoliation of jack pine: implications to host mediated interguild interactions and plant defense theories

Foliar feeding by the jack pine budworm Choristoneura pinus pinus altered the constitutive and induced monoterpene content and composition of Pinus banksiana phloem. In constitutive phloem tissue the ratios of alpha-pinene to beta-pinene decreased with defoliation intensity. Biochemical responses to simulated bark beetle attack were also influenced by defoliation intensity. Total monoterpene concentrations were always higher in induced than constitutive tissue, but the extent and rate of monoterpene accumulation, and also proportionate changes in monoterpenes during induction, varied with defoliation intensity. Quantitative and compositional changes in response to simulated bark beetle attack were also affected by host seasonal phenology and time since defoliation. There was a positive relationship between myrcene concentration and arrival by the bark beetle Ips grandicollis. Bark beetle arrival rates were more strongly associated with the chemical composition of constitutive than reaction phloem chemistry. However, chemical changes during induced host responses were also associated with beetle behavior. The ratio of alpha-pinene to beta-pinene during the first three days of induction was consistently associated with both defoliation by C. pinus pinus and susceptibility to I. grandicollis. These results show that direct injury to one plant tissue, foliage, cannot only cause chemical changes in another tissue, phloem, but such changes can be greater in the tissue not directly affected. The influence of defoliation intensity on monoterpene accumulation suggests a link between production and allocation of carbon within P. banksiana. The relationship between defoliation level and induced monoterpene concentrations was either parabolic or inverse depending on the interval following defoliation. Likewise, the relationship between plant growth and defoliation intensity varied with the length of time following defoliation. Thus, plant defense theories can be improved by incorporating the length of time during which the effects of a stress are exerted.