Genetic and environmental factors behind foliar chemistry of the mature mountain birch.

Previous studies of mountain birch (Betula pubescens spp. czerepanovii) repeatedly have found differences between individual trees in herbivory-related traits, but rarely have yielded estimates of the additive genetic variation of these traits or of their relationship to habitat. We used thirty-year-old birch half-sibs in a northern common garden to estimate the effect of genetics and local microhabitat on resistance-related traits. Genetic estimates of foliar chemistry have been studied only rarely with trees as old as these. Moth performance (Epirrita autumnata), rust (Melampsoridium betulinum) incidence levels, and the general level of natural herbivory damage to individual trees were used as direct measures of birch resistance. Chemical resistance-related traits in plant chemistry included 15 individual phenolics, 16 amino acids, and phenoloxidase activities in the foliage. We also followed birch phenology and growth. Our results show that the genotype of the birch was the most important determinant of phenolic composition and phenoloxidase activity, but that amino acid levels were best explained by the microhabitat of the birch. We also found that the phenology of the birch had a high heritability, although its variation was low. Our results reveal rich genetic variation in birch chemistry.

Immunological memory of mountain birches: effects of phenolics on performance of the autumnal moth depend on herbivory history of trees.

Plants have been suggested to have an immunological memory comparable to animals. The evidence for this, however, is scarce. In our study with the mountain birch -- Epirrita autumnata system, we demonstrated that birches exposed as long as 5 yr to feeding of E. autumnata larvae (delayed induced resistance, DIR), responded more strongly to a new challenge than trees without an herbivory history. Pupal weights remained lower, and the duration of the larval period was prolonged in the DIR trees, although immunity, measured as an encapsulation rate, was not affected. We further demonstrated that the effects of birch phenolics on performance of E. autumnata were different in the exposed (DIR) trees from naive control trees, although we found only one significant change in chemistry. The quercetin:kaemferol ratio was increased in DIR trees, suggesting that herbivory caused oxidative stress in birches. In DIR trees, phenolics, especially hydrolyzable tannins (HTs), affected pupal weights negatively, whereas in control trees, the effects were either nonsignificant or positive. HTs also prolonged the duration of the larval period of females, whereas peroxidase (POD) activity prolonged that of males. We suggest that the causal explanation for the induced resistance was an enhanced oxidation of phenolic compounds from the DIR trees in the larval digestive tract. Phenolic oxidation produces semiquinones, quinones, free radicals, and ROS, which may have toxic, antinutritive, and/or repellent properties against herbivores.

Foliar phenolics are differently associated with Epirrita autumnata growth and immunocompetence.

The quality of available food may affect insect herbivores directly (via growth and survivorship) and/or indirectly (by modifying insect vulnerability to parasitoids and pathogens). We examined the relationship between different phenolic compounds, belonging to various phenolic groups, in Betula pubescens spp. czerepanovii (mountain birch) foliage and the larval performance of the geometrid Epirrita autumnata (autumnal moth). Direct effects on insect performance were described by pupal weight, developmental rate, and survivorship; indirect effects were described by the encapsulation rate of an implant inserted into the insect hemocoel, a commonly used way to describe insect immune defense. We found profound differences in the effects of different phenolic categories: several individual hydrolyzable tannins were associated positively with larval performance but negatively with level of immune defense, whereas flavonoid glycosides were inversely related to larval survival but showed no association with the larvae immune defense.

Temperature as a modifier of plant-herbivore interaction.

Temperature directly affects the growth, survival, and development rates of poikilothermic insect herbivores; it may also have an important indirect impact, via the activities of plant defensive enzymes. The effects of wounding birch leaves and temperature on the growth and development rates of a Lepidopteran moth, Epirrita autumnata, were studied. We also examined the activities of a mountain birch (Betula pubescesns spp. czerepanovii) defensive enzymes, specifically the polyphenoloxidases (PPOs), in relation to temperature and wounding. The optimal temperature for early instars in terms of survival and developmental rates was between +15 and 20 degrees C. Wounding treatment had different effects on birch PPO activity depending on the temperature: at +12 degrees C, wounding decreased the activity, suggesting induced amelioration at that temperature, whereas at +25 degrees C, wounding increased the activity, suggesting induced resistance. However, larval growth was retarded slightly, but significantly, on the leaves of wounded twigs at both temperatures. Both PPO activity and larval growth rates were affected within 12 h, indicating the existence of a transcription- and translation-independent defense system in birch leaves. We suggest that underlying the increase in PPO activity and the decrease in larval growth rate may be H2O2, which has been shown to accumulate in response to wounding. Our results also provide a possible biological mechanism for the hypothesis that low temperatures promote the success of E. autumnata and other Lepidopteran larvae via decreased defensive enzyme activities of host plants at lower temperatures.