Evidence of Phenotypic Plasticity in the Response of <Emphasis Type="Italic">Fagopyrum esculentum</Emphasis> to Population Density and Sowing Date

Phenotypic plasticity is an important attribute that enables plants to survive across a range of environments. We conducted two experiments to investigate the plasticity of architectural traits and biomass ratios for Fagopyrum esculentum Moench in response to population density and sowing date. These included (1) inter-planting distances of 5, 10, 15, or 20 cm; and (2) sowing on 25 July, 1 August, 5 August, or 10 August. Many traits exhibited phenotypic plasticity that was coupled with changes in plant size. However, variations in leaf/mass ratio from either experiment, as well as leaf/root ratios in response to sowing date, were independent of size. When coefficients of variation were computed, some consistency was found in the magnitude of trait plasticity for both density and sowing date. For each experiment, leaf/root ratios, leaf/mass ratios, and stem/mass ratios were most plastic. Although this suggests that biomass ratios could be more responsive to environmental changes, a wide array of traits should be considered if we are to fully understand the mechanism for these phenomena.     

Determinants of growth rate on chemically heterogeneous host plants by specialist insects

Nine Passiflora species growing sympatrically in a Costa Rican rain forest were screened for the presence of alkaloids, tannins, cyanogenic compounds, saponins and cardenolides. The first three chemical classes were found to be present and the Passiflora species could be classified into five groups according to the presence/absence of each chemical class. In order to assess the effects of this chemical variability on specialist herbivores, larvae of Heliconius ismenius and H. melpomane were experimentally introduced to seven species of Passiflora host plants growing naturally in the rain forest. Survival and growth rates were subsequently monitored at 2 day intervals and were found to be independent of each other. Larval growth rate and larval survivorship were not significantly affected by cyanogenesis in the host plants, nor by the presence of alkaloids, tannins and non-tanning phenolic compounds. Larval growth rate was found to be correlated to percentage nitrogen in the host plants, but was independent of host water content and non-structural carbohydrate content. Although these insects would be classified as host-specific by most authors, the results indicate the presence of a sufficiently general detoxication apparatus to consume many different classes of potential toxins with no apparent ill effects. We conclude that Passiflora natural product diversity does not cause major deleterious effects in Heliconius growth characteristics and that these effects are not the major component maintaining the current pattern of larval feeding specialization in this plant-herbivore system.     

Seasonal age-specific measurements of the sesquiterpene lactones and chromenes of Encelia farinosa

Age-specific concentrations of farinosin, a sesquiterpene lactone, and encecalin and euparin, both chromenes, were measured in two semi-desert populations of Encelia farinosa in California over a three month period. All three natural products differed significantly in concentration. The average concentration of encecalin is 10 times that of euparin and eight times that of farinosin. Farinosin concentrations did not differ between the two populations, but both euparin and encecalin were in higher amounts in the UC Riverside population than in the Home Gardens population. Seasonal changes in natural products differed at the 0.05 level for all populations and compounds except euparin in the UC Riverside population. Percent nitrogen content of dried leaves was similar in both populations, but differed between collection dates. Water content did not vary significantly with leaf age on one collection date. The significance of these results is discussed in the light of herbivore history.