La relation entre l'allocation aux composes secondaires et le taux de croissance relatif chez les Asteracea

Differences in resource availability have been shown to generate variation in defense chemistry in some species, but proper interspecific studies are rare and give conflicting results. This study focused on the trade-off between growth and production of chemical compounds of plants grown under different resource availabilities. I tested the hypothesis that: (1) contrasting plant species grown under controlled and enriched environmental conditions will show a negative correlation between their relative growth rates and their secondary compounds; (2) there is a negative correlation between mobile defenses (measured by the toxicity bioassay) and immobile defenses (measured by total phenol concentration) of plants grown under suboptimal environmental conditions; (3) plants grown under light-stressed conditions but optimal nutrient conditions will produce less carbon-based secondary compounds than plants grown under nutrient-stressed conditions but optimal fight conditions; and (4) plants grown under optimal environmental conditions will have a high RGR but will produce less secondary compounds than plants grown under stressful environmental conditions. Hypothesis 1 was tested experimentally using 31 species of Asteraceae grown with high levels of mineral and light resource availability under controlled conditions. Hypothesis 2 was tested in 20 species of Asteraceae by examining if there is any correlation between relative growth rate and secondary metabolism, and if there is any correlation between the type of chemical defense (mobile and immobile) in plants grown under controlled conditions of high light intensity but suboptimal levels of mineral nutrients. To test hypotheses 3 and 4, I conducted an experiment using six species of Asteraceae grown under 11 different combinations of light and mineral resources availabilities. Also, I investigated if there was any difference concerning the amount of specific secondary compounds, measured with HPLC produced by high light intensity and two levels of mineral resources availabilities in two of the six species (Chrysanthemum leucanthemum and Rudbeckia hirta). This thesis provided evidence that the resources availabilities affect the growth and the chemical parameters in different ways, but the data of this thesis shows no trade-off between growth and chemical defense when comparing species within a given experiment. The relative growth rate and the specific leaf area are affected by reducing the mean values under nutrient stress. In contrast, the root:shoot ratio increased under such conditions. As expected, leaf nitrogen content was reduced under nitrogen stress. The complication occurs in the parameters related to chemical defense; while total phenolics content increased, toxicity decreased. Finally, I did not detect the presence of any polyacetylenes or sesquiterpene lactones by HPLC in the leaves of the two species tested. The amount of pentaynene produced in the flowers of Rudbeckia hirta decreased significantly with age but increased significantly when the plant was subjected to a reduced nutrient availability.