Influence of light quality on common lambsquarters adaptive strategies
Invasive weeds can reproduce in a wide range of environments regardless of competition because plastic physiological mechanisms can accommodate prevailing conditions. However, potential adaptive traits responsible for this plasticity are unknown. Common lambsquarters ('Chenopodium album' L.) was used as a model invasive weed because, once native to Europe, it is widely dispersed in natural and agricultural lands across North America and the world. The adaptive traits of common lambsquarters to temperature, nitrogen (N) availability, light quantity (photosynthetic photon flux density, PPFD), and light quality (red to far-red light ratio, R/FR) stresses were examined in growth cabinet experiments. Novel traits identified in these studies that contribute to the invasive capabilities of this species include increased shoot/root ratio and increased height, biomass, leaf area and main-stem leaf number per mol of accumulated incident PPFD. Seed weight was unaffected by the stress treatments, but the carbon-nitrogen ratio of the seed decreased under the low PPFD treatments which could enhance germination. A field experiment was conducted using a similar treatment structure as the growth cabinet study to determine the maternal effects of N and light (PPFD and R/FR). In one year, seeds matured in a Low LIGHT Low N environment had greater seed recruitment when placed back into a similar offspring environment whereas in another year, seeds matured under both Low N environments had less seed recruitment. The maternal environment also influenced seedling biomass. These results suggest that plants can sense and respond to the environments in which they are growing and pass on traits that may or may not be useful to subsequent progeny depending on prevailing conditions. These studies challenge previous work on the role of R/FR on adaptive shade avoidance traits. In general, the results presented in this thesis suggest that although R/FR modified the morphological responses of common lambsquarters, low PPFD also triggered many canopy survival traits. Therefore, it is the expression of plastic traits in response to low PPFD and R/FR that have allowed this species to survive and persist in a variety of natural and agricultural ecosystems despite management efforts. Understanding how PPFD, R/FR, temperature and N promote the expression of morphological and physiological traits will be useful in the development and refinement of models of crop-weed interactions and invasive weed biology.