Weeds are often portrayed as growing in resource-rich environments. However, weeds growing within crops often deal with variable nitrogen (N) availability and reduced levels of light quantity and quality as a result of the crop canopy. In order to explore how weeds adapt to such stressful growing conditions, growth-cabinet studies were conducted using common lambsquarters as a model weed to determine how light, defined in terms of photosynthetic photon flux density (PPFD) and quality (red to far-red light ratio [R/FR]), and N stress influence the expression of adaptive traits that contribute to survival. Development rate of common lambsquarters was not influenced by low N; however, low N in addition to low R/FR delayed the rate of leaf appearance. Main-stem leaf number was reduced by low PPFD but was insensitive to N and R/FR. Neither doses of N had any influence on the shoot-to-root ratio. Plants also responded to the interaction of light and N. Under low PPFD and high N, plants adapted by growing taller, increasing biomass allocation to leaves, and producing more leaf area per mol of accumulated incident PPFD. Plants adapted to the most stressful treatment combination of low PPFD and low N by producing thinner leaves and increasing inflorescences per mol of accumulated incident PPFD. Seed production was reduced under low PPFD, but 1,000-seed weight and carbon concentration was unaffected. Although reduced in number, the total N concentration of the seed increased under low PPFD treatments, especially under low N. The adaptive traits identified in this study provide a greater understanding of the survival and persistence of common lambsquarters.
Nomenclature: Common lambsquarters, Chenopodium album L.