Research was conducted to characterize the phenology of Chenopodium album growth parameters as influenced by biotic (crop environment) and abiotic (climate) factors, and to quantify the relationship between C. album and crop photosynthetic plasticity and C. album seed production. Weed-crop experiments included a Glycine max or Zea mays monoculture, and C. album at 4 plants m-2 with G. max, Z. mays , and in monoculture. Weed density experiments included C. album at three densities (4, 16, and 36 plants m-2 in Z. mays. C. album leaf area and plant height phenology differed among years and was heterogeneous within experiments. Conversely, crop leaf area and plant height phenology did not differ greatly among years or C. album densities and was less heterogeneous within an experiment than C. album. Weed relative leaf area and relative volume phenology differed among years and was due to differences in C. album leaf area and plant height phenology. Differences in C. album relative leaf area and relative volume among years may explain differences in crop yield response to weed infestations between sites and years. C. album photosynthesis at 4 plants m-2 was heterogeneous among plants within experiments and differed between years, but was greatest in monoculture and least in G. max. G. max and Z. mays photosynthesis per plant was less heterogeneous than that of C. album. C. album relative photosynthesis differed between years but was greater in Z. mays than in G. max. C. album seed production per plant at 4 plants m-2 was greatest in monoculture and least in G. max and was strongly correlated with early-season intercepted PAR (r = -0.73) and with maximum photosynthesis per plant (r = 0.91). These results indicate that the fitness of C. album was determined very early in the growing season, and was strongly influenced by the light quantity available and associated photosynthetic capacity.
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