Estimates of physiological determinants for Amaranthus retroflexus

Redroot pigweed is a troublesome weed in the sorghum-growing regions of Nor th America. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of redroot pigweed density and environmental conditions on physiological determinants of redroot pigwe ed growth: duration of plant growth, light interception, radiation-use effi ciency, and dry matter partitioning. In addition, specific leaf area was de termined. Redroot pigweed was seeded at monoculture densities of 2, 4, and 12 plants m(-1) of row each year at each location. Duration of redroot pigw eed growth was not influenced by plant density. Light interception was defi ned as a simple exponential function of leaf area index. Specific leaf area did not change over the season and averaged 135 cm(2) g(-1). Partitioning of redroot pigweed dry matter was not influenced by plant density or enviro nmental conditions but did not change within vegetative and reproductive st ages. Radiation-use efficiency was not influenced by redroot pigweed densit y; the most reliable estimate was 1.74 g dry matter MJ(-1) of intercepted p hotosynthetically active radiation. Physiological determinants described we re not affected by redroot pigweed density or environmental conditions and therefore provide a starting point for the development of a redroot pigweed growth module. The module could be coupled with available crop growth mode ls (e.g., the sorghum growth model SORKAM) to simulate redroot pigweed-sorg hum competition.