Light Absorption and Competition in Mixed Sorghum‐Pigweed Communities

Abstract

Abstract Plant productivity in a community is governed in part by its ability to absorb and utilize photosynthetically active radiation (PAR). Studies on weed competition with a crop for light are limited. The effect of pigweed ( Amaranthus hybridus L. and A. palmeri S. Wats) competition on leaf area development, light absorption, and dry matter production of fully developed grain sorghum [ Sorghum bicolor (L.) Moench] was evaluated in a field experiment on Pullman clay loam (a fine, mixed, thermic Torrertic Paleustoll) at Bushland, TX, in 1984. Profile measurements (0–0.3, 0.3–0.6, 0.6–0.9, and >0.9 m above ground) of absorbed PAR (APAR) and leaf area index (LAI) by species were taken at four densities of pigweed (0, 1, 4, and 12 plants m −2 ). APAR calculated for sorghum in mixed communities of 1,4, and 12 pigweed plants m −2 was 79,77, and 49% of the APAR in weed‐free sorghum. Sorghum LAI was reduced to 81, 65, and 37% of the LAI of weed‐free sorghum in canopies with 1, 4, and 12 pigweed plants m −2 . Sorghum LAI was concentrated in the 0.3‐ to 0.6‐m layer, while the taller pigweed plants had the greatest leaf area concentration above 0.6 m. By absorbing light in the upper canopy, pigweed reduced light penetrating into sorghum. Leaf measurements of photosynthesis and transpiration rates, leaf temperature, and stomatal resistance indicated a relatively minor degree of water stress under full canopy and high potential evaporation conditions; the level of water stress measured was not adequate to explain sorghum dry matter reduction in plots with 1, 4, and 12 pigweed plants m −2 to 78, 56, and 28% of that in weed‐free sorghum.