Competition between crops and weeds may be stronger at the root than at theshoot level, but belowground competition remains poorly understood, due tothe lack of suitable methods for root discrimination. Using a transgenicmaize line expressing green fluorescent protein (GFP), we nondestructivelydiscriminated maize roots from weed roots. Interactions betweenGFP-expressing maize, common lambsquarters, and redroot pigweed were studiedin two different experiments with plants arranged in rows at a higher plantdensity (using boxes with a surface area of 0.09 m2) and insingle-plant arrangements (using boxes with a surface area of 0.48 m2). Root density was screened using minirhizotrons. Relativeto maize that was grown alone, maize root density was reduced from 41 to 87%when it was grown with redroot pigweed and from 27 to 73% when it was grownwith common lambsquarters compared to maize grown alone. The calculated root: shoot ratios as well as the results of shoot dry weight and root densityshowed that both weed species restricted root growth more than theyrestricted shoot growth of maize. The effect of maize on the root density ofthe weeds ranged from a reduction of 25% to an increase of 23% for commonlambsquarters and a reduction of 42 to 6% for redroot pigweed. This studyconstitutes the first direct quantification of root growth and distributionof maize growing together with weeds. Here we demonstrate that theinnovative use of transgenic GFP-expressing maize combined with theminirhizotron technique offers new insights on the nature of the response ofmajor crops to belowground competition with weeds.