A two-stream description of the interaction of radiation with vegetation and an underlying surface is applied to experimental data from a sugar-beet canopy. The two parameters of the theory (reflectance of a dense canopy, and the attenuation coefficient for radiation in the canopy) are determined for chlorotic and normal sugar-beets, thereby allowing estimation of leaf area index from remotely-sensed reflectance measurements. Comparison with previous results for corn suggest that the parameters are species dependent.