This paper explores the use of the simple ratio of reflectances or the vegetation index to estimate gross primary productivity and canopy resistance to transpiration. The study employs a two-stream approximation radiative transfer model to calculate hemispheric canopy reflectance in the visible and near-infrared wavelength intervals. Simple leaf models of photosynthesis and stomatal resistance are integrated over leaf orientation and canopy depth to estimate canopy photosynthesis and bulk stomatal or canopy resistance. The ratio of near-infrared and visible reflectances is found to be a near-linear indicator of minimum canopy resistance and photosynthetic capacity but a poor predictor of leaf area index or biomass. The radiative transfer model is validated against observations, showing good agreement in the global albedo and diurnal variation. The model's predictions of canopy transmittance and absorption of PAR are also consistent with experimental data. The vegetation index, derived from the simple ratio, is found to be insensitive to leaf area index and biomass when the leaf area index exceeds 2-3, there are patches of bare ground, or the leaf-angle distribution is unknown. Photosynthesis and stomatal resistance are modeled for individual leaves and integrated over the canopy. The canopy photosynthetic rate and resistance are calculated for different leaf-angle distributions and are found to be relatively insensitive to the direction of incoming radiation. The predicted canopy photosynthesis and resistance are compared with experimental data, showing reasonable agreement. The diurnal variation of canopy photosynthesis and resistance is also analyzed, highlighting the saturation effect at higher leaf area indices.This paper explores the use of the simple ratio of reflectances or the vegetation index to estimate gross primary productivity and canopy resistance to transpiration. The study employs a two-stream approximation radiative transfer model to calculate hemispheric canopy reflectance in the visible and near-infrared wavelength intervals. Simple leaf models of photosynthesis and stomatal resistance are integrated over leaf orientation and canopy depth to estimate canopy photosynthesis and bulk stomatal or canopy resistance. The ratio of near-infrared and visible reflectances is found to be a near-linear indicator of minimum canopy resistance and photosynthetic capacity but a poor predictor of leaf area index or biomass. The radiative transfer model is validated against observations, showing good agreement in the global albedo and diurnal variation. The model's predictions of canopy transmittance and absorption of PAR are also consistent with experimental data. The vegetation index, derived from the simple ratio, is found to be insensitive to leaf area index and biomass when the leaf area index exceeds 2-3, there are patches of bare ground, or the leaf-angle distribution is unknown. Photosynthesis and stomatal resistance are modeled for individual leaves and integrated over the canopy. The canopy photosynthetic rate and resistance are calculated for different leaf-angle distributions and are found to be relatively insensitive to the direction of incoming radiation. The predicted canopy photosynthesis and resistance are compared with experimental data, showing reasonable agreement. The diurnal variation of canopy photosynthesis and resistance is also analyzed, highlighting the saturation effect at higher leaf area indices.