P. J. SELLERS (1985) investigated the relationship between canopy reflectance, photosynthesis, and transpiration using a two-stream approximation model of radiative transfer. The model calculates hemispheric canopy reflectance in the visible and near-infrared wavelength intervals by integrating simple leaf models of photosynthesis and stomatal resistance over leaf orientation and canopy depth. The ratio of near-infrared and visible reflectances is a near-linear indicator of minimum canopy resistance and photosynthetic capacity but a poor predictor of leaf area index or biomass. The paper explores how the simple ratio of reflectances or the vegetation index can be used to estimate gross primary productivity and canopy resistance to transpiration loss. The methods involve integrating existing formulations that describe radiation interception, photosynthesis, and transpiration by individual leaves over whole canopies. Simplifying assumptions are made to facilitate the solution of the relevant equations, but the effect of these simplifications is considered small. The discussion is divided into three parts. The first part describes a radiative transfer model for calculating spectral reflectance of vegetated surfaces. The second part reviews mathematical representations of photosynthetic and transpiration rates of individual leaves as functions of environmental variables. The third part integrates simplified versions of these formulae to calculate photosynthetic rates and surface resistance of whole canopies. The radiative transfer model uses equations to describe the vertical profile of upward diffuse radiative flux within the canopy and the profile of downward diffuse flux. The model considers the scattering of radiation by leaves and the effect of leaf angle distribution on albedo. The results are compared with observations from maize canopies, showing good agreement with measured data. The model is used to calculate the simple ratio and vegetation index by comparing near-infrared and visible albedos. The vegetation index is found to be a near-linear predictor of absorbed PAR. The results indicate that the vegetation index is an insensitive measure of leaf area index and/or biomass when the leaf-area index exceeds 2 or 3, or when there are patches of bare ground in the sensor field of view. The paper also discusses the relationships between photosynthetic rates and stomatal resistance of individual leaves and vegetative canopies. The model is used to estimate integral values of photosynthesis and stomatal resistance for whole canopies by integrating expressions over all the leaves in the canopy. The results show that the vegetation's response is relatively insensitive to the direction of incoming radiation over the middle range of solar angles. The model is validated against observed data, showing good agreement for canopy photosynthesis and resistance values. The results indicate that the vegetation index is a near-linear predictor of absorbed PAR and that the model provides a useful estimate of evapotranspiration when inserted in the Penman–Monteith equation. The study highlights the importance of considering leaf angle distribution and solar elevation in the analysis of canopy photosynthesis and resistance.P. J. SELLERS (1985) investigated the relationship between canopy reflectance, photosynthesis, and transpiration using a two-stream approximation model of radiative transfer. The model calculates hemispheric canopy reflectance in the visible and near-infrared wavelength intervals by integrating simple leaf models of photosynthesis and stomatal resistance over leaf orientation and canopy depth. The ratio of near-infrared and visible reflectances is a near-linear indicator of minimum canopy resistance and photosynthetic capacity but a poor predictor of leaf area index or biomass. The paper explores how the simple ratio of reflectances or the vegetation index can be used to estimate gross primary productivity and canopy resistance to transpiration loss. The methods involve integrating existing formulations that describe radiation interception, photosynthesis, and transpiration by individual leaves over whole canopies. Simplifying assumptions are made to facilitate the solution of the relevant equations, but the effect of these simplifications is considered small. The discussion is divided into three parts. The first part describes a radiative transfer model for calculating spectral reflectance of vegetated surfaces. The second part reviews mathematical representations of photosynthetic and transpiration rates of individual leaves as functions of environmental variables. The third part integrates simplified versions of these formulae to calculate photosynthetic rates and surface resistance of whole canopies. The radiative transfer model uses equations to describe the vertical profile of upward diffuse radiative flux within the canopy and the profile of downward diffuse flux. The model considers the scattering of radiation by leaves and the effect of leaf angle distribution on albedo. The results are compared with observations from maize canopies, showing good agreement with measured data. The model is used to calculate the simple ratio and vegetation index by comparing near-infrared and visible albedos. The vegetation index is found to be a near-linear predictor of absorbed PAR. The results indicate that the vegetation index is an insensitive measure of leaf area index and/or biomass when the leaf-area index exceeds 2 or 3, or when there are patches of bare ground in the sensor field of view. The paper also discusses the relationships between photosynthetic rates and stomatal resistance of individual leaves and vegetative canopies. The model is used to estimate integral values of photosynthesis and stomatal resistance for whole canopies by integrating expressions over all the leaves in the canopy. The results show that the vegetation's response is relatively insensitive to the direction of incoming radiation over the middle range of solar angles. The model is validated against observed data, showing good agreement for canopy photosynthesis and resistance values. The results indicate that the vegetation index is a near-linear predictor of absorbed PAR and that the model provides a useful estimate of evapotranspiration when inserted in the Penman–Monteith equation. The study highlights the importance of considering leaf angle distribution and solar elevation in the analysis of canopy photosynthesis and resistance.