Estimating photosynthetic capacity from leaf reflectance and Chl fluorescence by coupling radiative transfer to a model for photosynthesis

In photosynthesis models following the Farquhar formulation, the maximum carboxylation rate V_cmax is the key parameter. Remote-sensing indicators, such as reflectance ρ and Chl fluorescence (ChlF), have been proven as valuable estimators of photosynthetic capacity and can be used as a constraint to V_cmax estimation. We present a methodology to retrieve V_cmax from leaf ρ and ChlF by coupling a radiative transfer model, Fluspect, to a model for photosynthesis. We test its performance against a unique dataset, with combined leaf spectral, gas exchange and pulse-amplitude-modulated measurements. Our results show that the method can estimate the magnitude of V_cmax estimated from the far-red peak of ChlF and green ρ or transmittance τ, with values of root-mean-square error below 10 μmol CO₂ m⁻² s⁻¹. At the leaf level, the method could be used for detection of plant stress and tested against more extensive datasets. With a similar scheme devised for the higher spatial scales, such models could provide a comprehensive method to estimate the actual photosynthetic capacity of vegetation.