Validating Chlorophyll Fluorescence Measures of Efficiency: Observations on Fluorimetric Estimation of Photosynthetic Rate

In low light, chlorophyll excitation energy is used to drive photosynthetic electron transport with an efficiency close to 0.125 molecules of O₂ evolved per photon absorbed. At higher light intensities, increasingly large amounts of excitation energy are dissipated as light (fluorescence) or heat. This implies an intimate inverse relation between energy dissipation and electron transport, and this is partly seen when fluorescence emission is compared with the rate of oxygen evolution. Recently developed instruments and methods (for the measurement of modulated chlorophyll fluorescence) have led to the postulation of fluorescence quenching parameters that are linearly related to the quantum yield of carbon assimilation, thus facilitating fluorimetric estimation of gas exchange rates. Here we demonstrate methods of validating these linear relations, which need not rely on measurements of O₂ evolution or CO₂ fixation, and suggest a practical modification of one relation for general application to the problem of rate quantification.