Leaf photosynthesis models are used extensively in photosynthesis research and are embedded in many larger scale models. Typical photosynthesis models simplify light intensity as the integrated intensity over the 400 to 700 nm waveband (Photosynthetic Active Radiation; PAR). However, far-red light (700-750 nm, FR) also drives photosynthesis when supplied in addition to light within the PAR spectrum. Currently, it is unknown how much far-red light contributes to carbon assimilation under various spectral light conditions. We developed a combined experimental and computational method to quantify FR stimulation. Gas-exchange parameters and incident light spectra were measured simultaneously and analysed with wavelength-dependent modelling of light harvesting. Hereto, separate excitation of Photosystem I and Photosystem II was calculated from incident light spectra. The effect of FR supplementation on photosynthesis was subsequently modelled and expressed as a single parameter ρ. We tested our method on Solanum dulcamara, Lactuca sativa and Phaseolus vulgaris under various light conditions. Results show consistent species-specific ρ-values across a range of FR levels. Our method provides an approach to consistently quantify the effect of FR stimulation on photosynthesis and harmonize the interpretation of photosynthesis measurements under different light regimes, for example in (experimental) setups with artificial FR supplementation or in canopies.

Maximal sunlight slowly varies over a day due to earth’s rotation. However, it is unknown whether this predictable daily light pattern influences the short-term regulation of non-photochemical quenching (NPQ) responses across the day. Diurnal variation in NPQ at growth light intensity, NPQ capacity and kinetics during and after 30-minute high light period, were measured in tomato plants grown in diurnal constant (DC) or diurnal parabolic (DP) light intensity. We also measured leaf gas exchange and the violaxanthin de-epoxidation state (DES) during and after 30-minute high light period at different time points in the day. A clear diurnal pattern emerged in NPQ induction kinetics in DP-grown plants, which was characterized by longer time for NPQ to reach its maximal value at the end of the photoperiod compared to midday, but with the highest capacity. The observed diurnal variations in NPQ capacity were not explained by the DES, but were associated with slower relaxation of qE. These diurnal patterns in NPQ responses were less pronounced in DC-grown plants. Concurrently, CO 2 assimilation rate and stomatal conductance were lowest in the end of day but similar between light regimes. We conclude that the diurnal short-term responses of NPQ involves regulation of its ∆pH-dependent component, which in turn can be influenced by the partitioning of proton motive force.