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.

Drought and flooding occur at opposite ends of the soil moisture spectrum yet resulting stress responses that occur in plants share many similarities. Drought limits root water uptake to which plants respond with stomatal closure and reduced leaf gas exchange. Flooding limits root metabolism due to soil anoxia, which also limits root water uptake and leaf gas exchange. As drought and flooding can occur consecutively in the same system and resulting plant stress responses share similar mechanisms, a single theoretical framework that integrates plant responses over a continuum of soil water conditions from drought to flooding is attractive. Based on a review of recent literature, we integrated the main plant eco-physiological mechanisms in a single theoretical model with a focus on plant water transport, plant oxygen dynamics, and leaf gas exchange. We used the Soil-Plant-Atmosphere-Continuum model as “backbone” for our theoretical framework development, and subsequently incorporated interactions between processes that regulate plant water and oxygen status, levels of abscisic acid and ethylene hormones and resulting acclimation strategies in response to drought, waterlogging, and complete submergence. Our theoretical framework provides a basis for the development of mathematical models to describe plant responses to the soil moisture continuum from drought to flooding.