Modelling photosynthesis rates under variable light spectra

Background

Sunlight consists of all colours in the colour spectrum in roughly equal amounts. However, when light passes through leaves in a canopy, blue and red light are absorbed more than green and far-red light. This means that lower in canopy, not only is the light intensity lower the light spectrum is also enriched in green and especially far-red light.

In the so-called light reactions, the protein complexes Photosystem I (PSI) and Photosystem II (PSII) absorb light and convert the energy to useful chemical energy. For efficient photosynthesis the excitation of PSI and PSII needs to be balanced. However, as the photosystems absorb different colours differently, changes in light quality lead to changes in the excitation balance, which in turn lead to decreasing photosynthesis rates. Plants have evolved several ways to restore the balance. For example, in the timescale of minutes, light absorbing proteins move between PSI and PSII and attach to the less excited complex. In the timescale of hours to days, the relative amount of both photosystems is changed.

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Project description

So, the incident light spectrum directly influences photosynthesis and can induce several adaptation processes in plants. While the existence of these biological processes is known, it is unknown how these processes influence the final rate of carbon assimilation. We aim to quantify this effect through photosynthetic modelling.

We start with the commonly used Farquhar, Von Caemmerer and Berry (1980) model and an extension developed by Johnson and Berry (2021). These models explicitly model the influence of among others light intensity, carbon concentration and water exchange, but crucially do not include the effects of light quality. The models assume that the adaptive processes are activated to such a degree, that the excitation is balanced. We are adapting the models such that the excitation can be unbalanced and the adaptation processes are explicitly modelled to restore balance.

The project consists of a combination of model development and model validation. The model is experimentally validated by conducting gas exchange, chlorophyll fluorescence and absorbance change measurements under different incident light spectra. The light spectrum is generated by a special LED array, which can provide spectra resembling sun or shade light.

Results

As a first step in modelling the effects of light quality, we modelled the instantaneous effect of additional far-red on the carbon assimilation rate (Jans, 2024). The new model was validated by measuring carbon assimilation rates under a range of supplied far-red levels. These results have recently been published and can be found in the publication mentioned below.

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