Wavelength-dependent excitation ratio of photosystem I and II in Arabidopsis thaliana

Photosynthesis is a finely tuned process in which plants balance excitations between photosystem I (PSI) and photosystem II (PSII) to optimize energy conversion efficiency. The distribution of light energy between PSI and PSII across the full spectrum of photosynthetically active radiation is hard to quantify. Current attempts rely on estimating the PSI/PSII reaction center ratio and PS antenna sizes. In this study, we employed a streak-camera system to measure excitation distribution between PSI and PSII in Arabidopsis thaliana leaves over the wavelength range 430–630 nm, bypassing the need for indirect estimations of antenna size and pigment distribution. Our findings show that the absorption weighted PSII/(PSI + PSII) excitation ratio is 0.60 ± 0.01 in the 430–630 nm spectral range. By fitting the excitation spectrum with absorption spectra of PSI, PSII, and LHCII, we estimate the PSII/(PSI + PSII) reaction center ratio to be 0.58 ± 0.004. The excitation ratio shows that in the dark-adapted supercomplex organisation, PSII is overexcited. By recording a light response curve of ΦPSI and ΦPSII, we determined that the dark-adapted supercomplex organisation leads to overexcitation of PSII in low to medium light illumination intensities (∼372 μmol m⁻² s⁻¹). However, state transitions alone can rebalance the charge separation ratio. The quantitative excitation ratio and its correlation with reaction center ratios provide crucial parameters for refining photosynthetic models and understanding energy distribution across photosystems.