Photosynthetic Quantum Yield Dynamics: From Photosystems to Leaves

TLDR

The study quantitatively addresses how the wavelength dependence of CO₂ fixation quantum yield (α) and its acclimation to growth‑light spectra are governed, using combined in vivo physiological and in vitro molecular methods. Cucumber plants were grown under artificial sunlight, shade, and blue light, and PSI/PSII electron transport and α were measured across 20 wavelengths in vivo, while excitation balance was calculated from both in vivo data and in vitro photosystem composition and spectroscopic properties. Leaves grown under shade light showed higher α when excited at wavelengths favoring PSI, whereas sun and blue leaves had higher α when excited at PSII‑favoring wavelengths; the shade spectrum yielded the lowest PSI:PSII ratio, and the calculated excitation balance from in vivo and in vitro data matched closely, determining α above 580 nm; overall, leaves acclimate their photosystem composition to growth light, altering the wavelength dependence of excitation balance and quantum yield, and combining multiple wavelengths can substantially enhance quantum yields.

Abstract

The mechanisms underlying the wavelength dependence of the quantum yield for CO(2) fixation (α) and its acclimation to the growth-light spectrum are quantitatively addressed, combining in vivo physiological and in vitro molecular methods. Cucumber (Cucumis sativus) was grown under an artificial sunlight spectrum, shade light spectrum, and blue light, and the quantum yield for photosystem I (PSI) and photosystem II (PSII) electron transport and α were simultaneously measured in vivo at 20 different wavelengths. The wavelength dependence of the photosystem excitation balance was calculated from both these in vivo data and in vitro from the photosystem composition and spectroscopic properties. Measuring wavelengths overexciting PSI produced a higher α for leaves grown under the shade light spectrum (i.e., PSI light), whereas wavelengths overexciting PSII produced a higher α for the sun and blue leaves. The shade spectrum produced the lowest PSI:PSII ratio. The photosystem excitation balance calculated from both in vivo and in vitro data was substantially similar and was shown to determine α at those wavelengths where absorption by carotenoids and nonphotosynthetic pigments is insignificant (i.e., >580 nm). We show quantitatively that leaves acclimate their photosystem composition to their growth light spectrum and how this changes the wavelength dependence of the photosystem excitation balance and quantum yield for CO(2) fixation. This also proves that combining different wavelengths can enhance quantum yields substantially.