Extending the range of photosynthetically active radiation (PAR): the significance of far-red photons for photosynthesis

Photosynthetically active radiation (PAR) for oxygenic photosynthesis has traditionally been defined as the 400–700 nm wavelength range. However, recent studies revisiting the Emerson enhancement effect, an early demonstration of wavelength synergy in photosynthesis, have provided compelling evidence that longer-wavelength far-red (FR; 700–750 nm) photons contribute to plant photosynthesis far more effectively than previously thought when combined with shorter-wavelength photons. Nonetheless, the reported magnitude of this contribution varies among studies, prompting ongoing debate over whether the definition of PAR should be expanded to include FR photons. In this review, we (i) synthesize recent advances in our understanding of the photosynthetic role of FR photons and the physiological mechanisms underlying their synergistic interactions with shorter-wavelength photons; (ii) critically evaluate the factors contributing to differences among studies in the estimated photosynthetic contribution of FR photons, and (iii) discuss the implications of these findings for redefining PAR and improving the measurement and modeling of photosynthesis under both sunlight and electric lighting.