Why Far-Red Photons Should Be Included in the Definition of Photosynthetic Photons and the Measurement of Horticultural Fixture Efficacy

Abstract

Historical background. Photosynthesis has long been known to be wavelength-dependent (Hoover, 1937;Emerson and Lewis, 1943). At low photon flux densities, McCree (1971) and Inada (1976) found that red photons (600-700 nm) drive photosynthesis more efficiently than green (500-600 nm), followed by blue (400-500 nm) photons. Because green photons penetrate deeper into leaves, more recent studies indicate that at higher photon flux densities red and green photons are used more efficiently than blue photons (Terashima et al., 2009;Liu and van Iersel, 2021). Longer wavelength far-red photons (above 700 nm), on the other hand, are largely inactive for photosynthesis when applied alone (Emerson and Lewis, 1943;McCree, 1971) and have thus been excluded from the definition of photosynthetically active radiation (PAR; 400 to 700 nm).The rapid decline in photosynthetic efficiency at longer wavelengths (above ~685 nm) was first observed by Emerson and Lewis (1943) in green algae (the 'red drop'). Over a decade later, the same research group found that the photosynthetic rate under simultaneous illumination with photons above 680 nm and shorter wavelength light was greater than the sum of the rates from applying each light separately (Emerson et al., 1957). This is now known as the Emerson Enhancement Effect. This synergism among shorter and longer wavelength photons was later revealed to be due to the distinct excitation spectra of the two photosystems -PSI and PSII (Duysens and Amesz, 1962;Hill and Bendall, 1960). While the discovery of the Emerson Enhancement Effect contributed to the identification of PSI and PSII, the significance of the wavelength synergy in photosynthetic efficiency received little attention over the next 50 years and the spectral effects on photosynthesis continued to be studied under monochromatic lights. As a result, the now widely accepted definition of PAR was developed without taking account of synergistic effects on PSI and PSII photochemistry between far-red and shorter wavelength photons.This 400 to 700 nm range was selected by McCree (1972) as the best of multiple definitions of PAR that were in use at the time. He concluded that the photon flux density between 400 and 700 nm was 'an acceptable definition of photosynthetic flux' for nine commonly used broad spectrum lights. Interestingly, he found that photosynthetic rates, normalized based on PAR, were highest under a high-pressure sodium light (with most of the light in the red part of the spectrum) and a quartz-iodine light (rich in red and far-red photons). None of the definitions of PAR analyzed by McCree accounted for photons with wavelengths greater than 710 nm. His study did not test whether including far-red photons in the definition of PAR would improve the correlation with photosynthetic rate.Recent advances in light emitting diode (LED) technology have enabled researchers to re-visit the Emerson Enhancement Effect and study not only the short-term photosynthetic responses of single leaves under low light but also the long-term responses of plant canopies under higher photon flux densities. Zhen and van Iersel (2017) found that adding supplemental far-red photons from LEDs (peak at 735 nm) to red/blue or white LED light synergistically increased PSII photochemical efficiency and photosynthetic rate over a wide range of light intensities (also see Murakami et al., 2018). The enhancement was slightly larger under the red/blue background light than under a warm white LED, probably because the warm white LED light already had 4% far-red photons.Zhen, Haidekker and van Iersel (2019) studied the effects of photons from 678 to 752 nm using laser diodes that had a narrow spectral output (FWHM of 2 to 3 nm). As the wavelength of the photons increased from 678 to 703 nm, they increasingly excited PSI more efficiently than PSII. Photons up to 732 nm significantly enhanced photosynthetic efficiency by exciting PSI, but photons above 752 nm were not effective. There was a gap between 732 to 752 nm because laser diodes were not available in this region.In a subsequent study, Zhen and Bugbee (2020a) measured canopy photosynthesis in 14 diverse species and found that photons from far-red LEDs (700 to 750 nm; peak wavelength at 735 nm) were as effective as traditional 400-700 nm photons when applied at up to ~40% of the total photon flux. As expected, far-red photons alone were not effective. Zhen and Bugbee (2020b) followed this study with a long-term study with lettuce grown under either blue/red or white LEDs (each with and without 15% far-red photons from far-red LEDs). The total photon flux from 400 to 750 nm was equal among spectral treatments. Photon capture and canopy gas exchange were continuously measured, which allowed the analysis of canopy quantum yield (CQY; CO2 fixed per mole of absorbed photons) throughout the study. CQY was equal among treatments from planting to harvest, confirming the important role of far-red photons for photosynthesis.These findings provide compelling evidence for the photosynthetic value of far-red photons, as long as the far-red flux does not exceed about 40% of the total photon flux. This is equal to or higher than the fraction of far-red photons in sunlight under which plants have evolved.Unfortunately, the DesignLights Consortium (DLC, 2021), following ASABE Standard 640, recently decided to not expand the definition of PAR to include far-red photons, despite clear evidence of the photosynthetic efficacy of those photons. DesignLights Consortium cites our research in their decision and we do not believe that our research was interpreted accurately. Our combined findings provide compelling evidence for the value of far-red photons in photosynthesis, when combined with shorter wavelength photons. Defining PAR as the photon flux density between 400 and 750 nm will more accurately reflect the photosynthetic activity of horticultural fixtures and better correlate with plant growth than the current definition.Since the efficacy of horticultural fixtures (µmoles of photons per joule) is calculated as PAR/energy use, the efficacy of fixtures that include far-red photons will be counted as lower than fixtures without far-red. Fixture efficacy is used by energy efficiency programs to determine fixture eligibility for rebate or incentive programs. Until the definition of PAR is expanded to include far-red photons (700 to 750 nm), lighting manufacturers will be discouraged from including far-red LEDs in fixtures. Changing the definition of PAR to include photons from to 750 nm will facilitate the development of fixtures with higher efficacy and will encourage innovation (Kusuma et al. 2020).Practical Limitations. Far-red photons typically cause significant stem, leaf, and/or petiole elongation, which will likely limit the maximum fraction of far-red photons to less than about 20% of the total photon flux for most crops. Because of these powerful effects, we recommend that LED manufacturers clearly indicate the fraction of far-red photons in fixture specifications [(700-750 nm)/(400-750 nm)].