Greater Than the Sum of the Parts: Revisiting the Enhancement Effect in Photosynthesis Using Simulated Sun‐ and Shade‐Light

Far-red light (FR) alone drives photosynthesis poorly, but when combined with shorter wavelengths it enhances photosynthesis beyond the sum of their individual effects-a phenomenon known as the Emerson enhancement effect. This effect is well-established for narrowband PAR-FR mixtures, and recent results show it also occurs within broadband "white" light, though the spectra investigated differed from those observed under natural conditions. In this study, we used simulated sun and foliar shade spectra (SUN and SHADE) during growth and measurements of mature tomato leaves to determine quantum yield for CO₂ assimilation (Φ_CO2) on an absorbed light basis under SUN and SHADE, 17 narrowband irradiances, and combinations of SUN or SHADE with the 17 narrowband irradiances. Enhancement was calculated for each of the 53 unique spectra by comparing predicted and measured Φ_CO2. This study shows that a 23% enhancement occurs in the simulated SHADE spectrum light and involves the whole spectrum, and in the simulated SUN spectrum enhancement is absent. Further enhancement was observed when narrowband irradiances were added to the SUN or SHADE spectra. The exclusion of the far-red region (> 700 nm) by PAR-based light intensity measurement is particularly problematic in natural, far-red-rich, canopy environments where far-red has surprising photosynthetic utility.