2019 ASHS Annual Conference

Green light is traditionally considered to be less photosynthetic active than red and blue light. However, green light can penetrate deeper into leaves and the absorbed light can be used efficiently. We investigated the magnitude of enhancing photosynthesis by green light and the underlying photosynthetic mechanism. We constructed photosynthetic light response curves and CO₂ response curves (A/Ci curves) for different light spectra on 'Green Tower' lettuce (Lactuca sativa). The maximum quantum yield of CO₂ fixation (moles of CO₂ fixed per mol of incident photons at very low PPFD) of each spectrum was calculated as the initial slope of the light response curves. From A/Ci curves, maximum carboxylation rate (V_cmax) and maximum electron transport rate (J_max) were estimated. In this study, red, green, and blue lights were tested, as well as red/green and blue/green mixtures at different ratios. Among monochromatic lights, red light had the highest quantum yield of CO₂ fixation (0.066 mol·mol^-1). Mixing 20%-80% green light with either red or blue light did not affect quantum yield. At a PPFD of 200 µmol·m^-2·s^-1, red and 80% red+20% green resulted in the highest photosynthetic rates, ~ 7.5 µmol·m^-2·s^-1. Blue, green and mixtures of these two colors yielded the lowest photosynthetic rates (5.8 - 6.2 µmol·m^-2·s^-1). At high PPFD (1,000 µmol·m^-2·s^-1), red, green and mixtures of red/green light yielded the highest photosynthetic rate (~16.2 µmol·m^-2·s^-1), while blue and blue/green light mixtures had lower photosynthetic rates (14.2 µmol·m^-2·s^-1). Green light and red/green mixtures achieved the highest maximum net photosynthetic rate (19.6 µmol·m^-2·s^-1) among all spectra. Blue and 80% blue+20% green lights had the lowest maximum photosynthetic rates (16.8 and 17.8 µmol·m^-2·s^-1). We also found that green light is efficient at upregulating Rubisco activity (as indicated by V_cmax), and potentially has the highest electron transport rate at high PPFD. In conclusion, green light is photosynthetically efficient at high PPFD. At PPFDs relevant for supplemental lighting in vegetable production, red and 80%red+20%green lights have the highest photosynthetic rate. The benefit of green light on photosynthesis likely results from its deep penetration into leaves and canopies. Therefore, enhancement of photosynthesis by green light is assumed to be more pronounced in whole canopies than single leaves. The magnitude of benefit of green light on whole canopy photosynthesis needs further study, as do potential morphological and quality effects of different LED spectra.