2018 ASHS Annual Conference

Green light has lower leaf absorptance than red and blue light, and therefore is often considered to be less photosynthetically efficient at the same photosynthetic photon flux density (PPFD). However, green light can penetrate deeper into leaves and excite deeper cell layers more effectively than red and blue light. We hypothesized that, at high PPFD, green light may excite more leaf cells than red and blue light, and may achieve a higher leaf photosynthetic rate. To characterize the interactive effect of light quality and intensity, we explored the effect of light quality and PPFD on photosynthetic rate. Photosynthetic rate of 'Green Tower' lettuce was measured under different PPFDs of red, blue and green light, and photosynthesis-light response curves were fitted for the three colors. The maximum quantum yield of photosynthesis (moles of CO₂ fixed per mole of photons) was determined as the initial slope of the light-response curve. To account for differences in absorptance among the three colors, data was expressed both on an incident and absorbed PPFD basis. At low PPFD, plants under green light had the lowest photosynthetic rate among plants under red, blue and green light. The maximum quantum yield of photosynthesis was significantly higher under red than under blue or green light, indicating that the lower photosynthesis under green light is not only due to lower absorptance, but also to less efficient use of absorbed photons. At PPFD above 800 µmol/m²/s, however, photosynthetic rate under green light was higher than under red and blue light, likely because red and blue light were absorbed by the top cell layers of the leaf. This likely saturated the reaction centers in those cells, while deeper cell layers receive few photons. Green light, on the other hand, penetrates deeper into the leaf and excites more cell layers than red and blue light. Consequently, a higher PPFD is required for green light to saturate a larger numbers of reaction centers. Because green light can effectively excite more photosynthetic reaction centers, the light saturation point and light-saturated photosynthesis under green light are likely higher than those under red or blue light. In conclusion, green light is more effective at driving photosynthesis at high PPFD than red and blue light. This knowledge could lead to refined design of LED grow lights that will enhance the light use efficiency, reduce energy cost, and ultimately reduce production cost for greenhouse growers.