Red and blue light are traditionally believed to have a higher quantum yield of CO₂ assimilation (QY, moles of CO₂ assimilated per mole of photons) than green light, because green light is absorbed less efficiently. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. We hypothesized that, at high photosynthetic photon flux density (PPFD), green light may achieve higher QY and net CO₂ assimilation rate (A _n) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of PPFD and light spectrum on photosynthesis, we measured leaf A _n of "Green Tower" lettuce (Lactuca sativa) under red, blue, and green light, and combinations of those at PPFDs from 30 to 1,300 μmol⋅m^-2⋅s^-1. The electron transport rates (J) and the maximum Rubisco carboxylation rate (V _c,max) at low (200 μmol⋅m^-2⋅s^-1) and high PPFD (1,000 μmol⋅m^-2⋅s^-1) were estimated from photosynthetic CO₂ response curves. Both QY _m,inc (maximum QY on incident PPFD basis) and J at low PPFD were higher under red light than under blue and green light. Factoring in light absorption, QY _m,abs (the maximum QY on absorbed PPFD basis) under green and red light were both higher than under blue light, indicating that the low QY _m,inc under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high PPFD, the QY _inc [gross CO₂ assimilation (A _g)/incident PPFD] and J under red and green light were similar, and higher than under blue light, confirming our hypothesis. V _c,max may not limit photosynthesis at a PPFD of 200 μmol m^-2 s^-1 and was largely unaffected by light spectrum at 1,000 μmol⋅m^-2⋅s^-1. A _g and J under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and PPFD on photosynthesis was due to effects on J. No interaction between the three colors of light was detected. In summary, at low PPFD, green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high PPFD, QY _inc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.

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