Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms.

Red and blue light are traditionally believed to have a higher quantum yield of CO assimilation (, moles of CO 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 (), green light may achieve higher and net CO assimilation rate ( ) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of and light spectrum on photosynthesis, we measured leaf of "Green Tower" lettuce () under red, blue, and green light, and combinations of those at s from 30 to 1,300 μmol⋅m⋅s. The electron transport rates () and the maximum Rubisco carboxylation rate ( ) at low (200 μmol⋅m⋅s) and high (1,000 μmol⋅m⋅s) were estimated from photosynthetic CO response curves. Both (maximum on incident basis) and at low were higher under red light than under blue and green light. Factoring in light absorption, (the maximum on absorbed basis) under green and red light were both higher than under blue light, indicating that the low under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high , the [gross CO assimilation ( )/incident ] and under red and green light were similar, and higher than under blue light, confirming our hypothesis. may not limit photosynthesis at a of 200 μmol m s and was largely unaffected by light spectrum at 1,000 μmol⋅m⋅s. and under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and on photosynthesis was due to effects on . No interaction between the three colors of light was detected. In summary, at low , green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high , under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.