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 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 mu mol.m(-2).s(-1). The electron transport rates (J) and the maximum Rubisco carboxylation rate (V-c,V-max) at low (200 mu mol.m(-2).s(-1)) and high PPFD (1,000 mu mol.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,V-max may not limit photosynthesis at a PPFD of 200 mu mol m(-2) s(-1) and was largely unaffected by light spectrum at 1,000 mu mol.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.

Automated summary

The study demonstrates that green light has lower photosynthetic efficiency at low light intensity, while it may have higher efficiency at high light intensity compared to red and blue light, due to its more uniform absorption throughout the leaves.