Based on McCree’s work ( McCree, 1971, 1972), photosynthetically active radiation is typically defined as light with a wavelength range from 400 to 700 nm. The photosynthetic activity of light is wavelength dependent. Contrary, at high PPFD, QY inc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves. In summary, at low PPFD, green light had the lowest photosynthetic efficiency because of its low absorptance. No interaction between the three colors of light was detected. 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. 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. At high PPFD, the QY inc and J under red and green light were similar, and higher than under blue light, confirming our hypothesis.
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. 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. 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 CO 2 response curves.
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. We hypothesized that, at high photosynthetic photon flux density ( PPFD), green light may achieve higher QY and net CO 2 assimilation rate ( A n) than red or blue light, because of its more uniform absorption throughtout leaves. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. Red and blue light are traditionally believed to have a higher quantum yield of CO 2 assimilation ( QY, moles of CO 2 assimilated per mole of photons) than green light, because green light is absorbed less efficiently. Horticultural Physiology Laboratory, Department of Horticulture, University of Georgia, Athens, GA, United States.
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