![]() The limitations of McCree’s action spectrum were explained in his original paper: the quantum yield was measured under low photosynthetic photon flux density ( PPFD), using narrow waveband light, and expressed on an incident light basis ( McCree, 1971), but these limitations are sometimes ignored. The misconception that red and blue light are used more efficiently by plants than green light still occasionally appears ( Singh et al., 2015), often citing McCree’s action spectrum or the poor absorption of green light by chlorophyll extracts. Blue and red photons, therefore, may be used less efficiently and are more likely to be dissipated as heat than green photons. Absorption of photons by chloroplasts near the adaxial surface may induce heat dissipation of excess excitation energy in those chloroplasts, while chloroplasts deeper into the leaf receive little excitation energy ( Sun et al., 1998 Nishio, 2000). Leaf photosynthesis may benefit from the more uniform light distribution throughout a leaf under green light. Since red and blue light are absorbed more strongly by photosynthetic pigments than green light, they are predominantly absorbed by the top few cell layers, while green light can penetrate deeper into leaf tissues ( Nishio, 2000 Vogelmann and Evans, 2002 Terashima et al., 2009 Brodersen and Vogelmann, 2010), thus giving it the potential to excite photosystems in deeper cell layers. Green light is least absorbed by green leaves, which gives leaves their green appearance ( McCree, 1971 Zhen et al., 2019). Within the visible spectrum, green leaves have the highest absorptance in the blue region, followed by red. The low absorptance of green light is partly responsible for its low quantum yield of CO 2 assimilation. Light in the green region (500–600 nm) generally resulted in a slightly higher quantum yield than light in the blue region (400–500 nm) ( Figure 1 McCree, 1971). Within the 400–700 nm range, McCree (1971) showed that light in the red region (600–700 nm) resulted in the highest quantum yield of CO 2 assimilation of plants. Light with a wavelength shorter than 400 nm or longer than 700 nm was considered as unimportant for photosynthesis, due to its low quantum yield of CO 2 assimilation, when applied as a single waveband ( Figure 1). 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.
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