If we analyze the changes in the PPFD and spectrum of daylight with time of day in the natural environment, it would be possible to compare the efficiencies of green and other monochromatic lights in an ecological context. The method enables us to measure in situ quantum yield and opens the way to obtaining ecologically meaningful action spectra. Further studies are, of course, awaited.
Although the light absorption profiles calculated by Nishio (
2000) are spurious (Vogelmann and Evans
2002), his argument has nevertheless been proven experimentally to be correct using our differential quantum yield method. Namely, red light is more effective than green light in white light at low PPFDs, but as PPFD increases, light energy absorbed by the uppermost chloroplasts tends to be dissipated as heat, while penetrating green light increases photosynthesis by exciting chloroplasts located deep in the mesophyll. Thus, for leaves, it could be adaptive to use chlorophylls as photosynthetic pigments, because, by having chlorophyll with a green window the leaves are able to maintain high quantum yields for the whole leaf in both weak and strong light conditions.
Some green algae such as
Codium fragile and
Ulva pertusa, inhabiting the deepest part of the green algae zonation, appear very black, because they contain a keto-carotenoid, siphonaxanthin, which absorbs green light with a peak at 535 nm and transfers energy to chlorophylls with an efficiency of 1.0 (Kageyama et al.
1977, Akimoto et al.
2004, Akimoto et al.
2007). Because the peak of available PPFD shifts toward blue wavelengths with depth of sea-water, it has been argued that siphonaxanthin is a useful carotenoid to absorb green light. If leaves of land plants had black chloroplasts with siphonaxanthin, the leaves could close the so-called green window and increase their absorptance. If the carboxylation enzyme, Rubisco, were very efficient, land plants would indeed be able to have thin black leaves. However, having the inefficient Rubisco as their primary carboxylation enzyme, leaves receiving high light need considerable chloroplast volumes to contain it (Terashima et al.
2005, Terashima et al.
2006). Moreover, to supply CO[SUB]2[/SUB] efficiently to the chloroplasts, the leaf also needs a large cumulative cell surface area per leaf area, so the chloroplasts must be distributed throughout the leaf (Terashima et al.
2001, Terashima et al.
2005, Terashima et al.
2006). Given these constraints, it would be ideal to have chlorophyll that enables considerable light absorptance, due to the high absorptivity of blue and red light, but also penetration of green light to the lower chloroplasts. As Nishio (
2000) argued, this may explain why land plants adopted Chl
a and
b from green algae but did not develop other pigment systems.
If a gradient in the ratio of Rubisco to photosynthetic pigments freely changes in response to PPFD, leaves could exist with black chloroplasts containing both chlorophylls and siphonaxanthin. When light absorption is plotted against the cumulative black pigment content for such leaves, the gradient would be very steep, because absorption coefficients would now be high for green as well as blue and red light. In the upper chloroplasts, the ratio of Rubisco to black pigments would then need to be very large but to decrease drastically with depth. Noting that the dynamic range of acclimational modification of chloroplast properties is limited within a given species, it would be impossible to counterbalance the profile of light absorption by drastically changing the Rubisco/black pigment ratio. It is, therefore, worth mentioning again that, by having chlorophylls with a green window to the most abundant photosynthetically active wavelengths of solar radiation, green leaves have succeeded in moderating the intra-leaf light gradient to a considerable extent.
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