Absorbance spectra of chlorophylls or pigments extracted from green leaves show that green light is absorbed only
weakly. Action spectra of photosynthesis for thin algal solutions, transparent thalli of ordinary green algae, and leaves of
aquatic angiosperms also show that green light is less effective than red light. As has been pointed out by Nishio (2000) ,
these facts are often confused, and it is frequently argued that green light is inefficient for photosynthesis in green
leaves. However, many spectra of absorptance (the absolute value of light absorption) measured with integrating spheres
have shown clearly that ordinary, green leaves of land plants absorb a substantial fraction of green light ( McCree 1972 ,
Inada 1976 , Gates 1980 ). It is also known that green light, once absorbed by the leaves, drives photosynthesis with high efficiency ( Björkmann 1968 , Balegh and Biddulph 1970 , McCree 1972 , Inada 1976 ). On an absorbed quantum basis,
the effi ciency or photosynthetic quantum yield of green light is comparable with that of red light, and greater than
that of blue light. The difference between the quantum yields of green and blue light is particularly large in woody
plants grown outdoors in high light. The question of how much green light is absorbed and used in photosynthesis by
the green leaves of land plants has therefore been solved. In this mini-review, however, we aim at further clarifying
another important role of green light in photosynthesis, by considering the intra-leaf profiles of light absorption
and photosynthetic capacity of chloroplasts. First, we briefly explain light absorption by the leaf. Secondly, we
examine the light environment within the leaf. Thirdly, we compare the vertical, intra-leaf profile of photosynthetic
capacity with that of light absorption. We also discuss some serious problems with the use of pulse amplitude
modulated (PAM) fluorometry in assessing leaf electron transport rate and photoinhibition. Fourthly, we propose a
new method to measure the quantum yield of any mono-chromatic light in white light, and demonstrate the effec-
tiveness of green light in strong white light. Based on these arguments, we fi nally revisit the enigmatic question of why
leaves are green.
As an optical system, the leaf differs from a pigment solution in two aspects: the concentration of pigments into
chloroplasts and the diffusive nature of plant tissues. The first factor decreases the opportunity for light to encounter
pigments and generally decreases light absorption, and has been called the sieve or flattening effect. Once light that is strongly absorbed by chlorophylls, such as blue or red, encounters a chloroplast, most of the light is absorbed. Let us make the drastic assumption that the chloroplast is a sac containing a solution of chlorophylls at a concentration of 100 mol m–3
. This value is chosen because (i) ordinary green leaves are a few hundred micrometers thick; (ii) 50–80% of their volume comprises cells; and (iii) chloroplasts occupy 5–10% of the cell volume. Given that the values of ε for the mixture of chlorophylls at blue and red wavelengths are > 1.0×10 4 m 2 mol –1 , and the chloroplast thickness is 2μ m, then A of the chloroplast calculated
using Equation 1 is > 2. In other words, < 1% of the red or blue light is transmitted through the chloroplast. On the
other hand, for wavelengths that are weakly absorbed, such as green light, T is considerable. When ε for green light is
assumed to be 500 m 2mol –1 ,A and T would be 0.05 and 79.4%, respectively. Using a simple model shown in
Fig. 1, let us consider how the sieve effect is influenced by wavelength. In the left-hand cuvette, photosynthetic pigments are uniformly distributed, whereas the right-hand model comprises one half-cuvette with the pigments concentrated 2-fold and another half-
cuvette containing only the solvent. At wavelengths with strong absorption, the loss of absorptance by the sieve effect
is large. On the other hand, at wavelengths of weak absorption such as green, the loss is marginal. The sieve effect,
therefore, strongly decreases absorptance at wavelengths of strong absorption such as red and blue light. Because of this,
absorption spectra with strong sieve effects show flattened absorption peaks; hence the alternative term ‘flattening effect’.
The second point that distinguishes leaves from a simple pigment solution is that leaf tissues are diffusive. This is due
to the fact that the leaf consists of cells and intercellular air spaces. The refractive index, which depends on both the
material and wavelength of the light, of the bulk plant cells is around 1.48, compared with 1.33 for water and 1.0 for air.
The diffusive nature of leaf tissues increases the light path length (détour effect) and thereby the opportunity for light
to encounter chloroplasts, leading to the increase in absorptance ( Vogelmann 1993 ). On the other hand, the diffusive
nature of the leaf tissues inevitably increases the reflectance, R , of the leaf to some extent. Leaves appear to minimize
R of the adaxial side by having a greater contact area between the adaxial epidermis and palisade tissue cells per unit leaf
surface area than that between the abaxial epidermis and spongy tissue cells. In some species, palisade tissue cells are
funnel-shaped, which further increases the contact area with the epidermis ( Haberlandt 1914 ). By reducing the chances of
refraction at the interfaces between cells and air, R decreases to a considerable extent (compare the differences in R
between the adaxial and abaxial sides).
The increase in absorptance due to light diffusion (détour effect) is signifi cant in the spongy tissues in bifacial leaves
whose abaxial surfaces are paler than their adaxial surfaces ( Terashima and Saeki 1983 , Vogelmann 1993 ). In such leaves,
spongy tissues have cell surfaces facing various directions and fewer chloroplasts (or chlorophyll) per unit mesophyll
volume. In leaves of Camellia japonica, a typical example, lengthening of the optical path is more marked in the spongy
tissue than in the palisade tissue ( Terashima and Saeki 1983 ). On the other hand, in spinach, where the difference in the
chlorophyll content per unit mesophyll volume between the palisade and spongy tissues use is small, the optical path
length does not differ much between the tissues ( Vogelmann and Evans 2002 ). The consequence of lengthening the optical path can be shown using the same model ( Fig. 2 ). In this model, the path length increases by 3-fold (see Vogelmann 1993 ). At strongly
absorbed wavelengths, the increase in absorptance achieved by lengthening the light path is 11% (while the increase in
A is, of course, 3-fold). In contrast, for weakly absorbed wave-lengths such as green light, the increase in absorptance is
much greater. In summary, for strongly absorbed light such as red or blue, the sieve effect decreases absorptance considerably,
whereas the détour effect increases absorptance marginally. On the other hand, for green light, loss in the efficiency of
absorptance by the sieve effect is small, while gain in absorp-tance by the détour effect is large. Consequently, green
leaves absorb much green light. Typical values of absorp-tance at 550 nm range from 50% in Lactuca sativa (lettuce)
to 90% in evergreen broad-leaved trees ( Inada 1976 ). The corresponding absorptance values for blue and red lights
range from 80 to 95%. Moreover, as already mentioned above, it has been clearly shown that the quantum yield of
photosynthesis based on absorbed photosynthetically active photon flux density (PPFD), measured at low PPFDs, was
comparable between green and red light. When measured in leaves grown under natural conditions, particularly for those
of trees, the quantum yield of green light is considerably greater than that of blue light ( Inada 1976 ), because some
fraction of blue light is absorbed by flavonoids in vacuoles and/or carotenoids in chloroplast envelopes. Moreover,
some carotenoids in thylakoid membranes do not transfer energy to reaction centers, or transfer with an effi ciency sig-
nifi cantly less than 1.0 ( Akimoto and Mimuro 2005 ). For example, one of the most abundant carotenoids in thyla-
koids, lutein, transfers its energy to chlorophyll with an effi-ciency of 0.7 ( Akimoto et al. 2005 ). The effi ciency for
neoxanthin is even less, at most 0.09 (Akimoto et al. 2005 ). Accumulation of flavonoids and carotenoids is well known
to increase in response to ultraviolet and/or strong light ( Lambers et al. 2008 ). This probably explains to a considerable
extent why the quantum yield of blue light is low. Evans and Anderson (1987) reconstructed the absorbance spectrum of thylakoid membranes from those of the chlorophyll–protein complexes and estimated the relative excitation of PSII and PSI. Evans (1987) argued that imbalance of PSII/PSI excitation would occur at wavelengths where light is absorbed by Chl b because energy is preferentially transferred to PSII. This might also explain why the quantum yield of blue light on an absorbed quantum basis is low. If this effect is large, a decrease in the PSII quantum yield (Genty's parameter, see below) might be expected at wavelengths strongly absorbed by Chl
b . In a preliminary study with rice leaf discs illuminated with monochoromatic lights at a low PPFD of 5–12 μ mol m –2 s –1
Although the light absorption profi les 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 photo-
synthesis by exciting chloroplasts located deep in the mesophyll. Thus, for leaves, it could be adaptive to use chlo-
rophylls 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.
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