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Plant Cell Physiol. 39(10): 1020-1026 (1998)
JSPP © 1998
Green Light Drives CO2 Fixation Deep within Leaves
Jindong Sun, John N. Nishio2 and Thomas C. Vogelmann
Department of Botany, University of Wyoming, Laramie, Wyoming
82071-3165,
U.S.A.
Maximal l4CO2-fixation in spinach occurs in the mid-
dle of the palisade mesophyll [Nishio et al. (1993) Plant
Cell 5: 953], however, ninety percent of the blue and red
light is attenuated in the upper twenty percent of a spinach
leaf [Cui et al. (1991) Plant Cell Environ. 14: 493]. In this
report, we showed that green light drives 14C02-fixation
deep within spinach leaves compared to red and blue light.
Blue light caused fixation mainly in the palisade mesophyll
of the
leaf,
whereas red light drove fixation slightly deeper
into the leaf than did blue light. I4C02-fixation measured
under green light resulted in less fixation in the upper
epidermal layer (guard cells) and upper most palisade meso-
phyll compared to red and blue light, but led to more fixa-
tion deeper in the leaf than that caused by either red or blue
light. Saturating white, red, or green light resulted in simi-
lar maximal 14CO2-fixation rates, whereas under the highest
irradiance of blue light given, carbon fixation was not sat-
urated, but it asymptotically approached the maximal
14CO2-fixation rates attained under the other types of light.
The importance of green light in photosynthesis is discuss-
ed.
Key
words:
Carbon fixation gradient — Green light — Pho-
tosynthesis — Spinacia oleracea.
referred to as the upper part of the leaf), where the greatest
intensity of light is expected (Cui et al. 1991, Terashima
and Saeki 1983, Vogelmann 1993). These findings suggest
the possibility that maximal light absorption occurs in the
middle layers of the mesophyll rather than in those cell
layers nearest the adaxial leaf surface (Evans 1995), al-
though it has been shown that a steep light gradient exists
across spinach leaves (Cui et al. 1991).
Chloroplast adaptation to sun and shade conditions
within leaves is well-documented (Nishio et al. 1993, Out-
law 1987, Terashima 1989). However, the role of differ-
ent quality light at different depths within leaves has not
been established. Since blue and red light are rapidly at-
tenuated across leaves, and green light penetrates more
deeply into leaves, we tested the role of different light quali-
ty on CO2 fixation profiles across spinach leaves using
broad-band red, green, and blue light of different irra-
diance. We showed that green light, which Chi does not ab-
sorb as strongly as red or blue light, drives CO2 fixation
deeper within leaves than does red or blue light, and that
blue and red light contribute to most of the fixation that oc-
curs in the upper part of spinach leaves. The pigment com-
plement utilized by higher plants, as well as variations in
leaf thickness and pigment distribution within the leaf due
to growth conditions, allows dynamic utilization of light
within the
leaf.
The light microenvironment within leaves must have
profound effects on the distribution of carbon fixation with-
in leaves (Gates et al. 1965, Nishio et al. 1993, Osborne and
Raven 1986), as the light intensity, as well as quality, is
highly variable (Vogelmann 1993). Generally, red and blue
light are strongly attenuated in the upper part of the
leaf,
whereas green and far-red light are transmitted more deep-
ly into the leaf (Cui et al. 1991, Gates et al. 1965, Strain
1950,
Terashima and Saeki 1985, Vogelmann 1993).
We recently showed that in spinach leaves, CO2 fixa-
tion is disconnected from the light gradients across leaves
(Nishio et al. 1993). The maximum rates of carbon fixation
under white light occur deep within the leaf (mid-palisade
and lower) in bean (Jeje and Zimmermann 1983) and spin-
ach (Nishio et al. 1993); not at the top of the leaf (upper epi-
dermis plus upper 20-25% of the palisade mesophyll, also
1 Supported in part by grants from Competitive Research Grants
Office, U.S. Department of Agriculture (Nos. 91-37100-6672 and
93-37100-8855).
2 Corresponding author: nishio@uwyo.edu
Materials and Methods
Plant growth conditions—Spinach (Spinacia oleracea cv. hy-
brid 424, Ferry-Morse Seed Company, Modesto, CA, U.S.A.)
was cultured hydroponically in 0.5 x Hoagland solution (Hoa-
gland and Arnon 1950) in controlled environmental growth cham-
bers under
800
j/mol
quanta (400-700 nm) m~2s~' as previously
described (Nishio et al. 1993). The temperature was 23±2°C in
the light and 17±2°C in the dark, and the light period was 12 h.
Five-
to six-week old plants were used in the experiments. The
average thickness of leaves was 680|/m. Cabbage (Brassica
oleracea L., Fl hybrid Savoy King (Sakata Seed America Inc.,
Morgan Hill, CA, U.S.A.)), utilized for light saturation curves,
was grown in the greenhouse at the University of Wyoming,
Laramie, WY, U.S.A. The photoperiod was 16 h (supplemented
with 1,000 watt metal arc lamps), the temperature was 28±3CC,
and the relative humidity was not controlled.
I4CO2 fixation—Gradients of '4CO2 fixation across leaves
were measured as previously described (Nishio et al. 1993), except
that 700 ppm CO2 containing 1.48 x 104 Bq 14CO2 (specific activity
of 18.5 x 1010 Bq moP') was used. A leaf was placed under assay
light for 5 min, and then clamped in a small leaf chamber (1.27 cm
diameter) that allowed top illumination and gas exchange through
small ports on the bottom. The leaf in the chamber was illuminat-
1020
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Green light
and
CO2 fixation within leaves1021
ed by light from
a
1,000 watt xenon
arc
lamp directed with
a
fiber
optic cable (1.5 cm diameter). The light was filtered
to
provide
the
monochromatic light
as
described below. I4CO2 was injected into
the chamber,
and the
leaf labeled
for 5 s and
chased
for 5 s
(time
before freezing).
A
longer pulse and/or longer chase
did not
alter
the patterns
of
fixation across the leaves. Then,
a
flat leaf plug was
obtained from
the
center
of
the labeled leaf tissue using
a
frozen
(in N2(/)) paper punch with
a
copper block attached
to the bot-
tom, that flattened the leaf plug immediately prior
to
freezing and
"punching". The resulting flat, frozen leaf plug was stored
in
N2(/)
until
it was
transferred
to a
freezing microtome
and
sectioned
paradermally in 40/«n increments. Chlorophyll was bleached with
acetic acid before 14CO2 incorporation
was
measured
by
liquid
scintillation counting with quench correction (TriCarb 4430; Unit-
ed Technologies Packard, Downers Grove,
IL,
U.S.A.) (Nishio
et
al.
1993).
Photosynthetic CO2 gas exchange—Gas exchange was meas-
ured with
a
CIRAS-1 (PP Systems, Bedford, MA) portable gas ex-
change system.
Monochromatic light—Light from
a
1,000 watt xenon
arc
lamp was focused and directed onto
a
fiber optic cable that illumi-
nated
the
leaf.
Broad band single-color (monochromatic) light
was made
by
inserting various light filters between
the arc
lamp
and
the
fiber optic cable. Transmission spectra
of
the filters were
scanned
in an HP
8452A Diode Array Spectrophotometer (Hew-
lett-Packard, Waldbronn, Germany)
or a
Lambda
4B
Spectropho-
tometer (Perkin-Elmer Corp., Norwalk, CT, U.S.A.). Spectral
ir-
radiance
of the
filters
and
xenon source were measured with
an
Optronics 742 spectroradiometer (see below). Blue light
for
14CO2
fixation was obtained with
two
blue filters (Kopp 4-96
and
Hoya
B370,
Amax=411
nm,
half-band width
=
54
nm). Blue light
for
gas
exchange was obtained with
one
blue filter (Manostat, Filter #42,
New York, NY, U.SA., Amax=411 nm, half-band width=66 nm).
The wider band width allowed
us to
obtain higher photon fluxes.
The UV light that passed through the filters was mostly eliminated
by glass. Green light
was
obtained using
one
green filter (OCLI
green dichroic filter, Optical Coating Laboratory,
Inc.,
Santa
Rosa, CA, U.S.A., Amax=544 nm, half-band width
=
70
nm).
Red
light
for
14CO2 fixation was generated using
a
611
nm cut-off filter,
and
red
light
for
gas exchange was obtained using
a
648 nm cut-off
filter (Kopp,
CS
2-64, Pittsburgh,
PA,
U.S.A.).
Photon flux through
the
filters
for gas
exchange
and
carbon
fixation was measured with
a
LI-COR LI-185B using
a
quantum
sensor 190SB (LI-COR, Lincoln,
NE,
U.S.A.), whose sensitivity
to the monochromatic light was calibrated against curves
for
spec-
tral irradiance
of the
monochromatic light. Spectral irradiance
was measured with
a
spectroradiometer (Model
742,
Optronics
Laboratories, Orlando, FL, U.S.A.), that was calibrated against
a
1,000 W tungsten-filament quartz-halogen standard lamp
(Op-
tronics Laboratories, traceable
to a
standard
at the
National
Institute
of
Standards
and
Technology, Gaithersburg,
MD,
U.S.A.).
The I4CO2 measurements were based
on
incident light,
whereas,
the
quantum efficiency measurements
of CO2 gas ex-
change were based
on
absorbed light. Light absorption
by
leaves
was measured
as
described previously (Gorton
and
Vogelmann
1996).
Briefly, measurements
of
leaf reflectance (R)
and
transmit-
tance
(T)
were made using
an
integrating sphere,
10
cm
in
diame-
ter, coated with Kodak white reflectance coating (Eastman Kodak
Co.,
Rochester,
NY,
U.S.A.)
and
coupled
to a
LI-1800 spectro-
radiometer (Li-Cor Inc., Lincoln,
NE,
U.S.A.) with
a
fiber optic
cable. Collimated light
was
provided
by a
150 W xenon-arc lamp
(Bausch
and
Lomb, Ocean Springs, MS, U.S.A.) stabilized
by an
uninterruptible power source (model 450, American Power
Con-
version, West Kingston,
RI,
U.S.A.). Measurements
of R and T
were made from leaf discs
(1
cm diameter)
by
scanning from 400-
700nm. Absorbancy
(A) was
calculated
as
A=l—(R+T).
The
measured fraction
of
absorbed incident light
for
spinach was
0.98
for blue light, 0.93
for red
light, 0.91
for
white light,
and
0.81
for
green light.
For
cabbage,
the
fraction
of
incident light absorbed
was 0.956
for
blue,
0.79 for
green,
0.916 for red, and 0.89 for
white light.
The difference between
the
filter sets used
for gas
exchange
and carbon fixation had little impact on our analysis. The percent-
age
of
incident light absorbed by the leaf at wavelengths across
the
blue region
of the
spectrum was very flat (98-97% between
400-
495 nm). Whereas,
the
average absorption
by the
leaf within
the
wavelength range
of the
611 cut-off filter
was
92.3%,
and it was
93.5%
for the
648
nm
cut-off filter.
Results
Monochromatic red, blue, or green light caused differ-
ent patterns of carbon fixation across leaves (Fig. 1). Blue
light caused fixation mainly in the most upper portion of
the leaves, whereas fixation under green light, was shifted
more deeply into the leaves. Red light caused a fixation pat-
tern intermediate to those under blue or green light. Differ-
ent irradiances of each light type always caused the same
relative pattern of fixation across the leaf (Fig. 1).
Carbon fixation across leaves, measured under blue
light, increased from the upper epidermis to a maximum
about 150^/m deep within the leaf (Fig. 1A). The amount
of fixation deeper in the leaf dropped dramatically at
depths greater than 200 /im. It appears that on an absolute
basis,
fixation per paradermal leaf section under blue light
(Fig. ID) was higher at the top of the leaf compared to that
caused by red or green light (Fig. IE, F). Total fixation
(sum of fixation of all leaf sections across the leaf) driven
by 500^mol blue light m~2s~' (6,700 dpm (leaf plug)"1),
however, was lower than that under comparable red (7,000
dpm (leaf plug)"1) or green light (7,500 dpm (leaf plug)"1).
When 14CO2-fixation was measured under green light,
carbon fixation increased to a maximum approximately
250-300
/um
below the adaxial leaf surface, then decreased
with greater leaf depth. The absolute values of the slopes of
the increasing and decreasing gradients were similar so a
relatively gaussian distribution of fixation occurred under
green light (Fig. IB, E). The maximum fixation observed
under red light was about 150-200 fim below the adaxial
leaf surface (Fig. 1C, F); and the major decrease in fixation
did not occur until depths greater than
300
fim. Based on
the gas exchange data (see below), carbon fixation was not
saturated at SOO/zmolm^s"1; however, the relative pat-
tern of fixation was similar at all levels of irradiance given
(Fig. 1A-C).
While maximum fixation under all light types occurred
in the middle of the palisade mesophyll and not at the top
of the
leaf,
the slopes of the ascending and descending por-
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1022Green light and CO2 fixation within leaves
BLUEGREENRED
I i i i i i
150 300 450 600 750 0 150 300 450 600 750 0
LEAF DEPTH (pm)150 300 450 600 750
Fig. 1 Effect of different monochromatic light on carbon fixation per section across sun leaves of spinach. Carbon fixation across
leaves expressed on a relative basis with maximum fixation/section set to 100 (A, B, and C), or as dpm/section (D, E, and F). Irradiances
utilized were 500^mol m~2 s~'
(A),
200^mol m~2 s"1 (•), and 50/miol m~2 s"1 (•). A and D. Blue light,
(A,
n=
10);
(•, n=8). B and E.
Green light,
(A,
n =
10);
(•, n=4); (•, n =
4).
C and F. Red light,
(A,
n =
3);
(•,
n
= 3).
120
100
80
60
40
20
0
100
80
60
40
20
0
20 40 60 80 100
LEAF DEPTH (relative)
Fig. 2 Relative carbon fixation per section across sun leaves of
spinach under different monochromatic light. The maximum rate
per section was set at 100% under each light type. A. White light
800/imolm~
2s~' (D, n=ll). B. Blue light,
500
^mol m~2 s"1 ('
n=10);
Red light, 500/rniolm~
/imol m~2s~' (*, n=10).
(A,
n=3); Green light, 500
tions of the fixation profiles and the absolute depth of max-
imum fixation varied with the color of light (Fig. 2). The
overall shape of the fixation profile under white light
(Fig. 2A) was similar to the shape obtained by summing the
individual fixation patterns under blue, red, and green light
(not shown). The maximum peaks of fixation under blue
and red light were closer to the top of the leaf (at about
20%
leaf depth) compared to that under green light (max-
imum about 35-40% of leaf depth) (Fig. 2). The descen-
ding leg (abaxial side) of the pattern of fixation under blue
light was much steeper and not as deep into the leaf com-
pared to the pattern of carbon fixation caused by red light
(Fig. 2B). While the peak of carbon fixation under green
light was deeper in the leaf than that under red or blue
light, the fixation under green light at the top of the leaf
was low compared to that due to red or blue light (Fig. 2B).
In spinach and cabbage, carbon fixation under white,
red, or green light exhibited similar saturation levels
(Fig. 3). Photosynthetic CO2 gas exchange under blue light
was significantly lower than under red, green, or white light
at similar photon fluxes; however, extrapolation of the rate
of gas exchange under blue light suggests that the max-
imum rate attained under white light could be reached with
higher fluxes of blue light than we utilized. In spinach, red
by guest on September 20, 2011pcp.oxfordjournals.orgDownloaded from
Green light
and CO
2
fixation within leaves1023
I i ' | i i | i i | i i | i i
O
0
300 600 900 1200 1500
LIGHT (nmol/m2-s)
Fig.
3
Effect
of
wide band monochromatic light
on
CO2
gas ex-
change rates.
A.
Cabbage. White
(O, n=4);
Blue
(•,
n
=
4);
Red
(•, n=4);
Green
(A,
n=3). B. Sun
spinach. White
(O,
n=2);
Blue
(•, n=2); Red (•, n=2);
Green
(A,
n
=
2).
light exhibited
the
highest quantum efficiency; whereas
in
cabbage,
red and
white light
had
similar quantum efficien-
cies (Table
1). In
spinach,
the
quantum efficiency under
green light
was
about
10%
lower than that
for red
light,
and
for
blue light
it
was about 25% lower compared
to red
light.
In
cabbage,
the
quantum efficiency
for
blue light
was
20%
lower than that
for red
light.
Fig.
4
shows
the
relative carbon fixation
per Chi
under
the different colored lights. Plotting
the
data
in
this fashion
accentuates
the
role
of
green light
in
driving carbon fixa-
O
O
150
300 450 600
LEAF DEPTH (Mm)750
Fig.
4
Effect
of
monochromatic light
on
relative I4CO2 fixation
per
Chi
across spinach leaves.
The
photon flux
of
each light type
was 500/imolm"2s~'. Green light,
n=10,
(A);
Red
light,
n
=
3,
(•); Blue light, n
=
10,
(•).
tion more deeply
and
broadly across
the
leaf than either
blue
or red
light.
On a Chi
basis,
the
peak
of
fixation
by
green light
is
about 200//m into
the leaf,
whereas fixation
under blue light
and red
light
on a Chi
basis peaks just
below
the
epidermal layer. Carbon fixation under blue light
decreased towards
the
abaxial leaf surface more rapidly
compared
to
that under red light,
and
fixation
per
Chi is
re-
duced
to 10% of the
maximum about 450//m below
the
adaxial leaf surface.
On a
relative basis, carbon fixation
per
Chi under green light decreases
to a
minimum
of
about
40%
of the
maximal rate only
at the
bottom
of the leaf.
Discussion
Green light
is
effectively absorbed
by
green leaves
(Bjorkman
1968,
Gabrielsen
1948,
Gates
et al. 1965,
Kleshnin
and
Shulgin
1959,
Monteith
1965,
Moss
and
Loomis
1952,
Rabideau
et al. 1946,
Seybold
and
Weis-
weiler
1943,
Shibata
et al. 1954,
Stoy
1955,
Strain 1951),
and green light efficiently drives electron transport
(Ghi-
Table
1
Quantum efficiency (0)
and
yield
of
CO2 uptake
of
intact leaves under monochro-
matic illumination
Species
Spinach (n
=
6)
Cabbage
(n=4)
Light quality
Red
White
Green
Blue
Red
White
Green
Blue
Initial slope
(<f>)
0.062±0.001
0.059±0.001
O.O57±0.0O3
0.047 ±0.002
0.064 ±0.002
0.062 ±0.002
0.058±0.002
0.052 ±0.003
Quanta/CO2 uptake
16.1
16.9
17.5
21.3
15.6
16.1
17.2
19.2
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1024Green light
and CO
2
fixation within leaves
rardi
and
Melis 1984). Action spectra show that green light
is
an
effective spectral region
in
powering photosynthesis
in
higher plants, especially
in
leaves with high
Chi
content
(Bulley
et al. 1969,
Clark
and
Lister
1975,
Inada
1976,
Lundegardh
1966,
McCree
1972,
Pickett
and
Myers
1966,
Yabuki
and Ko
1973). Green light
is
also active
in the pro-
duction
of Chi
(Sayre 1928). Because many early action
spectra were determined with dilute algal suspensions,
the
action
of red and
blue light
has
been accentuated (e.g.,
Haxo
and
Blinks 1950).
As a
result,
it
is often assumed that
green light
is not
important
in
driving photosynthesis
be-
cause
of the low
extinction
of Chi at
green wavelengths
compared
to red and
blue light (e.g., Campbell
1996,
Raven
and
Johnson 1996).
Action spectra
of
higher plants (Clark
and
Lister 1975,
Evans
1987,
Inada
1976,
McCree 1972) also show that
the
quantum efficiency
of
carbon fixation
is
highest under
red
light,
and
generally, blue light
has a
lower quantum efficien-
cy than green light, even though
the
Chi extinction
is
higher
in blue light.
The
lower quantum efficiency under blue light
is
due in
part
to
blue light absorbing flavonoids, which
are
not
in the
chloroplast
and
cannot transfer energy
to the
reaction centers,
and
carotenoids, which
may not
efficiently
transfer energy
to the
reaction centers under certain condi-
tions.
Light saturation curves with
red and
green light caused
equal rates
of
photosynthesis
in
Sinapsis alba,
and the
rates
approached saturation under
the
maximum blue light
uti-
lized (Gabrielsen 1940).
We
also were unable
to
reach satu-
ration
of
photosynthesis under blue light,
as we
were
un-
able
to
produce fluxes
of
blue light greater than
1,000
^molm~2s~l. However,
our
data (Fig. 3B) corroborate
the findings
of
Gabrielson (1940).
The
lower rates
of
photo-
synthesis under blue light
at
comparable fluxes
of
the other
types
of
light
are due, in
part,
to a
combination
of
carote-
noid
and
flavonoid absorption. Since greater fixation
per
section occurred
at the top of the
leaf when illuminated
with blue light compared
to red or
green light (Fig.
ID, E,
F),
it is
likely that more blue light energy was absorbed
and
transferred
to the
reaction centers
in the
upper portion
of
the
leaf.
Thus,
it is
also possible that induction
of
non-pho-
tochemical quenching
and
photoinhibition caused
the de-
creased rates
of
photosynthesis under blue light, especially
at wavelengths between 380-430
nm
(Jones
and Kok
1966),
which
we
used
(see
materials
and
methods).
We showed that green light drives photosynthetic
car-
bon fixation deeper within leaves than does blue
or red
light.
In
leaves with high
Chi
concentrations,
the
action
in
the green region
of the
electromagnetic spectrum
is
high,
as
the relative absorption
of
green light compared
to red or
blue light increases
as the Chi
concentration increases
(Strain
1950,
1951).
The
higher contribution
of
blue
and
red light
to
carbon fixation
at the top of the
leaf
and
green
light
to
fixation deeper within
the
leaf
is due to the
wave-
length dependent transmission
and
absorption
of
light
by
the photosynthetic pigments.
Although
"sun"
leaves from spinach plants
are
signifi-
cantly thicker than shade leaves,
on a
relative basis, the
two
types
of
leaves exhibit identical carbon fixation profiles
(Sun
et al.
1996)
and
light gradients
(Cui et al.
1991).
The
distribution
of Chi
across
the
leaf greatly affects
the
light
microenvironment within spinach leaves,
so
the Chi concen-
tration
in the
upper part
of a
thick leaf must
be
lower than
in
the
upper region
of a
thin leaf
for
similar light distribu-
tion
to
occur
in
both leaf types. Indeed,
Chi per
volume
(paradermal leaf section)
is
lower
in the
upper portion
of
sun spinach leaves than
in
shade spinach leaves (Nishio
et
al.
1993). Also,
the
epidermis contains little
Chi
because
only guard cells contain
Chi, and the
maximum
Chi con-
tent
per
volume
is in the
middle
of the leaf.
It appears that mesophytic leaves, such
as
spinach
leaves, have
an
"optimal" leaf that exhibits photosynthetic
gradients that
are
dependent
on
both
the
distribution
of
photosynthetic capacity across
the
leaf
and
leaf anatomy.
There
is a
strong correlation between carbon fixation
and
Rubisco,
but not
between
Chi and
carbon fixation (Nishio
et
al.
1993).
Why
maximum carbon fixation (Rubisco)
oc-
curs
in the
middle
of the
palisade mesophyll
and not at the
top
of
the
leaf,
however, has
not yet
been clearly explained.
Under
low
photon fluxes
of
monochromatic light,
the
pattern
of
fixation across
the
leaves
was the
same
as
under
high photon fluxes
(Fig.
1).
It was not
possible
to
signifi-
cantly shift
the
relative amount
of
fixation towards
the top
of
the
leaf by illumination with
a
photon flux
of
blue light
as
low as
50/miol
m~2 s~'
(compared
to
fixation under
500
fimol blue light m~2 s~'). However,
the
relative fixation
in
the uppermost section under blue light
of
50^/mol
m"2 s~'
was higher than under 500^mol
m~2 s~'.
This large differ-
ence
was not
seen under
red or
green light,
so it
raises
the
possibility that
the
epidermal section
was
photoinhibited
by 500 fimol blue light
m"2 s~'.
Such
a
shift was
not
detect-
able when leaves were photoinhibited with white light,
but
we
did not
measure carbon fixation under
50
/^mol white
light m-2s"'(Sun etal. 1996).
Similar rates
of
CO2
gas
exchange under
low
levels
of
blue
and
green light were measured
(the
rate under green
light
is
slightly higher). While
the
quantum efficiency
of
ab-
sorbed green
and
blue light
are
somewhat similar,
the
utili-
zation
of
incident light
is
much lower
for
green light than
for blue because
the
fraction
of
light absorbed
is
lower
for
green light
by
aljout
20% in
spinach leaves. Thus,
at low
light intensities, both
red and
blue light drive more photo-
synthesis than does green light because relatively more blue
and
red
light
are
absorbed
and
used
to
drive electron trans-
port (compare
ID, E).
It appears that
on an
absolute basis, fixation under
blue light was higher
at the top of
the leaf compared
to
that
caused
by red or
green light
(Fig.
1D-F). Based
on the ab-
by guest on September 20, 2011pcp.oxfordjournals.orgDownloaded from
Green light
and CO
2
fixation within leaves1025
sorption data, fixation under blue light should
be 7%
higher than fixation under
red
light
and 17%
higher than
under green light
(if all
light
was
absorbed
by
Chi).
How-
ever,
the
total fixation under 500/anol blue light m~2s~'
was 6,700
dpm
(leaf plug)"1 (90%
of
green light), under
a
similar flux
of red
light
the
total fixation
was
7,000
dpm
(leaf
plug)"1
(94%
of
green light),
and
under green light
it
was 7,500
dpm
(leaf plug)"1 (100%) (Fig. 1).
CO2 gas ex-
change under
500
/miol blue light m""2
s~' was
also slightly
lower than under
the
other types
of
light (Fig. 3B).
Gas ex-
change under 500 //mol
red
light m~2s~1
was
about
15%
higher than under
an
equivalent flux
of
green light. Thus,
photosynthesis under
red and
green light fluxes
of 500
^molm~2s-1 were about
the
same, whereas under blue
light, fixation
was
lower
by
about 7-10%.
Since fixation under blue light occurred mainly
in the
upper portion
of the
leaf
and the
pattern
of
fixation
is
relatively fixed, we concluded that more fixation
per
section
does indeed occur
in the
upper part
of the
leaf under blue
light than under green
or red
light (Fig.
ID, E,
F), although
the standard deviation
of the
absolute data
for
blue light
was large.
Red
light also caused more fixation
at the top of
the leaf than green light,
but not to the
same extent
as
blue
light. Though unlikely,
it is
also possible that
the
difference
in absolute fixation
may be due to
differences
in
leaf
sam-
ples rather than
in
light quality.
Contrary
to
thinking that green light
is the
least
uti-
lized photosynthetically active radiation
as
often stated
in
biology text books,
our
data
and
that
of
earlier researchers
(e.g., Bulley
et
al. 1969, Clark
and
Lister 1975, Inada
1976,
Lundegardh
1966,
McCree
1972,
Pickett
and
Myers
1966,
Strain
1951,
Yabuki
and Ko 1973)
highlights that green
light
is a
significant energy source
in
driving photosynthe-
sis,
particularly
at the
whole plant level.
The
complement
of Chi
a
+
b
and
carotenoids that evolved
in
algae
and is uti-
lized
by
higher plants allows efficient utilization
of the
broad spectrum
of
visible light that impinges
on
plants.
Al-
though green light provides
the
greatest amount
of
radiant
energy that reaches
the
earth's surface (see Kirk 1994),
Chi
a
and Chi b
exhibit their lowest extinction
in the
green,
which allows green light
to be
transmitted deeply within
leaves. While
red
light clearly
has the
highest quantum
efficiency
and
is
the
most efficiently utilized light,
it
is main-
ly absorbed
in the
upper portion
of
leaves.
Based
on the car-
bon fixation rates (Fig.
1),
however, more blue light
is
absorbed
at the top of the leaf.
Since blue light
is
also
ab-
sorbed
by
flavonoids
and
carotenoids, blue light
has a
lower quantum efficiency than
red
light does. Green light,
on the other hand,
has a
quantum efficiency higher
or
equiv-
alent
to
that
of
blue light,
and it is
transmitted through
the
leaf and
to
underlying leaves where
it
can actively drive pho-
tosynthesis (Fig. 4).
Thus,
the low
extinction
of
green light
by Chi
allows
transmission
of
light required
for
photosynthesis that
can
be absorbed deeper within
the leaf. In
high light, leaves
often
are
thicker,
and the
absorption
of
green light
is in-
creased because
the
leaves have
a
higher concentration
of
Chi
per
leaf area. Green light absorption
is
also increased
by light scattering.
In the
shade,
the
leaves
are
thinner,
but
the
Chi
concentration
on a
weight
or
volume basis
may be
higher (Nishio
et
al. 1993).
By
varying
the
concentration
of
Chi,
plants
can
modulate
the
amount
of
green light that
is
absorbed,
as
most
of the red and
blue light
is
absorbed
even
in
leaves with
low Chi
content (Moss
and
Loomis
1952,
Rabideau
et al. 1946,
Strain
1951,
Yabuki
and Ko
1973).
Leave possess mechanisms
to
deal with light absorbed
in excess
of
that which
can be
utilized
by the
electron trans-
port chain (Demmig-Adams
and
Adams 1992). Absorption
of light
by
non-photosynthetic pigments decreases
pho-
tosynthetic efficiency. Conifers
in
particular exhibit
de-
creased action
in the
blue region
of the
visible spectrum
(Burns 1942). Carotenoids
and
flavonoids have
a
role in
the
decreased efficiency
of
blue light
in
driving photosynthesis
(Clark
and
Lister 1975, Gabrielsen 1948, Inada 1976). With
regard
to
non-photochemical quenching, carotenoids
may
be directly involved
in
dissipation
of
absorbed light energy
by plants (Demmig-Adams
and
Adams
1992,
Horton
and
Ruban
1994,
Niyogi
et al. 1997,
Owens 1994). Anthocya-
nins
can
also decrease
the
efficiency
of
absorbed light
for
photosynthesis, however many higher plants
do not
have
significant quantities
of
anthocyanin during the majority
of
the growing season.
Excitation
of
Chi
at the top of the
leaf
by
blue
and red
light
is
higher than with green light.
The
high extinction
of blue
and red
light suggests
the
possibility that
the top of
the leaf
has a
greater capacity
for
non-photochemical
quenching than underlying tissue. Under full
sun,
where
light
is 3-5
times saturation
for
many plants, nonphoto-
chemical quenching could
be
engaged
at the top of
the
leaf,
but deeper within
the leaf,
where mainly green light
is ab-
sorbed, nonphotochemical quenching would
not be
active.
Such separation
of
"efficient"
and
"down-regulated"
pho-
tosynthesis across
the
leaf would protect
the
upper cell
layers
of the
leaf from photoinhibition
(Sun et al.
1996),
yet
at the
same time allow carbon fixation
to
effectively con-
tinue even under greater than saturating light.
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