Which regions of the electromagnetic spectrum do plants use to drive photosynthesis?
Light: The Forgotten Region of the Spectrum
In the past, plant physiologists used green light as a
safe light during experiments that required darkness. It was assumed that
plants reflected most of the green light and that it did not induce
photosynthesis. Yes, plants do reflect green light but human vision sensitivity
peaks in the green region at about 560 nm, which allows us to preferentially
see green. Plants do not reflect all of the green light that falls on them but
they reflect enough for us to detect it. Read on to find out what the role of green light is in photosynthesis.
electromagnetic spectrum: Light
Visible light ranges from
low blue to far-red
light and is described as the wavelengths between 380 nm and 750 nm,
although this varies between individuals. The region between 400 nm and 700 nm
is what plants use to drive photosynthesis
and is typically referred to as Photosynthetically Active Radiation
(PAR). There is an inverse
relationship between wavelength and quantum energy, the higher the wavelength
the lower quantum energy and vice versa.
Plants use wavelengths outside of PAR
for the phenomenon known as photomorphogenesis, which is light regulated
changes in development, morphology, biochemistry and cell structure and function.
The effects of
different wavelengths on
plant function and
form are complex and
are proving to be an
interesting area of
study for many
plant scientists. The use of specific and adjustable LEDs allows us to
tease apart the roles of
specific areas of the spectrum in photosynthesis. Furthermore, the synergy between photosynthesis and
photomorphogenesis can be more accurately examined now. This paper focuses on
photosynthesis. Photomorphogensis will be covered in the future.
Photosynthetic pigments and light
The first step in photosynthesis is the absorption of
light by antenna pigments located within the thylakoid membrane in the
chloroplasts. Photosynthetic organisms contain an assortment of pigments
thereby allowing absorption of a maximum number of wavelengths. All
photosynthetic organisms contain chlorophyll a and this is the primary light harvesting pigment. Higher plants
contain accessory pigments that are also involved in light harvesting and
photochemistry. These are chlorophyll b and the carotenoids.
Light absorption by photosynthetic pigments is extremely
fast. It occurs within femtoseconds
(10-15 s) and causes a transition from the
electronic ground state to an excited
state and within 10-13 s the excited state decays by vibrational relaxation to the
first excited singlet state. Photosynthetic antenna systems are very efficient at
excitation transfer processes. Under optimum conditions over 90% of the absorbed quanta are transferred within a few
hundred picoseconds from the antenna system to the reaction center which acts
as a trap for the exciton. The exciton transferred to photosystem II results in
the extraction of an electron from water
that is passed along the photosynthetic electron transport chain to an excited
photosystem I which subsequently reduces NADP+ to NADPH which serves as an energy
source for plant metabolism. A second energy source used in plant metabolism, ATP, is also produced during
electron transport via an
ATPase driven by a proton gradient. There are several
alternative electron transport routes utilized by plants but these are outside
of the scope of this paper.
Absorption spectra versus Action spectra
Reading through the popular literature on the internet and
on LED lamp websites it is obvious that there is little understanding about
which wavelengths plants use for photosynthesis. It is apparent that there is
confusion between what an absorption spectrum and an action spectrum are and
what they represent. An absorption spectrum defines the wavelengths that are
absorbed. An action spectrum defines the wavelengths that are most effective
for photosynthesis. In other words, it is the
portion of the spectrum that does the work. This is what is most
important in plant growth and
metabolism. It is important to note that light absorption and light utilization are two different phenomena.
1. What is
Which regions of the visible light spectrum do plants
absorb light? This is different for extracted chlorophyll molecules, whole
chloroplasts (where the chlorophyll resides) and plant leaves. To complicate
matters, the solvent in which chlorophyll is extracted also has an effect on
the absorption spectrum.
The absorption spectra of chlorophylls a and b extracts is why LED grow lamps are typically made up of blue and
red LEDs. The absorption spectra of isolated pigments have been the foundation
for LED selection for most LED lamps. Furthermore, it has been ignored that
carotenoids play a role in light absorption and energy transfer to the
The absorption spectra of isolated pigments in vitro do not represent
what the whole
plant absorbing. Each pigment has a specific absorption spectrum and in living systems pigments never exist alone. They
are always bound to proteins and this shifts
their absorption spectrum. This is why wavebands are absorbed rather than a single wavelength. In vivo, the probability of a pigment absorbing light absorption depends on: 1) the specific protein
that the pigment is bound to; 2) the orientation of the pigment-protein
complex within the cell; 3) the forces exerted by the surrounding
medium on the
Figure reprinted with permission from Dr. Holly Gorton.(Absorptance spectra of isolated pigments, disrupted chloroplasts, intact chloroplasts, and whole leaves from spinach (Spinacia oleracea) Modified from (Moss & Loomis, 1952)).
2. What is an Action Spectrum?
An action spectrum describes the efficiency with which specific wavelengths produce a photochemical reaction. Photosynthesis involves the harvesting of light (absorption spectrum) and the subsequent photochemical and biochemical reactions. Thus, an action spectrum describes the wavelengths that actually drive photosynthesis.
The seminal paper describing the action spectra for 22 plant species was published by KJ McCree (1972). This work was originally done in order to provide an accurate definition of PAR, which had not been previously described empirically. The action spectra described in the McCree paper plot the efficiency or quantum yield of CO2 assimilation as a function of wavelength. Interestingly, similar action spectra were observed for the 22 plant species. However, there was slight variation between species in the blue end of the spectrum. The results from this work indicated that PAR was between 400 nm and 700 nm and that all wavelengths within this region were used in photosynthesis.
Action spectra for 22 plant species grown in the field (top plate) and a growth chamber (bottom plate).
reprinted with permission from Dr. Holly Gorton. ( Photosynthetic action
spectra for the green alga Ulva (two cell layers)
(Haxo & Blinks,
1950) and higher plants (multiple cell layers).
The curve for higher
plants represents the average of action spectra obtained for 22 crop plants (McCree, 1971/1972) recalculated on a
The Role of Green
Light in Photosynthesis
It is clear that green light is a player in photosynthesis
along with the other portions of the spectrum. How and where does this occur? Blue and red light are absorbed
preferentially at the adaxial (upper) side of leaves and are more efficient at
driving photosynthesis in this region compared to green light (Sun et al. 1998;
Nishio, 2000; Terashima et al., 2009).
As a consequence, green light is transmitted deeper into the leaf and is more
efficient than either blue or red light at driving CO2 fixation at the abaxial (lower) sides (Sun et al.
1998; Terashima et al., 2009). Indeed, on an absorbed quantum basis,
photosynthetic efficiency or quantum yield for green light is similar to that
of red light, and greater than that of blue light in the deeper layers of a leaf (Terashima et al. 2009).
Transverse section of a lilac leaf (left panel) and
schematic of the internal structure. Light is absorbed by pigments within the
various layers of cells. The different cell layers have different absorbance properties. (hcs.osu.edu/hcs300/anat3.htm).
Typical absorption values of green light (550 nm) range
from 50% in lettuce to 90% in evergreen broadleaf trees. As
observed above in the action spectra, the entire light spectrum is used to
drive photosynthesis. It appears as though green light is not a safe light and
that green light is required for optimum whole plant photosynthesis. Recent
studies have determined that green light is more photosynthetically efficient
than red or blue in the deeper layers of leaves. The experiments we have
performed at Heliospectra support the importance of green of green light for
optimal plant growth and have found that the amount of green required is
species dependent. The Heliospectra LED selection differs from most other LED
plant growth lamps and this was based on full understanding of photosynthesis and plant physiological processes.
McCree, K.J. (1972). The action spectrum, absorptance and
quantum yield of photosynthesis in crop plants. Agric.Meteorol. 9 : 191-216.
Nishio, J.L. (2000). Why are higher plants green?
Evolution of the higher plant photosynthetic pigment complement. Plant, Cell
& Environment 23(6): 539–548.
Sun JD, Nishio
JN, Vogelmann TC. 1998. Green light drives
CO2 fixation deep within
leaves. Plant and Cell Physiology 39, 1020–1026.
Terashima I, Fujita
T, Inoue T, Chow WS, Oguchi R (2009) Green light drives leaf photosynthesis more efficiently than
red light in strong white light: revisiting the enigmatic question of why leaves
are green. Plant Cell Physiol