Higher Plants, Photosynthesis and Green Light: Myths Debunked

Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, it is the energy from the red and blue light that are absorbed that is, thereby, able to be used to do photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis.

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...full sunlight provides significantly more energy than can be utilized by the photosynthetic electron transport system of most C3 leaves, so energy dissipative mechanisms are important (Demmig-Adams & Adams 1992), and such dissipative mechanisms are more prevalent at the top of the leaf. Hence, under greater than saturating light, the percentage of absorbed green light utilized for photosynthesis must be higher for green light than for blue or red light, since more blue and red light are absorbed by the top of the leaf.

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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 absorptance by the détour effect is large. Consequently, green leaves absorb much green light. Typical values of absorptance 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...

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Chlorophyll absorption spectra indicate a very low absorption of green compared to red or blue wavelengths. However, the photosynthesis action spectrum of an intact leaf indicates the rate of photosynthesis is roughly 60% as much with green light as with red and it may actually be higher than with blue (see Salisbury & Ross, Plant Physiology, 3rd, p. 185) Thus, leaves can use green light fairly effectively in photosynthesis. Some of the absorption may be due to accessory pigments. Chlorophyll in an intact leaf can also absorb green light much more effectively than the chlorophyll absorption spectrum (chlorophyll extract in a spectrophotometer) indicates. One reason is that although green light is absorbed with low efficiency, it has many chances to be absorbed because it is repeatedly reflected from cell to cell by the complex leaf geometry so it has many chances to be absorbed. Such geometry effects do not occur with chlorophyll extract in a spectrophotometer tube. This provides an excellent illustration of how in vitro can differ markedly from in vivo. Unfortunately, biology textbooks usually just publish the in vitro chlorophyll absorption spectrum rather than the in vivo photosynthesis action spectrum.

Thus, the common idea that leaves are green because they reflect ALL green light is incorrect. Most leaves reflect relatively more green light relative to red/blue wavelengths and appear green to our eyes. An exception is the blue Colorado spruce (Picea pungens 'Glauca') with bluish needles. The sensitivity of our eyes might have something to do with it too because our eyes are most sensitive to 550 nm wavelengths and much less sensitive to red or blue wavelengths.

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Photosynthesis

A widespread misconception states that leaves reflect all green light and do not use green light in photosynthesis. Leaves often absorb more than 50% of the green light and use it efficiently in photosynthesis.8,22 The origin of this misconception is probably the chlorophyll absorption spectrum in textbooks. The chlorophyll absorption spectrum is a graph of light absorption versus light color. It shows that chlorophyll absorbs much red and blue light but little green light. However, accessory pigments absorb green light and pass that energy on to chlorophyll.

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Photosynthesis is the basis of plant growth and development. The regulations of photosynthesis by light quality include regulations of stomatal movement, leaf growth, chloroplast structure, photosynthetic pigment, D1 protein and its gene and photosynthetic carbon assimilation by visible light, and effect of ultraviolet light on photosystem II in plant. Blue light and red light can promote the opening of stomata, while the green light can close stomata. Blue light can improve the development of chloroplast, complex light of red, blue and green lights can expand leaf area, and red light can increase the accumulation of photosynthesis production. Effects of different light quality differ in various plants, organs and tissues. Blue light and far red light can promote the accumulation of psbA gene transcription. Most higher plants and green algae have highest photosynthesis rate in orange and red lights, secondly in blue-violet light, and minimum in green light. Ultraviolet light can decline the electron transfer activity of photosystem II. Moreover, questions regarding the effect of light quality on photosynthesis and some topics for future study were also discussed in this paper.

PMID: 18839928 [PubMed - indexed for MEDLINE]

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Re: Why photosynthesis pigment of plant is green and not black?

Date: Fri Dec 21 20:47:32 2001
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1008863837.Bt

The first website cited describes an interesting idea that plants are green because purple Halobacterium evolved before chlorophyll-containing organisms. The photosynthetic Halobacterium absorbed green light so other organisms possibly evolved chlorophyll to absorb nongreen wavelengths and fill an ecological niche. The first two websites both argue that a black chlorophyll might absorb too much radiation and either overheat the plant or harm the plant by absorbing destructive UV and x-rays.

It is important to remember that leaves often absorb more than half the green wavelengths and use them in photosynthesis. It is a widespread misconception that leaves reflect all green light. That misconception is based on looking at a chlorophyll absorption spectrum (see second website cited), which is obtained by extracting chlorophyll into an organic solvent, such as acetone, and measuring its absorption in a spectrophotometer at wavelengths between 400 and 700 nanometers. The chlorophyll absorption spectrum does show that chlorophyll in a test tube absorbs only about 2 to 3% of the green light. However, that is very artificial because a leaf is highly structured. Salisbury and Ross (1985) note that in the intact leaf, a green photon may not initially be absorbed by a particular chlorophyll molecule but it is reflected and then gets another chance to be absorbed, and perhaps another, and another, etc. within the complex leaf structure that does not exist in a test tube. Thus, each green photon has many opportunities to be absorbed in the leaf so the total absorption of green light by chlorophyll is much higher in the leaf than in a test tube of extracted chlorophyll. Accessory pigments, such as carotenoids, also absorb green light (see second website cited) and funnel the energy to chlorophyll.

Instead of a chlorophyll absorption spectrum, people should be looking at a photosynthesis action spectrum (see third and fourth websites cited) which shows the amount of photosynthesis at each wavelength. Because plants do absorb substantial green and yellow wavelengths and use them in photosynthesis, plants are more efficient than they seem by assuming they only use the red and blue wavelengths that chlorophyll absorbs in a chlorophyll absorption spectrum. Photosynthesis is the most widely taught plant biology concept but it is often taught with numerous misconceptions.

References

Why Trees are Green

Photosynthesis Pigments

Photosynthesis Action Spectrum for Elodea

Generalized Photosynthesis Action Spectrum

Salisbury and Ross. 1985. Plant Physiology. Belmont, CA: Wadsworth.

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http://www.leffingwell.com/lhc.htm

The ubiquitous green color of plants, due to chlorophyllic pigments, is a key molecular participant in the light harvesting of plants. While the chlorophylls are efficient in absorbing the red and blue portions of the light spectrum, they do not efficiently absorb other parts of the sunlight spectrum. More hidden in growing plants is a second participating molecular class, carotenoids, which can absorb other portions of the strectrum. In green leaves the color of the carotenoids is masked by the much more abundant chlorophylls while in red ripe tomatoes or petals of yellow or orange flowers, the carotenoids predominate.

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http://www.leffingwell.com/caroten.htm

Carotenoids are the pigments responsible for the colors of many plants, fruits and flowers. They serve as Light Harvesting Complexes (with proteins) in photosynthesis...carotenoids are the precursors of many important chemicals responsible for the flavor of foods and the fragrance of flowers.

The primary odor constituents derived from carotenoids are C13 - C11 - C10 - and C9 derivatives formed via enzymatic oxidation and photo-oxidation of the various carotenoids found in plants, flowers and fruits. While other aroma constituents such as esters, terpenes, pyrazines, etc. are usually also present, these C9 to C13 compounds often are essential to the odor profile. Above you will see a common oxidative fragmentation pattern (shown for beta-Carotene). Examples of aroma compounds produced in nature are shown below:

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http://science.jrank.org/pages/5303/Plant-Pigment-Carotenoids.html

Carotenoids have two important functions in plants. First, they can contribute to photosynthesis. They do this by transferring some of the light energy they absorb to chlorophylls, which then use this energy to drive photosynthesis. Second, they can protect plants which are over-exposed to sunlight. They do this by harmlessly dissipating excess light energy which they absorb as heat. In the absence of carotenoids, this excess light energy could destroy proteins, membranes, and other molecules. Some plant physiologists believe that carotenoids may have an additional function as regulators of certain developmental responses in plants.

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http://en.wikipedia.org/wiki/Xanthophyll

The xanthophyll cycle involves the enzymatic removal of epoxy groups from xanthophylls (e.g. violaxanthin, antheraxanthin, diadinoxanthin) to create so-called de-epoxidised xanthophylls (e.g. diatoxanthin, zeaxanthin). These enzymatic cycles were found to play a key role in stimulating energy dissipation within light harvesting antenna proteins by non-photochemical quenching- a mechanism to reduce the amount of energy that reaches the photosynthetic reaction centers. Non-photochemical quenching is one of the main ways of protecting against photoinhibition.[1] In higher plants there are three carotenoid pigments that are active in the xanthophyll cycle: violaxanthin, antheraxanthin and zeaxanthin. During light stress violaxanthin is converted to zeaxanthin via the intermediate antheraxanthin, which plays a direct photoprotective role acting as a lipid-protective anti-oxidant and by stimulating non-photochemical quenching within light harvesting proteins. This conversion of violaxanthin to zeaxanthin is done by the enzyme violaxanthin de-epoxidase, while the reverse reaction is performed by zeaxanthin epoxidase[2]

http://en.wikipedia.org/wiki/Photoinhibition

Photosystem II is damaged by light irrespective of light intensity.[2] The apparent quantum yield of the damaging reaction in typical leaves of higher plants exposed to visible light,[19] as well as in isolated thylakoid membrane preparations,[23] is in the range of 10−8 to 10−7 and independent of the intensity of light.[19] Therefore, photoinhibition occurs at all light intensities and the rate constant of photoinhibition is directly proportional to light intensity. Some measurements suggest that a photon of dim light causes damage even more efficiently than a photon of strong light.[20]

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EVALUATION OF LETTUCE GROWTH USING SUPPLEMENTAL GREEN LIGHT WITH RED AND BLUE LIGHT-EMITTING DIODES IN A CONTROLLED ENVIRONMENT - A REVIEW OF RESEARCH AT KENNEDY SPACE CENTER

Authors: H.H. Kim, R.M. Wheeler, J.C. Sager, G.D. Gains, J.H. Naikane
Keywords: controlled environment, photosynthetic rate (Pn), stomatal conductance (gs), electric light source, lettuce (Lactuca sativa), light quality, light-emitting diode (LED)
Abstract:

National Aeronautics and Space Administration’s (NASA) Biological Sciences research group at Kennedy Space Center performed several experiments with lettuce, one of the Advanced Life Support candidate crops, to evaluate the effects of green light in a controlled environment. Lettuce showed similar growth and photosynthetic rates with the addition of 5 % supplemental green light compared to the red and blue LEDs only grown plants. The addition of green light provided an aesthetic appeal of a green appearance. However, light sources with a higher fraction of green photons (> 50 % of total PPF) resulted in the reduced plant growth. Among the levels of green photons tested, the addition of 24 % green light (500 – 600) to red and blue LEDs enhanced plant growth. Spectral quality during growth affected the leaf photosynthetic rates (Pn) and the pattern of diurnal stomatal conductance (gs). The studies provided new information showing that leaf Pn and gs are responsive to spectral quality during growth and in the short-term, but are not directly coupled to dry mass accumulation.

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severian said:

...tenuous. No added dry weight but maybe quicker growth. Possible vegetative benefits but at the cost of 30% more power? What does it do for flowers? Probably not much.

Maybe they get more comfortable, less stress, fewer resins?

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One reason is that although green light is absorbed with low efficiency, it has many chances to be absorbed because it is repeatedly reflected from cell to cell by the complex leaf geometry so it has many chances to be absorbed.

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mrred said:

so can a green light cause my plants to hermie from the light? what your saying its a bad ideal to use a green light to go in the room when its nighttime?

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severian said:

...tenuous. No added dry weight but maybe quicker growth. Possible vegetative benefits but at the cost of 30% more power? What does it do for flowers? Probably not much.

Maybe they get more comfortable, less stress, fewer resins?

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Might be an argument to add green LEDs to the ever changing LED grow panels.

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