Why the forests are green?

The question posed by the title may seem unexpected or even strange, but it has direct relation to the cutting-edge research in a few fields of bio-compatible nanotechnology, especially in bio-like solar cells; it has much to do with our understanding of evolution, and this question is not as simple as it seems.

The "common-sense" answer could be: "The green color of plants is the color of life". Taking this poetic sentence for truth, one would conclude that the leaves of living plants are green because the green band of solar spectrum plays the key role in physical chemistry underlying the plants' life. But does our poetically inspired common sense truly suggest a solution for the puzzle?

In general cycle of life, the plants are responsible for the primary transformation of inorganic matter into organics using energy of sunlight, first of all - transformation of carbon from individual atoms bonded by oxygen, i.e. primitive molecules CO₂, into long branched -C-C- chains serving as "skeleton" of multifarious organic molecules. Although specific mechanisms of photosynthetic factory of life are complex and diversified (the details may be found, for instance, in [1]- [12]), in terms of energy exchange, the pigments of leaves represent molecular antennas selectively receiving (absorbing) certain portion of the incoming solar spectrum and thus selectively reflecting the rest. From the "point of view of the plant", the entire green leaf serves as a receiving antenna; from an "external observer" point of view, each green leaf is a transmitting antenna which reflects (re-transmit from the sun) predominantly "green" electromagnetic waves.

Thus, plants absorb the photons to power photosynthesis, and the pigment molecules such as chlorophyll, function as the primary antennas-receivers of solar photons. A pigment shows the "color" of reflected photons, while the remaining portion of spectrum is absorbed. Hence, the better reflected, i.e. lesser effective, photons are responsible for the visible color of leaves. Hence, the plants are green because the green band of visible spectra is the least effective for photosynthesis.

All the shown spectra, although sufficiently accurate for this specific aspect of discussion, must be considered as only schematics. Any specific details may be found in original publications: Absorption spectra of carotenoids, chlorophyll and anthocyanin may be found in above referred publications. Average crown reflectance spectra are based on [13] and [14]. Details on human spectral sensitivity may be found in [15] and [16]. Note: On two top diagrams, the horizontal scales are linear vs. the wavelength, on the bottom diagram - vs. frequency. However, the ranges are the same, and all three diagrams are matched at wavelength 550 nm (shown with central axis). Although one of three major pigments (anthocyanin) possesses relatively high absorption in green-yellow bands, it does not change the entire picture, from northern Canadian maples to tropical species. Thus, essential portion of solar spectra is actually lost in photosynthesis, which the Nature carefully developed over billions years targeting the maximum effectiveness.

Accordingly to one hypothesis, this particularity of photosynthesis was caused by the very ancient evolutionary era in relatively deep waters where the green and shorter bands of light could not effectively penetrate. In the later era, the predominant evolution had relatively sharply shifted into the shallow waters, and the blue light had become more effective, while the long-wave based reactions (earlier developed) had been preserved leaving the green-yellow band gap. In the other words, green color of plants is an accidental deficit of photosynthesis, an "involuntarily" shortfall of evolution.

Although usually cited, this hypothesis has many weak points and is not considered as the satisfactorily convincing one. Thus, while not contradicting against the solid contemporarily knowledge, one could look for different explanation.

Assuming that over nearly four billions years of evolution on the Earth, the Nature had sufficiently long time to develop an optimal photosynthetic "technology", one would look for a hidden rational reason prompted the Evolution to leave a "green gap". May be the green signal retransmitted by the leaves-antennas is "intentionally" designated by the Nature to some recipients?

As soon as we assumed that the existing mechanism of photosynthesis is truly optimal, we may forget for a moment the past paths of evolution while focusing our attention on the possible useful function of green gap. Let's imagine that there is no any gap, the absorption spectra are even, approaching maximum effectiveness over the entire visible range. What color would posses the plants?

The answer is: they will be black (or dark grey) . The utmost effective and flat absorption spectrum would mean a weak and flat reflection spectrum leaving no color, no specific features for an external observer.

A human would definitely dislike such a forest idea. The opponents may argue that the human taste is not important in this consideration because as species we are a very recent product of evolution. But most of animals would be unhappy as well: the green color is the color of food, and as such the green light is the carrier of life-important signals. Some animals may still find the food by smell, but electromagnetic wave, in particular visible light (plus near UV for bees, bats, and etc., and near IR for snakes), is the absolutely dominant carrier of signal in the kingdom of animals. The life was recently discovered in deep darkness, but there is no much evolutionary progress. The Life needs equally: Matter - to build the living bodies, Energy - to act, and Information - for interaction with environment and communication between the living beings.

We have a very good reason to believe that trillions of random elementary attempts over billions years, and a free competition between the products had resulted with wise eventual solution. In the other words, Evolution of Life wisely sacrifices the portion of incoming sun energy nearly exactly in the center of visible band to use it for signals, data analysis and decision making by living beings.

To find a support for this assumption, let's compare the action spectrum of chlorophyll with the sensitivity spectrum of human eyes, and we may be surprised that the minimum of the first matches rather well the maximum of the second. (We do not consider here the specific physical-chemical mechanisms underlying the color vision, such as three types of cone in human retina, because we are interested in actually resulted spectral sensitivity. The detailed description of these mechanisms may be found, for instance, in [15], [16]). Both, photopic (in bright light) and scotopic (in dim light) spectral sensitivity in the monkey shows "detailed similarities" with humans one [17]; "Spectral sensitivity functions for each animal agreed well with results obtained from a human subject in the same apparatus" [18].

The ungulates, such as cows, goats, and sheep, have dichromatic color vision with one of the peaks "tightly clumped at about 552-555 nm"[19]. Perhaps, the herbivorous animals represent especial interest for this consideration, and it is appropriate to note that the location of peak sensitivity in cows, goats, and sheep varied from about 444 to 455 nm. i.e. in the proximity of the first peak of the overhead spectrum of blue sky [20]. In the other words, the herbivorous animals developed the first peak for the signal of food and the second one - for contrasting overhead color. The spectral sensitivity of other animals, from rats to fish, although they demonstrate some variation, mostly express a clear "attraction" to green range of spectrum as well [21]-[22].

The opponents may still argue that the animals' eyes could be "developed" to match the plants' spectrum. But, on the one hand, such an argument would generate even more questions about the evolutionary goal of such adaptation while not answering the original question. On the other hand, animals play essential role in the life of plants as the pollinators, as the fertilizers, as the seeds' distributors, as the regulators of specific plants' growth, and as the active participants of carbon-oxygen cycle. One may still argue: even the entire kingdom of all land animals is relatively recent product of evolution as well. How evolution could envision the conditions of the current environment? We will not hypothesize any longer in this short publication, but will limit ourselves with one note: the green forests and grasslands were evolving actually synchronically with the land animals; may be in lessen but still essential extend it is correct for much longer preceding evolution in water. The question - what was evolved first, the contemporary plants or the contemporarily animals? - may be semantically equal to question what had appeared first - chicken or egg?

The question remains open. Suggested hypothesis is not a mathematically proved theorem. But assumption that the nature is "EVENTUALLY WISE" leads to conjecture that "the green color of life" is the result of CO-EVOLUTION OF PLANTS AND ANIMALS.

Acknowledgement

This article was written by Benjamin Dorfman (published with permission from the author).

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