Each spring when the colorful mosaic of flowers comes to a close, red-leafed plants stand out in the sea of green. While green- and red-leafed trees, bushes, and flora consist of chloroplasts to undertake photosynthesis, the latter utilize anthocyanins to provide added benefits and to differentiate themselves. The question is, with the continued ozone depletion that allows harmful ultraviolet (UV) rays to penetrate the atmosphere at greater levels and intensity and subtle changes in sunlight ranging from brightness to the way it is refracted due to the continued buildup of emissions and pollutants, is the existence of red-leafed plants evidence of evolution in progress? Is a transformation underway in which they will become the dominant type?
While these questions cannot be readily answered, it appears that red-leafed plants hold several advantages. They absorb green and yellow wavelengths (two dominant colors of the spectrum), they attract “friendly” insects to assist with pollination, they repel “hostile” pests that would exploit them, and they can tolerate environmental stress better than green-leafed plants because of their slower metabolism. However, to gain these advantages, red-leafed plants must expend energy and utilize nutrients to produce the pigmentation responsible for their color.
Red-leafed plants “are common throughout all orders of the plant kingdom, from… basal liverworts [mosses, ferns, gymnosperms (cycads or conifers)][1] to the most advanced angiosperms (flowering plants with ovaries). They [exist] in habitats as diverse as the Antarctic shoreline and the tropical rainforests, are as abundant in arid deserts as in freshwater lakes, and seem equally at home in the light-starved forest understorey (ground-lower level) as in the sun-drenched canopy (upper level-top).”[2] While the existence of red leaves is transient in some plants (e.g. deciduous plants that change colors in the fall, others that start out with red hues in the spring), it is permanent in other species. The focus of this article is on the plants with red leaf pigments that exist for the duration of their lives.
The “Red” in Leaves
Anthocyanins (mainly cyanidin-3-O-glucoside)[3], which belong to the flavonoid family are the key water-soluble pigment responsible for giving a plant its red color. They are synthesized in the cytoplasm[4] and reside in the vacuole of leaf cells. Other contributing pigments or photoreceptor chemicals that emit “reddish” colors are thiarubrine A, the 3-deoxyanthocyanins, the betalains, some terpenoids, and certain carotenoids. These pigments too, may perform similar functions and provide similar benefits as anthocyanins.
Based on their properties, anthocyanins absorb the green and yellow wavebands of light, commonly between 500 and 600 nanometers (nm)[5] (each nonmeter is equal to one billionth (10-9) of a meter), making leaves appear red to purple as they “reflect the red to blue range of the visible spectrum”[6] of light. In addition, flavins absorb blue wavelengths of light [to some degree], also contributing to a “reddish” color in leaves.[7] “Interestingly [though], the amount of red light that is reflected from red leaves often… correlates [poorly] to anthocyanin content; leaf morphology (structure and form) and the amount and distribution of chlorophyll are… stronger determinants of red reflectance.”[8] Although chlorophyll is the pigment responsible for giving most plants their green color, an experiment showed that it can play a role in red reflectance. When a transparent pure chlorophyll solution was created from ground up spinach leaves mixed with acetone to dissolve chloroplasts and their membranes, it reflected a “reddish glow/flourescence” when a beam of light was directed at it.[9]
When it comes to Rhodophyta (Red Algae), phycoerythrin, a pigment belonging to the phycobilin family found in its chloroplasts is responsible for its color. Phycoerythrins absorb (between 500 and 650 nm. of)[10] blue wavelengths of light and reflect red wavelengths as Rhodophyta engage in photosynthesis.
Photosynthesis
Photosynthesis is the process that plants and some bacteria use to convert energy from sunlight into sugar (glucose); which cellular respiration converts into ATP (adenosine triphosphate), chemical energy or the “fuel” used by all living organisms. Photosynthesis uses six molecules of water (transported through the stem from the roots) and six molecules of carbon dioxide (that enter through a leaf’s stomata or openings) to produce one molecule of sugar (glucose) and six molecules of oxygen (6H2O + 6CO2 -> C6H12O6+ 6O2), the latter, which is released into the air (also through the leaf’s stomata). Although “sugar (glucose) molecules formed during photosynthesis serve as… the primary source of food”[11] for plants, excess sugar (glucose) molecules are converted into starch, “a polymer… to store energy”[12] for use at a later time when photosynthetic sources of energy are lacking.
While chlorophyll (green) is the best-known photosynthetic pigment, other pigments also play a role in converting sunlight into useable energy. They include carotenoids such as carotene (orange), xanthophylls (yellow), and phycoerythrin (red). When engaging in photosynthesis, chlorophyll “absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths,”[13] while carotenoids and xanthophylls absorb some energy from the green wavelength, and phycoerythrin absorbs a significant amount of its energy from the blue wavelength. Many plants use multiple pigments for photosynthetic purposes, enabling them to maximize use of sunlight that falls on their leaves.
When comparing photosynthesis that occurs within red and green leaves, the latter, which have greater concentrations of chloroplasts, scientific studies have shown that the rate of photosynthesis is higher in green-leafed plants. In one experiment, green and red leaves were collected from the same deciduous tree and exposed to 5-10 minutes of light and another 5-10 minutes of darkness. Afterwards the change in Carbon Dioxide (CO2) levels was measured to determine the rate of photosynthesis. The “results showed that green leaves [had] a higher mean rate of photosynthesis (-.5855 parts per million (ppm) CO2/minute/gram) than red leaves (-0.200 ppm CO2/minute/gram). [However] the differences in [the] average rates of photosynthesis were not significantly different.”[14]
Another experiment compared the photoperiodic sensitivity of green-leafed (Perilla frutescens) and red-leafed (Perilla crispa) Perilla (flowering Asian annuals) or how long it took each of the Perilla plants to reach the same level of growth or flowering based on exposure to different light conditions. When exposed to 8 hours of light, red-leafed Perilla took 4 days longer to reach the same growth stage as green-leafed Perilla. The results were more dramatic when each plant was exposed to continuous light – red-leafed Perilla took between 47 to 55 days longer to reach the same growth stage as green-leafed Perilla.[15]
A third experiment involved an in-depth study of photosynthesis in red- and green-leafed Quintinia serrata, a tree native to New Zealand. When the rate of photosynthesis was measured at the “cellular, tissue, and whole leaf levels to understand the role of anthocyanin pigments on patterns of light utilization” of red- and green-leafed Quintinia serrata, it was found that “anthocyanins in the mesophyll (photosynthetic tissue between the upper and lower epidermis of a leaf) restricted absorption of green light to the uppermost [section of the] mesophyll [and that] distribution was further restricted when anthocyanins were also present in the upper epidermis.”[16]
Accordingly, “mesophyll cells located beneath a cyanic (blue or bluish) light filter assumed the characteristic features of shade-adapted cells, [with red leaves showing] a 23% reduction in CO2 assimilation under light-saturating conditions, and a lower threshold of irradiance (density of radiation occurrence) for light-saturation, relative to those of green leaves.”[17] In short, the findings were consistent with the previous two experiments in which red leaves displayed slower rates of photosynthesis, exhibiting photosynthetic characteristics of “shade-acclimated plants.”[18]
Although green-leaves appear to hold the advantage when it comes to photosynthesis, this advantage should not be overstated since to compensate for their slower rate of photosynthesis, red-leafed plants exhibit slower metabolism as established by an experiment using Iodine to test for the presence of starch. When I tested red leaves of an Acer Palmatum Japanese Maple and Acer Rubrum Red Maple in May 2006 (when the leaves were young and after the respective trees had expended most of their excess reserves over the winter), red Iodine turned a dark blue when placed on their sub-epidermal tissue, indicating that each leaf tested held this polymer. Thus, their photosynthetic activities were not only producing sufficient amounts of sugar (glucose) but excess reserve amounts. Had the metabolic rate of red-leafed plants been comparable, the same, or faster than that of green-leafed plants, when their rate of photosynthesis is slower as established by the above three experiments, it is unlikely that they would have been able to produce excess sugar (glucose) as evidenced by the presence of starch.
In addition, the presence of anthocyanins, which slow photosynthetic rates also provide an advantage. They give red-leafed plants a higher tolerance for harmful UV rays, which can reduce photosynthetic productivity in green-leafed plants. This is especially important when considering the ongoing depletion of the stratospheric ozone layer that filters harmful UV rays, minimizing the level that reaches the earth’s surface.
Anthocyanins
The presence of anthocyanins provide important benefits (which are discussed below) to red-leafed plants. Had their presence been for mere extravagance (e.g. solely to provide color – red, pink, purple, and blue hues to enable these plants to stand out from the sea of green) it is unlikely that red-leafed plants would make “the considerable metabolic investment” and expend energy to synthesize and accumulate the pigment in their leaf cells. As Kevin S. Gould, one of the world’s leading botanists and researchers wrote, “…a wealth of… evidence, ascribes a remarkable diversity of functions to anthocyanins… many of them associated with stress responses and some potentially critical to a plant’s survival.”[19] In his words, “Anthocyanins are arguably the most versatile of all [plant] pigments, their multifarious (diverse) roles in plant stress responses stemming as much from the physicochemical property of light absorption as from their unique combination of biochemical reactivities.”[20]
U.S. Department of Agriculture (USDA) Forest Service Researcher Paul Schaberg, a plant physiologist agrees. “There are all kinds of strategies that [plants] have to protect themselves and increase their chance of survival. …some trees have deep roots to get extra water and survive droughts. Others survive by just using less of the water they get. …red leaf coloration may be one of those specialized adaptive traits…”[21]
Protection from Hostile Environments
“When leaves receive more light energy than can be used in photochemistry, they show a characteristic decline in the quantum efficiency of photosynthesis, termed photoinhibition. Under severe conditions [critical leaf structures such as chloroplasts, thylakoid membranes (the area where photons from sunlight initiate photosynthesis), DNA, and proteins essential for photosynthetic activities can be harmed or destroyed].”[22]
Anthocyanins, through their absorption of blue-green light, “have been shown to protect plants from excess light during periods of high light stress (as occurs when plants are exposed to high light in combination with drought or cold temperatures)”[23] by providing a physicochemical barrier to protect a leaf’s chloroplasts and other critical structures. “Chloroplasts irradiated with light that has first passed through a red filter have been shown to generate fewer superoxide radicals (highly oxidized compounds)” that could damage a plant’s “photosystems (group of structures that perform photosynthesis)”[24] and impair its ability to transfer and use necessary sugar (glucose) to sustain its metabolism.
Numerous studies have shown that anthocyanins can effectively “reduce both the frequency and severity of photoinhibition [and] expedite photosynthetic recovery. [For example] in red-osier dogwood (Cornus stolonifera), a 30-minute exposure to strong white light [was found to have reduced] the quantum efficiency of photosynthesis by 60% in red leaves [and] by almost 100% in acyanic leaves (those without anthocyanins). [Then] when the plants were returned to darkness, the red leaves recovered to their maximum [photosynthetic] potential after… 80 minutes [while] their acyanic counterparts [still had not fully recovered] after six hours.”[25]
Another study involving the Setcreasea purpurea, a plant that can grow well under both extremely low light and high light conditions also illustrated this. When Setcreasea purpurea plants were kept in low light conditions, they did not synthesize anthocyanins. Accordingly their leaves were green. However, when these same plants were exposed to bright light conditions, they defensively accumulated anthocyanins in the epidermal layer of their leaves, transforming the color of their leaves to red. Accordingly, when the epidermal layer consisting of anthocyanins in the red leaves was removed, they exhibited a photoinhibitory effect similar to that experienced by green leaves, in which their photosynthetic yield was reduced by between 2-4 times from that of leaves where anthocyanins were present in their epidermal layer.[26]
Second, because of their ability “to absorb strongly in the UV region [of the spectrum]” anthocyanins have been proven to protect critical structures from “potentially damaging [amounts of] UV-B radiation – red-leafed Coleus varieties [were found to have retained] higher photosynthetic efficiencies… than green-leafed varieties” after exposure to UV radiation.[27] However, anthocyanins’ ability to absorb UV radiation is also a double-edged sword since if damage occurs, such absorption impairs DNA repair.
Third, since red-leafed plants have slower rates of photosynthesis and metabolism, they can also thrive well in low-light, high or low temperatures (anthocyanins “function as antifreeze by protecting leaves during a frost”[28] and from extreme heat) as well as from drought and desiccation (severe drying of tissues). For example, “New Zealand’s liverworts, [a primitive and simple land species] are remarkably resilient to environmental stresses such as intense sunlight, high temperatures, UV-B radiation, and desiccation.”[29]
In addition to protecting against harmful lighting, UV radiation, temperature extremes, droughts and desiccation, anthocyanins have been found to enhance plant resistance to heavy metal contamination (e.g. mercury) and wounds (e.g. leaf punctures).
Protection of Photolabile (Light-Sensitive) Defense Compounds and against Herbivory
Anthocyanins because of their absorptive qualities also play an important role in protecting photolabile (light-sensitive) molecules from degradation and damage caused by exposure to bright light and UV radiation. An example of this is illustrated in the silver beachweed (Ambrosia chamissonis), a plant that grows in sunny areas and contains large amounts of thiarubrine A, “a potent defense compound that is toxic to insects, bacteria, and fungi.” Without protection from two anthocyanic pigments – cyaniding-3-O-glucose and cyaniding-3-O-(6’-O-malonylglucoside) – this plant’s defenses would be rendered impotent since thiarubrine A is photolabile; even short exposures to visible light and/or [UV] radiation render it inactive.”[30]
Second, red color alone has also been found to protect plants from herbivory (harmful insects and organisms that feed on them). For example, “California maple aphids… readily [and harmfully] colonize [and exploit] yellow-orange leaves of Japanese maples [while ignoring] red-leafed [Japanese maples]. Similarily, leaf-cutting ants from tropical forests… browse significantly less on red leaves than on green leaves, [indicative that] athocyanins may serve as aposematic signals (warning of a special defense against enemies such as poison and toxins), [mimick the appearance of dead, inedible foliage and/or] simply render the leaves unpalatable.”[31]
Attraction of “Friendly” Creatures
While serving as a potent repellant to herbivores that could potentially cause serious or mortal harm through ingestion of plant foliage and compounds, anthocyanins also play the opposite role when it comes to “friendly” creatures. “The red colors of anthocyanic leaves”[32] also serve as a means to attract “birds, insects and mammals… for pollination and reproduction [purposes].”[33] When the fruit and background foliage of a Canadian shrub were manipulated, it was found that “red-orange [coloring accentuated] the conspicuousness of [its] black-colored fruits to birds [enabling them to remove the fruits at a higher rate, accordingly enhancing the plants’ reproductive efforts since when the fruits were consumed, their inedible seeds fell to the ground for possible germination].[34]
Antioxidants and Scavenging Free Radicals
Another important role of anthocyanins is that they serve as antioxidants to scavenge free radicals (highly oxidized compounds that could damage proteins, membrane lipids, DNA and other botanic structures) for salvageable compounds and diffuse their harmful energy. They reduce the amount of oxidized compounds by filtering out yellow-green light since “the majority of reactive oxygen in plant cells is derived from the excitation of chlorophyll.”[35] In one study, anthocyanins were found “to have the strongest antioxidizing power of [a group] of 150 flavonoids.” In another study, conducted by the USDA, anthocyanins in blackberries were found to have a potent antioxidant capacity against “superoxide radicals, hydrogen peroxide, and other oxidants.”[36] A third study, involving Arabidopsis plants (a species belonging to the mustard family that have white, yellow or purplish flowers) found that exposure “to strong light and low temperatures caused more lipid peroxidation in anthocyanin-deficient [plants]… Similarly, upon gamma irradiation, only those Arabidopsis plants that contained both anthocyanin and ascorbic acid were able to grow and flower normally.”[37]
Likewise, in a study involving Elatostema rugosum plants it was “established that red-leaved morphs held a significant antioxidant advantage over green morphs.”[38]
In addition a study reported by Kevin S. Gould also showed that anthocyanins served as a chemical remedy to repair damage caused by oxidation and free radicals. “Microscopic examinations of wounded leaf peels have shown that red-pigmented cells eliminate Hydrogen Peroxide (H2O2) significantly faster than… green cells.”[39]
Conclusion
While it appears red-leafed plants with their slower metabolic rate (to compensate for the edge green-leafed plants hold regarding photosynthesis) have an advantage because of the powerful presence of anthocyanins that give them an edge in hostile environments and assist with healing, the case cannot be made that “red” is biologically superior to “green.” Many green-leafed plants also utilize anthocyanins to reduce climactic stress – the only difference being is that they only expend energy to produce this pigment when most needed – spring to resist freezes and in autumn against heightened sensitivity to light exposure, thus ensuring that their leaves can “function long enough to unload nutrients and sugars” to store for the approaching winter and to give them a “head start for the next growing season.”[40] Last, if advantages of red-leafed plants were materially overwhelming (which, if their presence constitutes evolution in progress, could some day be the case depending on global warming, ozone depletion, and other adverse ecological changes), the vast majority of the Earth’s land would not be clothed in green.
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[1]Kevin S. Gould. Plant ecophysiology research. (Otago, New Zealand: Department of Botany, University of Otago). 19 May 2006. [http://www.botany.otago.ac.nz/plant_ecophysiology/]
[2]Kevin S. Gould. Nature’s Swiss Army Knife: The Diverse Protective Role of Anthocyanins in Leaves. Biomedicine and Biotechnology. 15 July 2004. 19 May 2006. http://www.pubmedcentral.gov/articlerender.fcgi?artid=1082902
[3]Kevin S. Gould. Nature’s Swiss Army Knife: The Diverse Protective Role of Anthocyanins in Leaves.
[4]Kevin S. Gould, et. al. Functional role of anthocyanins in the leaves of Quintinia serrata A. Cunn. Journal of Experimental Botany, Vol. 51, No. 347 (June 2000). 22 May 2006. http://jxb.oxfordjournals.org/cgi/content/full/51/347/1107
[5] Kevin S. Gould. Nature’s Swiss Army Knife…
[6]Anthocyanin. Wikipedia.com. 13 May 2006. 19 May 2006. http://en.wikipedia.org/wiki/Anthocyanin
[7]Seeing Red. NewScientist.com. 29 October 1994. 22 May 2006. [http://www.newscientist.com/backpage.ns?id=lw63].
[8]Kevin S. Gould. Nature’s Swiss Army Knife…
[9]W.P. Armstrong. Photosynthesis & Cellular Respiration: Supplements To Biology 101 Cell Unit. 2001. 19 May 2006. http://waynesword.palomar.edu/photsyn1.htm
[10]Lecture 5 Photosynthesis I: Light, Pigments, and Leaves. 22 May 2006. [http://biology.wright.edu/courses/304/Lect5PhotoI.html]
[11]Karin Tanino. Photosynthesis: Leaf Coloration. 19 May 2006. http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/Photosynthesis/LeafColoration/LeafColoration.htm
[12]Starch. Polymer Science Learning Center, Department of Polymer Science, University of Southern Mississippi. 2003. 22 May 2006. http://pslc.ws/macrog/kidsmac/starch.htm
[13]M.J. Farabee. Photosynthesis. 2001. 19 May 2006. [http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html]
[14]Courtney Marne et. al.. The Effects of Leaf Color on Rates of Photosynthesis. Colorado University Boulder (Boulder, CO. 2004). 19 May 2006. http://www.colorado.edu/eeb/courses/1230jbasey/abstracts/29.htm
[15]William P. Jacobs. Comparison of Photoperiodic Sensitivity of Green-Leafed and Red-Leafed Perilla. Plant Physiology Vol. 70. July 1982. 19 May 2006. http://www.plantphysiol.org/cgi/content/abstract/70/1/303
[16]Kevin S. Gould et al. Profiles of photosynthesis within red and green leaves of Quintinia serrata. Plant Sciences Group, School of Biological Sciences, University of Auckland (Auckland, New Zealand: September 2002). 19 May 2006. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12207671&dopt=Abstract
[17]Kevin S. Gould et. al. Profiles of photosynthesis…
[18]Kevin S. Gould et al. Profiles of photosynthesis…
[19] Kevin S. Gould. Nature’s Swiss Army Knife…
[20] Kevin S. Gould. Nature’s Swiss Army Knife…
[21]Glenn Rosenholm. Maples gamble on antioxidant’s value, says New England researcher. News Release USDA Forest Service Northeastern Area. 18 October 2005. 24 May 2006. http://www.na.fs.fed.us/nanews/archives/2005/oct05/mapleantiox/mapleantiox.htm
[22] Kevin S. Gould. Nature’s Swiss Army Knife…
[23]Anthocyanin. Wikipedia.com. 13 May 2006. 19 May 2006. http://en.wikipedia.org/wiki/Anthocyanin
[24] Kevin S. Gould. Nature’s Swiss Army Knife…
[25] Kevin S. Gould. Nature’s Swiss Army Knife…
[26]David Dewez et. al. Photorotective role of anthocyanins regulating PSII activity
[27] Kevin S. Gould. Nature’s Swiss Army Knife…
[28]Glenn Rosenholm. Maples gamble on antioxidant’s value, says New England researcher. [29]Kevin S. Gould. Plant ecophysiology research. Department of Botany, University of Otago (Dunedin, New Zealand). 19 May 2006. [http://www.botany.otago.ac.nz/plant_ecophysiology/]
[30] Kevin S. Gould. Nature’s Swiss Army Knife…
[31] Kevin S. Gould. Nature’s Swiss Army Knife…
[32] Kevin S. Gould. Nature’s Swiss Army Knife…
[33]Sean Henahan. Time to Leave. The National Health Museum. 19 May 2006. http://www.accessexcellence.org/WN/SUA06/leaves.html
[34] Kevin S. Gould. Nature’s Swiss Army Knife…
[35] Kevin S. Gould. Nature’s Swiss Army Knife…
[36]Marilyn Sterling, R.D. Got Anthocyanins? Nutrition Science News. (Penton Media, Inc. 2006). 19 May 2006. [http://www.newhope.com/nutritionsciencenws/nsn_backs/Dec_01/antho.cfm]
[37] Kevin S. Gould. Nature’s Swiss Army Knife…
[38]Samuel O. Neill et. al. Antioxidant capacities of green and cyanic leaves in the sun species, Quintinia serrata. Functional Plant Biology. Abstract. 19 May 2006. http://www.publish.csiro.au/nid/102/paper/FP02100.htm
[39] Kevin S. Gould. Nature’s Swiss Army Knife…
[40]Glenn Rosenholm. Maples gamble on antioxidant’s value, says New England researcher.
William Sutherland is a published poet and writer. He is the author of three books, "Poetry, Prayers & Haiku" (1999), "Russian Spring" (2003) and "Aaliyah Remembered: Her Life & The Person behind the Mystique" (2005) and has been published in poetry anthologies around the world. He has been featured in "Who's Who in New Poets" (1996), "The International Who's Who in Poetry" (2004), and is a member of the "International Poetry Hall of Fame."
