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 Botany at Dairy Farm. 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)] 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). ” 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), 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 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) (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” of light. In addition, flavins absorb blue wavelengths of light [to some degree], also contributing to a “reddish” color in leaves.  “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. ” 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. 
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) blue wavelengths of light and reflect red wavelengths as Rhodophyta engage in 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” for plants, excess sugar (glucose) molecules are converted into starch, “a polymer… to store energy” 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, ” 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. ”
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. 
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. ”.