Why Leaves Turn Red

A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society

This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Why Leaves Turn Red

Pigments called anthocyanins probably protect leaves from light damage by direct shielding and by scavenging free radicals

David W. Lee and Kevin S. Gould

any forests, like those spread the forest’s 70 percent of tree species gave them their name, deriving antho- Mthroughout New England, have that contain anthocyanin pigments cyanin from the Greek anthos, meaning just changed color in a spectacular (which produce colors ranging from flower, and kyanos, meaning blue. way, as they do each fall. The phenom- brown to red, depending on how Many long-standing misconceptions enon is familiar as well as dramatic, yet much chlorophyll the leaves retain), about anthocyanin function date from why it should happen has been a long- the story is quite different. For exam- these early observations, notably that standing enigma. When we were in ple, the brilliant fall foliage of the red these pigments arise from the break- school, the standard textbooks said oak (Quercus rubra) results from the ac- down of chlorophyll during autumn. that foliage changes color because the cumulation of anthocyanin in the vac- Given how striking and attractive breakdown of green chlorophyll mole- uoles (large, fluid-filled cavities) of red foliage is, it may seem baffling that cules unmasks other pigments, like the cells lying just under the leaves’ upper botanists remained ignorant about the yellow-to-orange xanthophylls and the epidermis layer. phenomenon for so long. There are red-or-blue anthocyanins, which, we Anthocyanins are elaborate pigment various reasons for this. First, because were told, serve no particular function molecules, widespread among land anthocyanins are responsible for the during the autumn senescence of plants. They account not only for the colors of fruits and flowers as well as of leaves. Now botanists know better. autumn hues of temperate woodlands, leaves, it was natural to concentrate on Indeed, a completely new apprecia- but also for the flushes of developing pigmentation in the former economi- tion for these colorful pigments has de- red foliage seen in tropical forests, on cally important organs, for which the veloped over the past decade or so, in the undersurface of shaded leaves and function of anthocyanin seems obvi- part from our studies of trees in the in crop plants suffering drought or nu- ous—to attract animals for pollination Harvard Forest, a nature sanctuary in trient deficiency. But plants can also and seed dispersal. Second, because central Massachusetts maintained for have other red pigments. Carotenoids, the discoveries of Richard Willstätter scientific research. There, during Sep- often rhodoxanthin, produce red color and his colleagues about the molecu- tember and October, one sees the in the senescing leaves of some conifers lar structure of anthocyanins from 1912 leaves on dozens of woody species as well as in the common box (Buxus to 1916 were made shortly after the re- changing color. In some plants, such as sempervirens), which decorates many discovery of Mendel’s laws of inheri- the witch hazel (Hamamelis virginiana), suburban lawns. Betalain pigments tance, the anthocyanins became an ear- it is indeed the loss of chlorophyll that color leaves red in a single order of ly subject of research in molecular reveals yellow carotenoid pigments, flowering plants, and a few other mis- genetics, rather than physiology. just as the textbooks say. However, for cellaneous pigments produce bur- (Mendel’s peas had distinctively col- gundy hues in very rare cases. But of ored flowers because of anthocyanins.) David Lee is a professor in the Department of Bio- all the red pigments, the anthocyanins Third, the discovery that light can in- logical Sciences at Florida International Universi- are the most widespread. duce anthocyanin production inspired ty and Research Collaborator at Fairchild Tropical We have collaborated in studying an- molecular biologists to study how light Garden in Miami. He has studied leaf color and thocyanin pigments since 1993 and are exposure activates genes involved in function since 1973. He received his Ph.D. in beginning to develop some working anthocyanin synthesis, again at the ex- botany from Rutgers University in 1970. Kevin hypotheses about their function. It’s cu- pense of research into anthocyanin Gould is an associate professor in the School of Bi- rious that an understanding has been function. ological Sciences at the University of Auckland in so long in coming, given the fact these Botanists of the late 19th-century, most New Zealand. He received his Ph.D. in botany red pigments have been subjected to notably the Germans who studied plant from the University of Manchester in 1985. He has applied his background in plant cell and devel- scientific scrutiny for nearly 200 years. anatomy and physiology, noticed that opmental biology to the mysteries of the functional anthocyanin production rises when a ecology of New Zealand’s diverse plants. Address The Discovery of Anthocyanins plant is subjected to low temperatures for Lee: Department of Biological Sciences, Florida Anthocyanins had been observed for and high light conditions. This observa- International University, Miami, FL 33199. Inter- centuries as “colored cell sap.” In 1835 tion led to the popular explanations that net: [email protected] the German botanist Ludwig Marquart anthocyanins protect the photosynthetic

© 2002 Sigma Xi, The Scientific Research Society. Reproduction 524 American Scientist, Volume 90 with permission only. Contact [email protected]. Figure 1. Anthocyanins are common in the autumn leaves of mid-latitude trees, but these pigments also add flashes of red color to the foliage of tropical forests and to crop plants exposed to drought or nutrient deficiency. Occasionally, anthocyanins are present in leaves year-round, as in the variegated leaves of Horopito (Pseudowintera colorata), a common tree in New Zealand forests (above). The patterns of coloration in this species make the leaves ideal for studying the antioxidant properties of these intriguing molecules. (Except where noted, all photographs by the authors.) structures against intense sunlight and Red Sunscreen variety of factors can contribute to help to warm leaves by increasing their When surroundings are bright and photoinhibition: intense sunlight; low rates of metabolism. These scientists cold, photosynthetic efficiency often temperature; acclimation of leaves to lacked the instrumentation and detailed declines. The phenomenon, known as extreme shade with a subsequent ex- knowledge of photosynthesis to test their photoinhibition, has been attributed in posure to high light; and inadequate ideas. In the mid-20th century, investiga- part to impairment in one of the func- phosphorus, which is important in the tors became aware that ultraviolet (UV) tional elements of photosynthesis. Nor- production of two energy-rich com- radiation could induce anthocyanin syn- mally, two units consisting of pig- pounds crucial for photosynthesis— thesis, leading to the hypothesis that an- ments, proteins and electron-transfer adenosine triphosphate (ATP) and the thocyanins protect plant tissues against molecules—known as photosystems I reduced form of nicotinamide adenine UV damage. But, as it turns out, antho- and II—absorb light energy. Photoinhi- dinucleotide phosphate (NADPH). cyanins absorb rather weakly in the UV- bition apparently involves a block in When chloroplasts are overwhelmed B region of the spectrum (wavelengths photosystem II. Unchecked, this im- with energy, the excess causes chemical of 285–320 nanometers), which is most pairment can permanently damage and, ultimately, physical damage. responsible for damage to biological tis- chloroplasts, cells and tissues. Plants have evolved several strate- sues; other colorless flavonoid pigments Investigators can observe photoinhi- gies to prevent photoinhibitory damage that are equally, or more, abundant in the bition because when photosynthetic tis- from intense light, in particular the in- leaves absorb UV-B much more strongly. sues receive a pulse of intense light, terconversion of certain xanthophyll Furthermore, anthocyanins are most they immediately emit a pulse of visi- pigments as a way of quenching the commonly produced in the interiors of ble light—that is, they fluoresce. De- overload of energy. Anthocyanins also leaves and hence are poorly placed to tailed analysis of this flash reveals efficiently protect against photoinhibi- protect leaves from the widespread ef- much about photosynthetic function. tion because they soak up radiant ener- fects of UV-B. These weaknesses were re- New techniques to measure this fluo- gy at wavelengths poorly absorbed by futed by one of us (Lee) in 1987. So what rescence have helped investigators test other accessory pigments, such as in good are anthocyanins to a leaf? Two re- the efficiency of the light reaction of the green waveband at around 530 cent discoveries have shed light on the photosynthesis under different condi- nanometers. In intact tissues, the range mystery. tions and to detect photoinhibition. A of absorbance also extends into shorter

© 2002 Sigma Xi, The Scientific Research Society. Reproduction 2002 November–December 525 with permission only. Contact [email protected]. Figure 2. Some senescing autumn leaves, such as the witch hazel (Hamamelis virginiana, left) are yellow. A transverse section of a leaf (right) shows that the yellow pigments are clustered exclusively in the chloroplasts, which are degrading. The loss of chlorophyll reveals the yellow carotenoid pigments. (Except where noted, all microscopic images are magnified 250 times.)

(blue) wavelengths, overlapping with of us (Lee) found support for the pro- layer could not protect the photosyn- the absorbance of chlorophyll, particu- tective function of anthocyanins in the thetic tissues, and both red and green larly with chlorophyll b, one of the two red-osier dogwood (Cornus stolonifera). showed the same degree of photoinhi- major forms of the pigment. In addi- We were able to compare the photosyn- bition. Workers in other laboratories tion, anthocyanins are very stable com- thetic responses of senescing leaves that have evidence that anthocyanins also pounds in the mildly acidic environ- varied only in the presence or absence protect Antarctic mosses, pine needles ment of the cell vacuoles that contain of anthocyanin near their surfaces. We and understory plants found in tropi- them. The hardy anthocyanins can thus hypothesized that this pigment layer cal rainforests. shield the more delicate chlorophyll would protect the photosynthetic tis- molecules housed in the chloroplasts. sues underneath. And we were right. Antioxidants by and for Plants During the past decade fluorescence When exposed to intense light, the In addition to protecting plants from measurements have yielded growing red leaves in our experimental collec- the usually short-term problem of evidence that the anthocyanins indeed tion suffered less and recovered more photoinhibition, anthocyanins appear protect against photoinhibition. Tay- quickly than the green leaves. The de- to protect plants from permanent dam- lor Field, then a graduate student at gree of protection was even greater at age by acting as antioxidants. One con- Harvard University and now a faculty low temperatures. When we illuminat- sequence of the absorption of intense member at the University of Toronto; ed the red and green leaves from their sunlight in leaves is increased produc- Noelle Holbrook at Harvard; and one green undersurfaces, the anthocyanin tion of reactive oxygen species and free

Figure 3. Many senescing leaves are red, as in the red oak (Quercus rubra, left). A transverse section (right) shows that the anthocyanin pigments responsible for this color are located in vacuoles of the long, thin palisade cells, which are stacked upright just under the upper epidermis.

© 2002 Sigma Xi, The Scientific Research Society. Reproduction 526 American Scientist, Volume 90 with permission only. Contact [email protected]. radicals (molecules with unpaired elec- 1 trons), such as singlet oxygen ( O2), • – superoxide ( O2 ) and the extremely toxic hydroxyl radical (•OH). The presence of unpaired electrons makes most β free radicals unstable and highly reac- -carotene tive. But not all of them are damaging 100 1.0 to plants. Some are involved in the normal formation of lignins, which strengthen cell walls; others fight off 80 0.8 pathogens. However, when the con- centration of free radicals exceeds the ability of natural antioxidants to quench 60 0.6 them—particularly during periods of stress—these reactive molecules can de- stroy the biological machinery around them, including membranes, proteins 40 0.4 and DNA, potentially leading to cell death. Leaves, with their relatively high percent leaf absorbance concentrations of oxygen from photo- 20 0.2 relative pigment absorbance synthesis and with their frequent expo- leaves sure to intense sunlight, seem particularly vulnerable to oxidative damage. 0 0 Anthocyanins have the potential to 300 400 500 600 700 800 curtail photooxidative damage by ab- wavelength in nanometers sorbing green wavelengths of light, thus shielding chloroplasts beneath from a portion of the spectrum that they cannot exploit for energy pro- OH duction. Thomas Vogelmann, who re- OH cently moved from the University of Wyoming to the University of Ver- HO O mont, and his colleagues have shown glucose that the light energy that anthocyanins absorb does not subsequently transfer O to the chloroplasts. Instead, the energy cyanidin-3-glucoside either is retained in the vacuole containing the anthocyanins (preliminary 100 1.0 evidence suggests that energy makes the molecules vibrate and even hum) or, more likely, is dissipated gradually 80 0.8 as heat. Consequently, the chloroplasts in red leaves produce fewer free radicals. Samuel Neill, a Ph.D. student at 60 0.6 the University of Auckland, measured levels of superoxide produced by a suspension of chloroplasts taken from 40 0.4 leaves of lettuce. When the chloroplasts were irradiated with white light, the percent leaf absorbance amount of superoxide rapidly in- 20 0.2 relative pigment absorbance creased, whereas they produced much leaves lower levels when irradiated with light that was of a similar intensity but had 0 0 been passed through a green-absorb- 300 400 500 600 700 800 ing filter. Clearly, the absorption of wavelength in nanometers green light by anthocyanins impedes superoxide production. Anthocyanins may also help a plant Figure 4. Beta-carotene molecules (top) add yellow color to senescing leaves. The light absorbance spectrum of leaves containing beta-carotene is compared with the absorbance spec- because they are potent antioxidants. trum of the molecule alone. The most common anthocyanin molecule in leaves is cyanidin-3- Solutions of cyanidin, the most preva- glucoside (bottom). Again, the absorbance spectrum of leaves containing the pigment is lent anthocyanin in leaves, have about compared with the wavelengths of light absorbed by the isolated molecule. Because of the four times more antioxidant capacity scattering of light within leaves, the absorbance spectra of the molecules are broader when than ascorbic acid or vitamin E. People they are in their native environment than when the pigments are isolated.

© 2002 Sigma Xi, The Scientific Research Society. Reproduction 2002 November–December 527 with permission only. Contact [email protected]. who ingest anthocyanins from fresh the accumulation of anthocyanins. fruit and red wine can improve the an- When we punctured the leaves with a tioxidant status of their blood plasma. fine needle, chloroplasts in the injured In 1999 James Joseph and many collab- area produced strong bursts of the re- orators at the United States Department active oxygen compound hydrogen of Agriculture and at Tufts University peroxide (H2O2), which we could de- demonstrated dramatic reversals in the tect and monitor over time. Chloro- decline of nervous system activity in plasts in both the green and red areas mice that were fed anthocyanin-rich produced hydrogen peroxide, but dif- blueberries (Vaccinium corymbosum). ferences between the two regions be- Widespread reporting about these re- came apparent within minutes of in- sults led to a run on fresh blueberries jury. Hydrogen peroxide continued to in supermarkets across the country. accumulate in the green portions for 10 Now a growing body of evidence minutes and then decreased only slow- shows the effects of anthocyanins as ly. In contrast, levels in the red regions antioxidants, raising the possibility declined rapidly to background counts that perhaps these molecules are as within the first five minutes. Figure 5. Red or purple color is often seen on health-promoting for the plants that How exactly do the anthocyanins the undersurfaces of leaves growing in the produce them as for the animals that protect from oxidative damage? The lower, more shaded parts of forests, especially in the tropics. The herb shown (Triolena ingest them. phenomenon is rather enigmatic. hirsuta) grows in the tropical understory of One of us (Gould) and his colleagues Whereas the troublemaking oxygen Central American forests. Whether or not studied the potential for such protec- molecules do their damage in the chlor- their undersides are red varies, making them tion in a New Zealand shrub, Pseudo- oplasts or in the cytoplasm, the antho- a good subject for research. (Photograph wintera colorata. Parts of the upper sur- cyanins are largely sequestered in cell courtesy of George Valcarce.) faces of its leaves are blotched red from vacuoles. Perhaps the anthocyanins, which are produced in the cytoplasm, carry out their antioxidative function there, before moving into vacuoles. Or maybe these organelles act as sponges for destructive hydrogen peroxide, which, unlike other reactive oxygen molecules, can migrate across the membrane of a vacuole. Even if biologists figure out how it is that anthocyanins can protect plant cells from oxidative damage, it will be hard to say whether these pigments evolved primarily to scavenge free radicals or to combat photoinhibition. It is possible that these functions act simul- taneously to protect plant tissues from intense sunlight. By absorbing light, anthocyanins would reduce the rate of both photoinhibition and photooxidative damage, and they could also neu- tralize whatever free radicals still ended up being produced. Both mechanisms could serve to protect plant tissues that are especially vulnerable to such damage, such as those under environmen- tal stress or in developing leaves when the photosynthetic structures are being assembled. Despite the protective functions of anthocyanins (particularly in bright, cold conditions), it is difficult to under- stand why plants invest energy to protect foliage that is about to fall off. Shielding the photosynthetic appara- Figure 6. Investigators in the 19th century carefully observed the distribution of anthocyanins tus of a dying leaf would add little to in different plants. Belgian botanist Édouard Morren published these microscope observations the carbon supply of a tree and is un- in 1858, showing the distribution of anthocyanins in the various organs of young seedlings of likely to justify the metabolic cost of red cabbage (Brassica oleracea). synthesizing an elaborate pigment

0.6

0.4

0.2 green leaves

relative photosynthetic efficiency relative red leaves 0 0 20 4060 80 100

exposure to time (minutes) intense light

Figure 7. Red-osier dogwood (Cornus stolonifera), photographed in the Catskill Mountains of New York, has red leaves in the autumn as well as some persisting green leaves (left). Green and red leaves on the plant differ in their relative photosynthetic efficiencies (right). Red leaves lose less efficiency when exposed to intense light and recover more rapidly after the exposure. molecule. But it is possible that the clear support for it. Anthocyanic con- Ecological Functions of Anthocyanins benefits continue into the next grow- centration in foliage, which varies both Some biologists have speculated that ing season, if, for example, the antho- among and within species, such as red the red colors in leaves may protect cyanins in autumn leaves act to allow maple (Acer rubrum), should be corre- them from being eaten. Anthocyanins the coordinated disassembly of the lated with lower nitrogen levels in leaf are members of a class of plant com- photosynthetic apparatus, particularly tissues and higher efficiencies of ab- pounds (polyphenols) that sometimes the breakdown of chlorophyll, before sorption into the plant. But the efficien- defend against predators, insects and the leaves drop. cy of nitrogen absorption can vary a microbes, but anthocyanins themselves The critical issue may be the move- great deal, both among individual don’t seem to act as poisonous deter- ment of nitrogen back into the plant. plants within the same species and in rents. However, the red appearance of The photosynthetic apparatus in leaves the same plant from year to year. It’s leaves may warn animals that the contains much of the total nitrogen, possible that the increase in absorption leaves are unpalatable. In many tropi- which is wasted if the plant does not may be rather small, and therefore hard cal trees, for example, young red-pur- recover it before the leaves detach. to detect, and yet still provide signifi- ple leaves hang from branch tips until Anthocyanins could protect this dis- cant advantage. We have obtained some at the end of their development they mantling process in leaves and in- preliminary evidence supporting this rapidly become green. Phyllis Coley of crease the amount of nitrogen shifted hypothesis from shrubs and trees at the the University of Utah has shown that into the woody tissues of the parent Harvard Forest, but much more re- these anthocyanic leaves have very low tree. Although the explanation is sim- search will be required to properly test nitrogen content and are seldom dam- ple enough, it may be difficult to obtain this prediction. aged by herbivores. She believes that

a b c d 200 micrometers

Figure 8. Puncturing the leaves of Pseudowintera colorata (the plant shown in Figure 1) with a needle leads to the release of hydrogen peroxide

(H2O2), which can be revealed using a blue dye that loses its fluorescence when exposed to this highly reactive compound. When a green leaf blade was tested (a), it was flooded with hydrogen peroxide, as shown by the lack of fluorescence around the puncture (b), and levels decreased only slowly. But when a red portion of a leaf was pricked (c), hydrogen peroxide levels returned to normal within five minutes (d).

© 2002 Sigma Xi, The Scientific Research Society. Reproduction 2002 November–December 529 with permission only. Contact [email protected]. Figure 9. Anthocyanins are common in the developing leaves of tropical plants, such as the cocoa plant (Theobroma cacao). The shrub (left) and leaves (middle) were photographed in French Guiana. An even closer view through the leaf surface (right) reveals that anthocyanins are located exclusively in the tips of hairs and in cells surrounding leaf veins. the anthocyanins may inhibit the mammals. In rare cases, anthocyanin the following year, when the eggs growth of fungi that leaf-cutting ants color in leaves may attract animals to would have hatched. So far this research cultivate and thus reduce leaf collec- consume fruits and disperse seeds, but is based on literature surveys and mod- tion by the ants. few of the red-senescing plants we have eling, with experiments and direct field Paul Lucas of the University of Hong studied produce fruits that persist so observations yet to be completed. Kong and his colleagues have argued late in the growing season. that young, red leaves in tropical forests It’s conceivable that autumn col- Problems Remain are edible, and indeed important in the oration evolved to deter herbivores, but The research we have described in this diets of some primates. They have the deterrence would only lead to a se- article is a dramatic shift toward the so- shown that chimpanzees and monkeys lective advantage if it protected the trees lution of a long-standing mystery, but of the Kibale Forest, in Uganda, prefer from damage in subsequent seasons. it is just a beginning. Many questions young, red leaves, which are highly The late William Hamilton of Oxford remain: For example, if anthocyanins palatable because of high protein lev- University and Samuel Brown, currently fulfill an important physiological or els and tenderness. Field tests of feed- at the University of Montpellier II in ecological function for certain plants, ing activities suggested to them that France, argued that autumn coloration why then do some plants not produce red-to-green shifts in leaf color are im- could warn aphids against laying eggs such pigmentation? And why aren’t portant cues that led to the evolution of on trees defended with plant com- more leaves red all the time? Interest- three-color vision in certain primates pounds. Thus, the autumn coloration ingly, one large group of flowering (including humans), unique among all could prevent leaves from being eaten plants, in the order Caryophyllales, are

β-D-glucose O COO– + HO N

COO– NH COO–

Figure 10. In a single order of flowering plants, red leaf color arises not from anthocyanins but from the presence of nitrogen-containing betalain pigments. Bougainvillea (Bougainvillea spectabilis) is a tropical, shrubby vine whose developing leaves and flower bracts are red because of betalains, specifically the compound betanin (top right). A section through a leaf shows that the pigment is located in the epidermis and hairs (lower right).

© 2002 Sigma Xi, The Scientific Research Society. Reproduction 530 American Scientist, Volume 90 with permission only. Contact [email protected]. incapable of synthesizing anthocyanins, learned much from many collaborators and declines in neuronal signal transduction, cognitive, and motor behavioral deficits although they produce the precursor from a symposium, “Why Leaves Turn with blueberry, spinach, or strawberry di- flavonoid pigments. Still, these plants Red,” that they organized at the Botanical etary supplementation. The Journal of Neuro- produce red colors, in both vegetative Society of America Meetings in Albu- science 19:8114–8121. and reproductive organs, but with a to- querque, New Mexico, in 2001—which led Lee, D. W., S. Brammeier and A. P. Smith. 1987. tally different class of pigments—the to the edited book cited in the Bibliography. The selective advantages of anthocyanins in betalains. Beets are a good example. developing leaves of mango and cacao. Biotropica 19:40–49. The nitrogen-containing betanin has Bibliography Lee, D. W., and T. W. Collins. 2001. Phyloge- Brown, S., and W. Hamilton. 2001. Autumn similar absorbance characteristics to netic and ontogenetic influences on the dis- leaf colours and herbivore defense. Proceed- the anthocyanins and also is a strong tribution of anthocyanins and betacyanins ings of the Royal Society of London B. Biological in leaves of tropical plants. International Jour- free-radical scavenger. Do these mole- Sciences 268:1489–1493. nal of Plant Science 162:1141–1153. cules function as an equivalent to an- Coley, P. D., and J. A. Barone. 1996. Herbivory Neill, S. O., K. S. Gould, P. A. Kilmartin, K. A. thocyanins in this order of flowering and plant defenses in tropical forests. An- Mitchell and K. R. Markham. 2002. Antioxi- nual Reviews of Ecology and Systematics plants? If so, how did they originate? dant activities of red versus green leaves in 27:305–335. In some cases, genes that promote Elatostema rugosum. Plant Cell and Environ- anthocyanin production in leaves may Dominy, N. J., and P. W. Lucas. 2001. Ecological ment 25:537–549 importance of trichromatic vision to pri- also promote production in flowers Simms, E. L., and M. A. Bucher. 1996. Pleiotrop- mates. Nature 410:363–366. and other organs. This connection may ic effects of flower color intensity on herbi- Field, T. S., D. W. Lee and N. M. Holbrook. 2001. confound efforts to interpret field tri- vore performance in Ipomoea purpurea. Evo- Why leaves turn red in autumn. The role of lution 50:957–963. als that test whether plants with antho- anthocyanins in senescing leaves of red-osier Tsuda, T., Y. Kato and T. Osawa. 2000. Mech- dogwood. Plant Physiology 127:566–574. cyanins are more likely to survive than anism for the peroxynitrite scavenging those without these pigments. But Gould, K. S., and D. W. Lee (eds.) 2002. Antho- activity by anthocyanins. FEBS Letters these problems are not insurmount- cyanins and Leaves. The Function of Antho- 484:207–210. cyanins in Vegetative Organs. London: Acad- Wheldale, M. 1916. The Anthocyanin Pigments of able. For instance, investigators can emic Press, Advances in Botanical Research, Plants. Cambridge: Cambridge University compare the performance of normal vol. 37. Press. plants with those of natural mutants— Gould, K. S., J. McKelvie and K. R. Markham. Winkel-Shirley, B. 2001. Flavonoid biosynthe- In press. Do anthocyanins function as an- or genetically engineered ones—vary- sis: a colorful model for genetics, biochem- tioxidants in leaves? Imaging of H O in red ing in the production of anthocyanin. 2 2 istry, cell biology and biotechnology. Plant and green leaves after mechanical injury. We expect some exciting research on Physiology 126:485–493. Plant, Cell and Environment. the functions of anthocyanins in vege- Gould, K. S., T. C. Vogelmann, T. Han and M. J. tative organs in the near future. Clearwater. In press. Profiles of photosynthesis within red and green leaves of Quin- Links to Internet resources for further Acknowledgments tinia serrata A. Cunn. Physiologia Plantarum. exploration of “Why Leaves Turn Red” Much of Gould’s research was supported by Hoch, W. A., E. L. Zeldin and B. H. McCown. are available on the American Scientist a Royal Society of New Zealand Marsden 2001. Physiological significance of antho- Web site: cyanins during autumnal leaf senescence. grant. Lee’s research on autumn leaf color Tree Physiology 21:1–8. http://www.americanscientist.org/ change, following earlier work in the tropics, Joseph, J. A., B. Shukitt-Hale, N. A. Denisova, articles/02articles/Lee.html was supported by a Bullard Fellowship at D. Bielinski, A. Martin, J. J. McEwen and P. the Harvard Forest in 1998. Both authors C. Bickford. 1999. Reversals of age-related