The colour red
Every secondary school pupil knows that leaves are green because chlorophyll, the pigment plants use in photosynthesis, is green. So why are many leaves, such as those on this liquidambar, scarlet for at least part of their lives?
The floor of Malaysia’s tropical rain forest is cool, damp and exceptionally dark. Less than half a per cent of the sunlight that strikes the forest’s canopy reaches the soil, and for the plants that live in these lower levels, capturing enough light for photosynthesis is a perpetual struggle. Able to generate only limited chemical energy from the scant light which falls on them, they grow slowly. Most crawl horizontally along the litter surface, their leaves thin, fragile, lacklustre.
And red. The under surface of the leaves of many of these shade-growing plants are pigmented an intense red. From ancient ferns to more recently evolved begonias, the crimsons, carmines, maroons and purples of these plants—revealed only when their leaves are turned over—provide a surprising contrast in an otherwise green-on-green environment.
Red leaves also present an intriguing question to botanists. What does a light-starved plant gain by throwing precious energy resources into the manufacture of red pigments? And why confine them to the underside of leaves, where they are largely hidden from view?
Showy autumn pageants in the forests of New England are no less intriguing. Red leaves generate big business in America. Every October, thousands of “leaf peepers” flock to small towns from Massachusetts to Maine to witness the dazzling array of reds, scarlets, purples and golds which ignite the canopy during the last stages of the deciduous leaf life cycle. The visitors bring their dollars, and their questions. Why do dying leaves turn red shortly before they detach from a branch? Do the pigments serve a useful function, or is the whole extravagant show a waste of a plant’s resources?
New Zealand’s forests are not famed for red foliage, though the summer leaves of our southern beeches would rate well on the international stage. Nonetheless, red leaves are surprisingly common here. Red mosses, ferns, conifers and flowering plants can be found in forests, marshes and sand dunes from Kaitaia to Stewart Island. And our displays of vermilion have recently begun to attract the attention of overseas scientists, for native New Zealand plants are providing clues to the function of red leaf pigments.
My own interest in red leaves was kindled several years ago during a walk with friends along the Fairy Falls track in Auckland’s Waitakere Ranges, where cascading stands of parataniwha cling precariously to the banks along the path’s steep descent. Parataniwha normally occupies shady, damp habitats, and can easily be overlooked, but there was a moment late in the afternoon when the angle of the filtered sunlight revealed the plant’s intrinsic beauty. The iridescent pinks and reds of the young leaves contrasted strikingly with the green background of mosses, ferns and sedges. As we stopped to admire the botanical mural, one of my companions asked me, “Why are parataniwha leaves this colour?”
As a botanist, I felt I should have known the answer, but in fact I had never given the issue any thought. My mumbled reply was something about protecting leaves from ultraviolet radiation—a reasonable guess, as it turned out—but I made a mental note to look into the subject further.
A search on the internet raised numerous possibilities but produced no definitive answer. I emailed David Lee, a botanist at Florida International University, Miami, who told me that red leaves had been a subject of debate since Aristotle and a focus of scientific examination for more than a century, but that scientists were still far from understanding the biological purpose of red pigments. In the course of our correspondence, Lee and I gradually realised that subtle differences between New Zealand and overseas red-leafed flora held the potential to resolve the mystery of why leaves turn red. We decided to study the phenomenon further, and made a successful bid for a research grant from New Zealand’s Royal Society.
Red pigments in leaves are called anthocyanins, derived from the Greek words anthos, meaning flower, and kyanos, meaning blue. Why blue? A remarkable property of anthocyanins is that while they are red in acidic solutions, they turn blue in alkaline solutions. This explains why red-wine stains on clothes often turn blue in the wash. It may also explain why hydrangea flowers vary from pink to blue depending on the acidity of the soil.
Anthocyanins—the same pigments that provide colour in strawberries, camellia flowers and red grapes—have long been regarded as a natural advertisement to insects and birds. “Come, visit my beautiful flowers (pollinate me) and eat my delicious berries (disseminate my seeds),” they seem to be saying. Judging by the high numbers of red flowers and red fruits that have evolved worldwide, the strategy works.
Such a “shop window” approach may explain the usefulness of anthocyanins in berries and petals, but it does not explain the presence of these pigments in leaves. Indeed, in some plants, just the opposite is true: anthocyanins effectively conceal the leaves from the eyes of herbivores. One of our most distinctive trees, the lancewood (horoeka), produces brown leaves at the seedling stage, when it is most vulnerable to browsing. The brown colour—a mixture of red anthocyanins and green chlorophylls—causes the leaves to blend with debris on the forest floor.
Many of the hypotheses that have been put forward to explain the occurrence of red leaves overseas do not stand up to scrutiny when applied to New Zealand’s flora. For example, some of our native species—in common with those in the Malaysian rainforest—hold red pigments exclusively on the underside of the leaf. But, unlike the Malaysian plants, ours do not usually grow under conditions of extreme shade. The leaves of toro (Myrsine salicina), mamangi (Coprosma arborea) and makomako (wineberry) have a rich pink undersurface, yet these species thrive in sunny, exposed positions at the forest margins. If it is to account for the New Zealand situation, any hypothesis intended to explain the purpose of undersurface coloration in leaves must accommodate both shade-loving and sun-loving plants.
And, unlike the blazing autumn colours of New England’s forests, New Zealand’s botanical reds are not confined to a leaf’s death throes. Some of our plants hold red leaves throughout their lives, while others generate anthocyanins only at the infant stage of leaf development. The fronds of pukupuku (rasp fern) and the tips of rimu branches, for example, are red only in the young, expanding leaves.
From a botanical standpoint, the most fascinating feature of New Zealand’s red-leafed flora is its exceptional variation in pigmentation patterns. While the entire leaf blade of some species is coloured red, in others anthocyanins occur as irregular blotches, or are restricted to the leaf margins, the mid-vein or the leaf stalk. Individual plants can even vary from leaf to leaf. The canopy tree tawherowhero, a prominent feature along the shores of the Bay of Islands and in montane forests, bears a mixture of red and green leaves in unpredictable proportions; both leaf types are apparently healthy, and the trees thrive.
Researchers in my own laboratory have used this variability as the basis for a set of experiments. They imposed various stresses on plants, such as high light, drought and extreme temperatures, and compared the performance of a red leaf versus that of a green leaf, a red shoot versus a green shoot, and even, at the microscopic level, a red cell versus a green cell.
The hypothesis we were testing in these experiments arose from a surprising source. In the early 1990s medical researchers began publishing reports of a bizarre statistical quirk that has since become known as the French Paradox. The paradox is this: despite the presence of risk factors such as high fat intake, hypertension and smoking, the French population as a whole has an unusually low incidence of cardiovascular disease compared with the populations of other industrialised nations. The phenomenon has been attributed, at least in part, to the French fondness for red wine. Brimming with natural antioxidants, red wine apparently confers a measure of protection on the human body against the malign effects of other factors. Phenolic components in wine, including anthocyanin pigments, have been identified as the chief sources of antioxidant protection. Anthocyanins, it turns out, are powerful antioxidants, four times more potent than vitamins C or E.
Antioxidants work by scavenging free radicals—highly reactive atoms and molecules such as hydrogen peroxide, nitric oxide and hydroxyl—and neutralising them before they cause damage to DNA and cells. Damage of the sort caused by free radicals is thought to lie behind some genetic diseases and cancers. Humans are constantly in contact with free radicals—the air itself contains several different kinds, and pollutants such as those found in cigarette smoke and traffic fumes add to the exposure. Antioxidants are a vital line of defence against these damaging agents.
Like us, plants are also exposed to free radicals, and their cells are no less vulnerable. The detrimental effects are compounded when plants experience harsh environmental stresses, such as the high levels of ultraviolet radiation typical of New Zealand’s summer months, water shortages when the soil is frozen, predation by insects and infections by fungi, bacteria or viruses. Under these conditions, free radicals are somehow generated inside cells, and it is these “enemies from within” which pose perhaps the greatest danger. Protection by antioxidants is not a luxury: it is critical for the plant’s survival.
If anthocyanins can protect the human body from the effects of free radicals, it seemed possible to us that they might operate similarly in leaves. Our early investigations revealed that the same stresses which lead to free-radical damage in plants can also cause green leaves to turn red. Green parataniwha leaves, for example, produced anthocyanins in response to drought. Similarly, horopito (New Zealand pepper tree) leaves acquired red blotches when they were grazed by insects. Were the anthocyanins that these plants produced in response to stress actually protecting the leaves from attack by free radicals?
It was an intriguing question, and two of my students and I set to work to answer it. As a first step, Sam Neill, a PhD student, compared the levels of antioxidants in red and green leaves, and found that red parataniwha leaves were loaded with powerful antioxidants while green leaves had virtually none.
It is one thing to show that antioxidants are present in a leaf extract, but quite another to demonstrate that they protect living tissues. Ideally, I wanted to watch the generation of free radicals under a microscope, and then witness their capture by anthocyanins inside living cells. Nothing similar had been attempted previously, either in plant or animal tissues.
Horopito leaves, with their irregular patchwork of reds and greens, offered ideal test material. Using razor blades, BSc Honours student James McKelvie painstakingly dissected the outer layers of cells from the red and green portions of horopito leaves. He then impregnated these sections with a dye which fluoresces intensely under ultraviolet light only if free radicals are present. The leaf sections, fragile and translucent, were carefully mounted on glass slides and examined under a microscope.
We then stabbed the leaf sections with a needle. Our intention was to simulate the damage caused by small insects browsing on a leaf’s surface, as herbivorous insects have long been thought to cause the production of free radicals in plants.
The wounded leaf tissues responded both immediately and dramatically. An explosion of free radicals, visible under the microscope as an intense burst of fluorescence, surged from the site of injury and diffused rapidly into neighbouring cells. Within seconds, a fluorescent halo encircled each wound, a miniature tornado of free radicals spreading outward and engulfing everything—cell walls, chloroplasts, all vital organelles—in their path.
Free radicals attacked both the green and the red portions of the horopito leaves, but the difference in response astounded us. The fluorescence from free radicals in the anthocyanic cells waned rapidly. We saw cell after cell extinguish the light from the harmful free-radical molecules, sequestering them in the vacuole—an empty space in the centre of a cell—out of harm’s way. Within five minutes of injury, the battle had been won; concentrations of free radicals in the tissues had diminished to normal levels.
Cells from the green leaf tissues, by contrast, fared less well against the free-radical army. Fluorescence levels increased for 10 minutes following injury, and started to abate only after 20 minutes. In the green portions of the leaves, free radicals lingered long enough to cause potential cell damage.
Our antioxidant hypothesis is unlikely to account for all instances of red leaves in the plant kingdom. It does, however, provide a possible explanation for some of the more phenomenal occurrences of red leaves overseas. In the forests of New England, the dying autumn leaves have one final task to perform. They must return all of their nutrients to the branches of their tree to provide resources for growth the following spring. Chloroplasts, which provide the energy for this nutrient movement, are extremely sensitive to free-radical attack at what is a stressful time, and it is at least possible that anthocyanins provide them with antioxidant protection—military protection, as it were.
And what of the red-leafed understorey plants in the Malaysian rainforest? As the sun moves over the canopy, occasional stray beams of light penetrate the dim interior and strike these plants with full force. Chloroplasts that are adapted to the extreme shade suddenly experience light up to 4000 times more intense than normal, which causes an explosive build-up of free radicals in the leaves. Effective antioxidant protection, such as that provided by anthocyanins, could be essential for survival under such conditions.
It makes sense for these deep-shade plants to house the anthocyanins on the leaf undersurface. Anthocyanins are very effective absorbers of light, soaking up as much as 20 per cent of the visible radiation that would otherwise be used for photosynthesis. In their light-starved environment, shade-dwelling plants cannot afford to waste such a precious resource. By storing the red pigments in cells that are beneath the photosynthetic tissues, the leaves can give themselves adequate antioxidant protection without compromising photosynthetic light capture.
I remain obsessed with red leaves. I see them wherever I walk in New Zealand, and continue to marvel at natural permutations in pigmentation from species to species. But I have a new problem now. Since anthocyanins are so effective at protecting leaves from the ravages of free radicals, why aren’t all leaves red?