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Why Are Plants Green?
Before we get into an explanation of fall colours we need to understand why plants are green in the first place. During the growing season plant leaves appear green to us because they reflect green light. Or more correctly, the chlorophyll inside the leaves reflects green light. Most of the materials that make up a leaf are colourless or clear. Leaves and the cells they are constructed of are mostly water, just like us. Chlorophyll is that miraculous molecule that captures sunlight and allows plants to make their own food, a process called photosynthesis. The energy of sunlight is used to recombine water and carbon dioxide molecules into simple sugars and oxygen: that rather important plant waste product! Chlorophyll molecules absorb, and make use of sunlight from the red and blue ends of the spectrum. They're not good at absorbing green, so this is transmitted or reflected, and bounces to our eyes. Plants are green because they waste green light. Actually, they waste a fair bit of the sunlight that hits them; no plant absorbs all the light hitting it. It's just that more green light is reflected than any other colour, so we see the plant as green. If plants could absorb all the light that hit them, what colour would they be? That's right, black.
Chlorophyll is a pigment. Any substance, regardless of whatever else it does, that influences the colour of another substance is a pigment. Say, for example, you're making Kool-Aid. If you add the sugar to the water first, the water remains clear. Sugar is not a pigment. When you add the contents of the Kool-Aid packet it produces a strong colour in the water. In this instance Kool-Aid is a pigment. Or rather, the food colouring in the Kool-Aid is a pigment. Chlorophyll is a "photosynthetic pigment": a substance involved in the process of making food from light that also produces colour.
Chlorophyll molecules are very large and complex, and (and this is important!) quite unstable when exposed to sunlight. Chlorophyll breaks down rapidly with use, so throughout the growing season plants must continue to produce new chlorophyll. A steady state, where production equals loss, is maintained until the end of summer.
OTHER PIGMENTS IN PLANTS.
Photosynthesis involves an array of photosynthetic pigments, not just chlorophyll. There are, in fact, two kinds of chlorophyll (referred to as "a" & "b"), as well as another class of photosynthetic pigments called carotenoids. These molecules also absorb light for the photosynthetic process, but instead of using that energy themselves they pass on the energy to the chlorophylls. The various carotenoids (the main ones are carotene and xanthophyll) tend to reflect (waste) light in the yellow to red end of the spectrum. They would make leaves look yellow if they were the only pigments present. There can be a number of different photosynthetic pigments in a leaf at any one time, but chlorophyll is the most common, so we see most leaves as green in colour. Carotenoid molecules are large and complex, too, but they do not break down as fast as chlorophyll. These different rates of decay are crucial to the development of fall colours, as we shall see.
In addition to photosynthetic pigments (chlorophylls and carotenoids), there can be several kinds of non-photosynthetic pigments present in leaves. These are compounds that have varying functions in the leaf, but also act as pigments. The relative proportions and types of all these pigments present during the growing season will determine the colour and patterning of the leaves at that time. And they will all have an impact on the fall colours, as well. Plants that have reddish or purplish leaves during the growing season, such as Coleus or Shubert's Chokecherry, have large concentrations of anthocyanins: a group of plant pigments that produce red, purplish or even blue colours. The anthocyanins are present is such concentrations that they swamp the green colour of the photosynthetic pigments. The chlorophylls are still there, but they are "out-shone" in the same sense that the carotenoids are normally "out-shone" by the chlorophylls. Each species of plant will have a unique array of pigments in it's "colour palette", based on the genetics of that species.
PREPARING FOR WINTER.
The shortening day length of autumn, and the approaching winter, triggers a series of changes in plants aimed at preparing them for winter dormancy. Actually, it's the increasing length of the night that most plants measure and respond to. Deciduous plants, those that drop their leaves for winter, allow their leaves to die in an orderly fashion, a process called "leaf senescence". These photosynthetic factories are shut down and much of the water and nutrients within are reclaimed by the plant, and transported to the root system for storage.
LEAF ABSCISSION: How Leaves Fall.
Senescence covers the period of time from when a leaf is functioning normally to when it falls from the plant (abscission). The process of leaf abscission begins at the base of the leaf petiole, the stalk attaching the leave to a stem. A barrier begins to form at the point of attachment. On the leaf side tissues grow softer, while on the stem side they harden and fill with corky, wax-like compounds. The flow of water and nutrients, once a two-way affair, both into and out of the leaf, becomes increasingly one way, out of the leaf. By the time the plant has withdrawn all that it can from a leaf the corky barrier between the leaf and the stem will be complete. On the leaf side the tissues will have softened and broken leaving the leaf attached to the plant by its vascular bundles: the series of tubes through which water and sap flow into and out of leaves, the leaf "veins". With the action of wind and frost these soon break and the leaf falls to the ground. But what falls is merely a hollowed out shell. Where the leave was attached to the stem is now an impermeable leaf scar, which will be incorporated into the stem bark. Often right next to the leaf scar on the stem are the buds that will produce new shoots and leaves in the following spring.
Fall Colours Emerge During Senescence.
It is during leaf senescence, the shutting-down and recycling of the leaf's contents, that we witness fall colours. One of the first events in senescence is that production of photosynthetic pigments stops. As the existing chlorophyll in the leaf breaks down, it is not replaced. As a result, the green colouring begins to fade. The always-present carotenoid pigments, which break down more slowly, are now revealed. The green of chlorophyll gives way to the yellow of carotenoids. For plants whose leaves turn yellow in the fall, the explanation of fall colours is just that simple.
Green Ash (Fraxinus pennsylvanica)
A tree with yellow fall colour.For plants that produce darker orange, reddish or purplish colours the story is more complex. During senescence these plants are producing anthocyanins, a red pigment, inside the leaves. Sugars, which continue to be produced until all the chlorophyll breaks down, can become trapped in the leaves as the transport system that would normally move them out of the leaves shuts down. In the presence of sugars a group of already-present, colourless compounds called flavonols are converted to anthocyanins by the action of sunlight. Leaves which contain roughly equal amounts of yellow carotenoids and red anthocyanins can appear bright orange. If the anthocyanins predominate the leaf will appear bright red. Because sunlight must hit a leaf in order to produce anthocyanins and there is little movement of this pigment within the leaf, shaded or partially shaded leaves may develop unique patterns.
Red-Osier Dogwood (Cornus alba)
A shrub with bright red colour.Virginia Creeper (Parthenocissus quinquefoila )
A uniquely patterned leaf.WEATHER AFFECTS FALL COLOURS.
Colours resulting from anthocyanin production, unlike those resulting from the always-present carotenoids, can be greatly influenced by weather conditions. Cool nights with sunny days in early autumn makes for good anthocyanin production. Low, but not freezing temperatures, help slow the movement of sugars out of the leaves at night. Sunny days help produce sugars initially, then drive the process that converts sugars and flavonols into anthocyanins. A cloudy autumn with warm nights will not produce good fall colours, at least not for plants that develop red colouring.
Weather can affect fall colours in other ways, too. An early frost can help to break down chlorophyll more rapidly and bring about a more coordinated fall colour display. If the frost affects all the plants in a given region they will tend to become more synchronized in the development of their colours.
INDIVIDUAL VARIATION.
Variations in fall colour between different species of plants are largely the result of their genetic ability to produce or concentrate the various pigments. Within each species, though, there can still be great variation in the fall colours produced. Some variation can be accounted for by genetic differences in individual plants, but most is likely the result of differences in the environmental conditions they experience. For example, a shrub growing on the north side of a forest bluff may not get enough sunlight to produce anthocyanins and could appear brown or yellow, while one on the south side, where it receives sunlight, might be bright red! Soil and moisture conditions and general health can all impact on the fall coloration of an individual plant.
Even within the leaves on a single plant, there can be variation in the nature and extent of their coloration. Each leaf will be exposed to its own unique set of environmental conditions. Some leaves will be more shaded than others, others may be more exposed to frost or be on a part of the plant that has been damaged or is diseased. The result can often be rather striking differences in leaf coloration on a single plant.
Colour variation on a single branch: Pin Cherry
(Prunus pennsylvanica)THE FINAL STAGES - Why Leaves Turn Brown.
As senescence progresses further even the carotenoids and anthocyanins decay and their colour fades. By the time the leaf falls, or shortly thereafter, the once vibrant colours will have faded to a dull tan or light brown.
Dead leaves on ground.
As with the other colours there are various compounds in leaves that act as brown pigments. One of the most common is tannin. Tannin and other such compounds are among the slowest to decay, giving colour to leaves even after they have fallen. Plants whose leaves turn brown before they fall often contain large concentrations of such compounds. In Manitoba, mature Bur oaks (Quercus macrocarpa ) often show little coloration in fall, the leaves turn from green to light brown, because of the high concentrations of tannin. Oddly though, in younger oaks a startling array of colours may be formed. Tannin tends not to be produced in such concentrations in seedling oaks.
Bur Oak (Quercus macrocarpa) leaves in varying shades.
PLANTS THAT DON'T TURN COLOUR.
Ever wonder why some plants don't produce fall colours and don't drop their leaves until after the snow flies? Anyone who's seen a Siberian elm or French lilac in late autumn knows what I mean.
Siberian elm (Ulmus pumilla) in late October.
The smug answer to that question is that these plants don't belong here. Many introduced, horticultural varieties of plants are grown here in Manitoba, but they are not adapted to our climate and growing season. Fall colours are dependent on the orderly procession of leaf senescence. Senescence is triggered by photoperiod changes that are specific to the latitude and growing season where the plant evolved.
Plant species rapidly adapt, through evolution, to become "local ecotypes", closely adapted to the conditions of their immediate environment. As plants spread north after the last ice age they had to adapt to the shorter growing season and begin senescence earlier than they would further south. An introduced plant that originates too far south or from a region with a much longer growing season will be overcome by winter before it can go through its orderly shut-down. Their leaves are frozen before the fall colours can develop. They are killed by frost rather than being reclaimed by the plant, and they flutter to the ground, still green. They often hang on the trees past the first snows as the process of leaf abscission has not progressed properly either.
If you're thinking of planting a tree or shrub and want something that will enhance the fall colours in your yard consider a native species that is a local ecotype To choose a plant for fall colours you should select it in the fall to be sure it produces the colour you want! Autumn is a good time to plant trees and shrubs anyway.
FALL COLOURS - A Brief Summary.
- Fall colours develop during leaf senescence, the gradual death of the leaves.
- There are three main factors that account for fall colours:
- The genetic make-up of the plant that determines what kind of photosynthetic and other pigments it has or can produce.
- The different rates of decay of the various pigments.
- Weather conditions influencing the production or decay of various pigments during leaf senescence.
- As the green of chlorophylls fades, other colours are expressed. The yellows of carotenoids, the reds of anthocyanins and the browns of tannin (and other compounds) become visible.
- Leaves fall as a result of a corky barrier being laid down between the leaf and stem. By the time of leaf-fall, the bright fall colours will have faded as did the original green colour.
A WONDROUS COINCIDENCE.
Most things in nature are the way they are for a reason; evolution is a demanding master. Fall colours, however, seem to be an exception. They are not (to my knowledge, at least) adaptive; that is, they do not help plants to survive, they are merely artifacts of leaf senescence, part of an orderly shutting-down of plants for the winter. The compounds that remain in leaves and create the attractive colours are the "left-overs" that the plant could not resorb or recycle. Unlike the familiar animal signs of autumn, such as squirrels burying nuts or flocks of geese heading south, which are clearly beneficial to those species, the spectacular scenery afforded by the death of leaves appears not to have any benefit to the plants. The answer to the question of why fall colours are so beautiful lies more in the eyes of the beholders, us, than in some grand scheme of nature. Fall colours are a wondrous coincidence!
LOOK FOR FALL COLOURS EVERYWHERE
Attractive fall coloration is not limited to trees and shrubs. We are most familiar with this phenomenon in woody plants (trees, shrubs & vines), but the same thing occurs in many herbaceous (non-woody) plants, too. Herbaceous perennial plants loose all their above ground growth each year and regenerate new stems and leaves in spring. Their leaves and stems go through senescence, just as the leaves of woody plants do. Many grasses and forbs (herbs, wildflowers, etc.) produce striking displays of colour as they prepare for winter dormancy. The fall colours in a marsh or on a prairie can be just as beautiful as in any forest. You just have to look a little closer!
And speaking of looking closer, take a little time and browse through our gallery of fall colours. But more importantly, get out and witness the splendour of autumn in Manitoba, in person!