The annual show of tree leaf colour

Constance Nozzolillo, BA Hon., MA Queen’s, PhD UOttawa
Retired Professor of Biology, UOttawa

Life member of the Phytochemical Society of North America, emeritus and founding member of the Canadian Botanical Association and the Canadian Society of Plant Biologists, emeritus member of the Botanical Society of America and the American Society of Plant Biologists, member of Groupe Polyphenols

As the end of summer nears, the green leaves of our deciduous trees undergo a change in colour.  Those of some become a brilliant red like the red maple while others change to a yellow or golden colour like the birch and all of them ultimately become a brown colour.  Despite the economic importance to our tourist industry of the autumnal display of colour of the trees in our forests, relatively little scientific study has been made of this annual phenomenon.  As a result, we are subjected in the newspapers and on websites to an imaginative description of the process too often only distantly based on established facts.  Of a half dozen of the many websites turned up by a Google search of “autumn color”, not one explanation conforms entirely to the present incomplete state of knowledge.  However, in the past two decades, ground-breaking work in the north-east USA and in northern Sweden has yielded a deeper understanding of the coordinated processes ongoing in the leaves as deciduous trees prepare for winter.

The reason for the autumnal change is variously attributed to lower light, cool temperatures, or various other environmental factors, although many descriptors have now grasped the fact that daylength is significant.  In point of fact it is not the daylength itself that is important.  The trees and shrubs recognize the increasing length of time between sunset and sunrise, i.e. the dark period, as the summer progresses and thus begin to make preparations for winter survival. They are able to do this because of an invisible blue-green pigment called phytochrome in their cells.  This pigment serves as a timepiece, enabling the plant to ignore the suggestion of warm autumn days that summer may never end and to realize that ambient temperature is not a reliable marker of the seasons. Thus the process called leaf senescence is begun.

Autumn_Scene_Bulte_October_2014
Autumn scene at the Queen’s University Biological Station. Photo by Grégory Bulté.

The colour change of the leaves is only one of many processes going on as the leaf senesces but it is the one most obvious to us.  The green leaf of the maple and other trees contains several pigments in addition to phytochrome.  Most visible are the green chlorophylls present in tiny intracellular structures called plastids.  These pigments are the key to our survival on earth because of their ability to trap the energy of sunlight for food production by the leaf.  Also present in the plastids but masked by the chlorophylls are several kinds of orange and yellow pigments called carotenoids, a name derived from the Latin name of the orange carrot root.  Over the course of a few days or weeks, the green of chlorophyll disappears.  The carotenoids also are destroyed but not so rapidly as the chlorophylls, thus the leaf becomes a yellow colour. They are yellow, not orange, because the xanthophylls, the yellow carotenoids, are three times more abundant than the carotenes , the orange carotenoids.  At the same time, the leaves of some species begin to produce red-coloured pigments called anthocyanins.  (These are the same pigments that are in the healthful blueberry and cranberry fruits we are encouraged to eat.)  If nearly every cell of the leaf contains these pigments in high concentration, the leaf will be a brilliant red colour like those of the red maple, the carotenoids again masked from view.  If the concentration is low, the leaf will appear orange like those of the sugar maple because we see the red of the anthocyanins mixed with the yellow of the carotenoids.  Thus the orange colour seen in sugar maple leaves is not due to orange carotenoids as frequently stated, even in some botanical textbooks, but to the mixing of the primary red and yellow colours. Didn’t we all learn that as kids?  Newly formed carotenoids do result in a red colouration in some species that retain their leaves in winter, red cedar being an example.

Since the above discusses the mixing of colours, at this time I will also point out that there are some tree leaves that produce anthocyanins all summer, a horticultural variety of Norway maple for example.  It cannot be ignored that the leaves of these trees are very dark in colour, and certainly not a clear green.  That is because nearly all the light passing through the leaf tissues is absorbed by the mixture of pigments thus yielding a very dark purplish or blackish colour.

How the annual disappearance of an amount estimated to be a billion tons of the green chlorophylls is brought about has been subjected to rigorous study only in the past three decades.  Some popular reports state that chlorophyll is broken down and remade all summer long, that it is “used up” in the photosynthetic act.  It is true that chlorophyll is easily broken as evidenced by the rapid bleaching of the leaves of an over-wintered houseplant abruptly put out into bright spring sunshine, but a leaf properly adjusted to bright light conditions does not bleach.  One of the roles of the carotenoids is to protect the chlorophyll from such destruction.  And of course, new chlorophyll must be made as new leaves are formed on the tree or to replace that damaged due to excessive light exposure, but the deliberate breakdown of chlorophyll in a senescing leaf seems to be a one-time event.  It is now known to be a highly controlled, multi-step process associated with major changes in the structure of the plastids.  An important result for the tree is the release of nitrogen, the most essential element for plant growth, from the proteins to which the chlorophylls had been attached.  The removal of this nitrogen from the leaf for storage in the trunk or other parts of the tree before the leaf falls off is a significant activity.  Results available to date demonstrate that trees vary considerably in their ability to do this.  Some are able to remove as much as 80%, whereas others remove relatively little before the leaves fall. Surprisingly, the nitrogen bound into the chlorophyll molecule itself is not made available for transport and so remains in the fallen leaf.

And what is the purpose, if any, of synthesizing new complex chemical structures like the anthocyanins?  Scientific study directed to answering this question has begun only in the past few decades and provides some evidence to indicate that the red pigmentation may protect the leaf from the damaging effects of light so that more of the valuable nitrogen and other nutrients can be transferred to the trunk.  Another question frequently asked is “Won’t the process of anthocyanin synthesis require energy that the leaf will no longer have freely available because of the loss of chlorophyll?”  Of course energy will be required both to make the anthocyanins and also to transport the nutrients but this energy can be provided by “burning” the residual sugars in the cells of the leaf in the tiny “furnaces” called mitochondria.  These bodies provide the energy necessary for the activities of every living cell, plant or animal.

And how are the red pigments made?  Contrary to statements frequently made in the past, these red pigments are not present in the leaf blade during the summer in the green leaf, although the leaf stalk may be red in some species, but are made anew in the autumn in those species equipped with the necessary genes.  That fact is now recognized in most popular explanations.  The answer usually provided to the “how” question, however, completely ignores the current state of knowledge of the process by stating that the sugars remaining in the leaf as a product of photosynthesis are converted into anthocyanins.  This may sound reasonable especially since some sugars are incorporated into the final structure.  However, innumerable studies over the past several decades have shown that building of the part of the anthocyanin molecule that is responsible for the red colour involves sugars only as a source of energy to drive the process.  It occurs in the cytoplasm of the cell in a tightly coordinated multi-enzyme process, the final step of which is to add one or more sugars.  Only then is the molecule so formed called an anthocyanin.  It does not leave the cell in which it is formed but is deposited into the vacuole, a large sac filled with a slightly acidic watery solution that occupies most of the cell volume.  The amounts of anthocyanins subsequently produced depend on many factors, including temperature, light exposure, and nutrient status.  Scientific studies back up the observation that bright sunny days and chilly nights result in the best display of colour.  A current “fashion” in popular explanations is to add that “chilly” cannot include sub-zero temperatures, but a touch of “jack frost” overnight does wonders to enhance the colouration of our hardy native species!  It is true that freezing will kill the leaf cells but no one seems to have tested the frost hardiness of our maple leaves.  If the weeds in our gardens can survive overnight temperatures of -1C or -2C with no ill effect, it seems logical that so can the leaves on a native tree.  On the other hand, in warm weather, especially if the days are overcast, much of the chlorophyll remains and little of the red pigments is made.  As any casual observer may note, more red colour is present in parts of the tree fully exposed to sunlight.  It is important to keep in mind that a typical leaf is comprised of many individual cells each responding more or less independently to its environment.  Although the overall process of preparing for winter is synchronized and ultimately leads to leaf fall, some cells may retain normal functioning much longer than others.  Thus the colour of the leaves is not the same in all parts of the tree or even in all parts of the same leaf.

While all these colour changes are ongoing, the leaves are prepared for the phenomenon familiar to us as leaf fall.  Preparation for separation from the tree is done so as not to leave an open wound at the point of attachment.  An impermeable corky layer, called an abscission layer, is gradually built across the base of the leaf stalk. Such a layer is obviously incompatible with the already-mentioned desirability of removing as much as possible of the valuable nutrients in the leaf for storage in the trunk; how can nutrients be moved past an impermeable layer?  The solution lies in the fact that the tissues through which the nutrients are moved, the vascular tissues which make up the veins of the leaf, are the last part of the leaf stalk to be cut off.  The statement usually made in popular explanations is that the sugars (sucrose, the sugar we put in our coffee, is the form in which sugars are transported in the plant) are trapped in the leaf by this layer and are thus available for conversion to anthocyanins.  But is it true that sugars are indeed trapped?  How long into the autumn is water carried into the leaf?  And how long does the part of the veins responsible for moving nutrients out of the leaf remain functional?  All we know for certain is that finally the leaf is held onto the tree only by the veins which ultimately break off to allow it to float away. Its point of departure is marked by a characteristic leaf scar that can be used to identify the tree in winter, its smooth corky surface marred by the broken remnants of the vascular tissues.

And now we come to the matter of the final brown colour seen in the fallen leaves and also in leaves of some species that are still attached as in the white oak, for example.  Again, explanation of the brown colour on websites is highly variable and mostly unrelated to the probable truth.  There is no brown pigment already present in the tree leaves under discussion, or at least none that any phytochemist has isolated and reported on.  The most likely cause of browning is the phenomenon familiar to anyone who has left a slice of apple or potato exposed to air.  The exposed surface soon becomes brown because the tissues of apple and potato contain an abundance of colourless phenolic substances that are polymerized by enzymes to yield a brown shade.  (There is now a genetically engineered variety of apple and another of potato that do not brown when cut, but will anti-GMO activists manage to prevent them from reaching the market?  Personally, I hope not!)

Tree leaves also contain a complex mixture of phenolics, most of them formed at various steps of the same pathway that leads to anthocyanins as well as the enzymes noted above.  Notable among them are two classes of vegetable tannins, condensed tannins, polymers of catechins whose synthesis branches off at a late stage of the pathway and hydrolysable tannins that are polymers of gallic acid combined with glucose.  (The chemistry of these compounds is exceedingly complex but that has not prevented their use over countless centuries in the production of leather from animal skin!) Thus there are ample compounds available for the browning phenomenon to occur in senescing leaves.
Our trees must maintain the delicate balance of retaining normal functioning of the leaves for as long as possible consistent with making adequate preparations for winter!  Beginning the process of leaf senescence too soon will rob the tree of the ability to make food during the warm days of autumn but, on the other hand, starting too late may result in an abrupt cessation of all metabolic activity due to a killing frost before the tree has time to recover nutrients from its leaves.