This paper will explain why some leaves turn red, some blue, and some yellow. You will learn what causes some leaves to turn a clear yellow or orange, while others turn a single mixed wine crimson, or a rainbowof unmixed colors. You will learn how some processes create colors late in the season, while others were chemically created months prior. You will find out that colors can be hidden and then revealed. You will learn which factors affect leaf color change: genetics, chemistry, external factors, or all of these.
Leaf color is the wild display of a larger process called senescence or leaf death. You could say coloration is a form or manifestation of senescence. Several other internal changes inherent to dying leaves help bring about color change. These are all ultimately tied to receding sunlight. Perhaps death ranks high in scales of ecological significance and this significance inherently implies a wide-range of causal factors. At any rate wide-ranging factors affect leaf color change.
The many factors that affect leaf color change are multiple and somewhat interrelated. The larger external causes are temperature, light levels, wind, and precipitation. Internal factors include photosynthetic rate, respiration rate, and protein levels. In some sense, the more significant the life process affected in plants, the more factors seem intertwined. Another way to say this is that major plant functions seem to have multiple causes. Plants don’t rely on one system, but possess back-up systems as well. Perhaps, the grander-scale the process, the more causes, back-up systems, are involved. Impressively, a huge span of causes can affect one change, as in the case of fall leaf coloration.
There are two theories of color change. One is competition theory, which states that color loss is related to diversion of nutrients to other events like flowering and fruit formation. This divergence slows synthesis. Nutrients are in demand for multiple functions and this lessens the ability to produce chlorophyll.
Another way to look at color change is the inverse of competition, mutual enhancement. There also seem to be various mutually enhancing mechanisms at work. Hormones involved in leaf color change seem to work in conjunction rather than independently. Numerous experiments of Larry Nooden suggest that soybean fruits produce a senescence factor, a hormone, which moves to the leaves where it causes leaf coloration and senescence. In cocklebur plants short-day and long-night conditions induce flowering and leaf senescence, but even if all flower buds are removed, leaf senescence still occurs.
The major cause of leaf color change involves both external and internal factors. Absorption of light from the environment spurs internal production of flavonoids, groups of pigments. Flavonoids also facilitate absorption of light. Furthermore, flavonoids in the leaf epidermis absorb ultraviolet light and protect chlorophyll from photooxidation. It follows that high elevation plants receive a greater amount of UV radiation and have more flavonoids. This is why alpine flowers have such intense color.
As the days get shorter, chlorophyll production lowers and falters. Carotenoid production also slows and stops. However, some changes affect chlorophyll but not carotenoid. These cause chlorophyll to slow before carotenoid causing the green pigment to be reduced while the yellow, oranges and reds remain. Various external changes lessen chlorophyll but not carotenoid such as: reduction of minerals, lowering of temperatures and dimming of sunlight, as the sun arcs shortens and recedes.
Disturbance is another external factor that affects color change, among other things. While this paper cannot fully explore this crucial topic, I will highlight one illustration. In 1979 researchers Bormann and Likens experimented with disturbance in Hubbard Brook. They found that exploitive species on a disturbed site maintained green leaves longer than either leaves of conservative species occurring in the canopy of the adjacent intact forest or saplings and sprouts in the cut-over site. After the disturbance, pin cherry dominated the site, in contrast to the American beech, sugar maple and yellow birch dominance of the adjacent intact forest. Rates of conductance and photosynthesis depend upon such species shifts. The study shows that exploitive species have greater leaf-level rates of physiological gas exchange and remain physiologically active for longer into autumn. This could affect recovery after disturbance. To further explain such dynamics I will turn to a general discussion of internal factors of leaf color change.
Internal factors relate to absorption and chemical change of light.
The processes of photosynthesis, pimentation, chemical make-up of sap,
and transport of sugars and minerals are all internal factors of color
The predominance of green in nature testifies to the predominance of chlorophyll. The amount and rate of chlorophyll production depends on supply of sunlight and minerals. Deficiencies of magnesium and other mineral lead to a yellowing of the leaf known as chloroses. Scientists think that most likely these minerals activate enzymes necessary for synthesis of chlorophyll. Thus amount of chlorophyll correlates to ability to carry out photosynthesis. As sunlight becomes abundant in summer chlorophyll levels rise and leaves turn a darker green.
What goes on inside these changing tree spectacles? Chemical processes that nourish leaves transform. Leaf functions slow and cease. Photosynthetic rates drop to one half to one third of the previous level. Respiration decreases. RNA and protein production level drops, including production of many enzymes. Phloem transport of sugars and minerals across the abscission layer slows and ceases. Potassium, nitrogen and phosphorous are lost. Nitrogen especially is lost in large amounts. Betula samples lost one third to one half of their nitrogen specimens in two weeks.
Certain minerals lost in large amounts are transferred back into twigs and branches. As in the death of other plant parts senescence of leaves is preceded by hydrolysis of RNA and protein and the product’s conversion into mobile amides and amino acids. These are degraded into smaller, more readily movable forms. As such nutrients are conserved by storage in other parts of the plant. The nutrient economy helps forest trees to survive on unfertile soils.
Further, the structure of molecules such as anthocyanin pigments, acidity of plant sap and nutritive elements of sap, all contribute to color change and ultimately leaf senescence. Ordinary leaf cell structure holds chlorophyll and carotenoids in the plastids.
Carotenoids are two kinds of fat-soluble pigments. Carotenoids accumulate
in the chloroplasts with the chlorophyll, and also in other plastids called
the chromoplasts. These carotenoid chromoplasts arise from plastid initials
and also from chloroplasts that lose their chlorophyll.
There appear to be two kinds of carotenoids, carotene and xanthophyll, both of which function to prevent the destruction of chlorophyll in the presence of light and oxygen. Xanthophyll may collect light and transfer it to chlorophyll by inductive resonance. The ubiquitous presence of carotenoids in plastids with chlorophyll indicates their functions are closely connected. It seems that progressive disruption of chloroplast organization at the ultrastructural level reveals the yellow and orange colors in some species.
A dominance of chlorophyll over carotenoids masks the color of carotenoids – red, orange, yellow and brown. Thus the leaf appears green. An average summer leaf has three times more chlorophyll than carotenoid. Some carotenoids are actually synthesized in the fall. Generally, throughout the fall chlorophyll production falls and carotenoids dominate, exposing the vivid hues of carotenoids to view. Thus for a time trees glow brightly.
Other important internal factors are hormones producing specific colors. In the 19th century, scientists thought specific hormones affected specific organs. We now see that plant hormones interrelate. The rule is that most physiological processes require interactions among several hormones, and a single hormone has several functions. Moreover, each process depends on cell types and species involved. It seems that different species use different hormones or rely on different interactions among them to accomplish their various functions.
Tree species as well as the individual tree’s make-up both affect coloration, and hormones are critical actors. The flavonoides that create the most brilliant colors are anthocyanins, the red pigments. For instance, anthocyanin in combination with other flavones, produce the flaming colors of sugar maples. Brilliant greens are caused by chlorophyll. Yet, a mix of flavones allow yellows, oranges and reds mingle on a single sugar maple tree. There is nothing like the sensation of lying beneath a sugar maple at the height of color brilliancy; as much as you try, you cannot truly absorb the sight. In contrast, some species boast darker hues. Cherries and Bradford pears turn subtle hues of gorgeous plum and wine.
In the latter part of leaf life the carbohydrate levels rise dramatically. Trees with the genetic capacity will use excess carbohydrates to produce anthocyanin. Anthocyanins are glycosides formed by reactions between sugars and cyclic compounds called anthocyanidins. Color of anthocyanins in cell sap vary depending on the acidity of cell sap. In acidic solution they are usually red. As pH is increased they may become purple or blue. Regardless of anthocyanin production chlorophyll levels must still drop before the new colors are unveiled.
Anthocyanins also affect leaf color in some species during the growing phase. For instance, Japanese maples, or "Red Maples" are red or purple because of the presence of anthocyanin in cell sap. This same anthocyanin might be masked in a species that only turns red, blue, purple or violet during senescence.
The production of flavones requires carbon. Because nitrogen is translocated back out of the leaf, there is less nitrogen available for protein formation. Carbon skeletons are used to form cinnamic acid a step along the way to shikimic and chorismic acids. Thus, what promotes formation of soluble sugars also helps form flavonoids.
As the season progresses and nitrogen levels drop low the translocation pathway becomes blocked. Yet photosynthesis in the leaf continues. This causes a build-up of soluble sugars. These sugars can be used for anthocyanin production for species that produce additional anthocyanin in the fall.
The actual colors a tree turns depends on genetic capability of the species and the variables of external factors. If trees do not produce anthocyanin the drop in chlorophyll will unmask the stable yellow or orange carotene and xanthophylls pigments that were there all along. This results in a clear yellow color, as in yellow poplar or hickory. A mixture of anthocyanin and yellow carotene will give a bright orange color. Mixes are variable and produce endless gradations of colors in the New England landscape.
Some species like alders and locusts show little color change. Others show beautiful shades of yellow including: poplars, tulips, ginkos, honey locusts, beeches, and birches. Most people find the most dazzling displays to be reds of trees such as: maples, sassafras, tupelo, staghorm sumac, white oak, and shadbush. Following color change leaves abscise and fall off the tree. You can visually observe the progression of senescence that signals abscission. The outer edges of the leaf will appear in vivid hues while the interior and base is still green. The last cells of the leaf to reach senescence are the cells located just along or above the base of the petiole. This is the abscission zone made up of short and compact cells without intercellular spaces. As senescence reaches them vessels become blocked by tyloses. Protoplasm and starch deposition occurs, the cell walls swell, and pectic and cellulosic materials are digested. At this point separation occurs between rows of cells, sometimes through the middle lamella or the primary cell walls. Sometimes the entire cells of one or more layers dissolve.
Often the shedding of foliage occurs quickly. Sometimes a tree will drop the bulk of its leaves in just an hour or two. Species vary in timing of leaf fall, as they did in coloration. For instance, many Quercus stay green later and abscise later. In some species leaves senesce, turn brown and shrivel but do not abscise. For instance, some Quercus and Carpinus leaves hang on through winter and abscise the following spring. The last reminders of summer’s lush, green canopies are these whispering rattles of winter.