Physiology of Autumn Colors
|Standard Cyclopedia of Horticulture|
Physiology of Autumn Colors. When the conditions in almost any locality are favorable for the rapid growth of plants, the prevailing color of the vegetation is green. The leaf-green, or chlorophyll, is a conspicuous part of vegetative organs. Green is normal, so that one does not regard a green plant as "colored." It is true that in some species of plants, chlorophyll is partially or completely veiled by the presence of other pigments, and in the blossoms it may practically fail; but in the latter case the life of the brilliant structures is fleeting, and green is promptly predominant. "Color" is more or less restricted to blossoms, to particular species, or to seasons. The great seasonal change is here the center of interest.
In the autumn the vegetation of the usual temperate landscape loses gradually its distinctive green, while striking yellows and reds are substituted. With favorable conditions, the climax of this transformation is such a riot of color as is not seen at any other time. It is noteworthy that this change is an immediate forerunner of leaf-fall and death. The vegetation that is suddenly cut off by severe frost seldom exhibits true autumnal colors, but instead the dry brown or blackened effects of rapid death, characteristic of any season. On the other hand, autumn tints of leaves may appear in the summer, as when limbs of the hard maple or peach are ringed. This suggests that the production of color is susceptible of experimental study. On the whole, the layman may regard the autumn colors as a necessity to the wholesome rounding out, and a fitting terminus, of a season of usefulness. Coloration is, however, an evidence of fundamental physiological changes; and it is appropriate to ask regarding the climatic or other conditions which bring this about, as well as concerning the nature of these internal changes which also make for the development of color in the autumn.
For the most part, the autumn leaf-colors fall into two groups—yellows and reds. These colors are produced by two groups of pigments essentially different in chemical and physical properties; yet these pigments are frequently blended in the same leaf, yielding such gorgeous effects as may be seen in the sumach.
The yellow pigment (more correctly pigments) of leaves occurs in the chlorophyll bodies of the cell. It is present hi conjunction with the leaf-green in the healthy leaf, but not infrequently it seems to increase in quantity as the chlorophyll disappears. It belongs to a group of substances often called xanthophylls. These are carotin-like compounds, that is, related to carotin, the orange or orange-red pigment of the carrot root. Carotin-like bodies are widely distributed in plants and are also responsible for the yellow, orange, and orange- red colors of a large number of blossoms. These pigments do not occur in solution in the cell-sap, but may be present either in the healthy chlorophyll bodies (plastids) or outside of them. In the latter case, they form crystals, or are in solution in droplets of fatty oils. Carotin-like compounds are more permanent than chlorophyll, so that any green plant may exhibit a yellowish color upon the gradual disappearance of the chlorophyll.
The red pigments of autumn leaves are cell-sap colors, substances soluble in the aqueous solution constituting plant juices. They are supposed to be tannoid compounds, and are generally referred to as anthocyanin. The pigments of red beets and dark grapes are similar compounds. It is significant that those plants exhibiting conspicuous red coloration in the autumn are usually those which give some indication of red during the growing season, as in the possession of red petioles or twigs; and, more especially, they are those in which red is more or less conspicuous as the buds open and the leaves unfold in the spring. The attractive tints of unfolding hard maple buds are therefore an indication that the maple has the capacity to develop a coloration of the cell-sap in the autumn. Some plants develop no anthocyanin under normal conditions of growth.
It is then evident that the yellow colors of autumn leaves may be due to both a greater visibility of the yellow in the chlorophyll body when the chlorophyll disappears, and also to actual increased development of carotin-like compounds. The reds and purples in autumn leaves are a result of the formation or increased formation of tannoid compounds. The question then is: What are the conditions which make the autumn season particularly favorable for the development of these substances?
Some careful studies have been made that bear upon this question. It appears that the production of autumnal reds in many species is related to the sugar- content, and color may be induced or heightened in the shoots of many plants by growing them for a time in strong solutions. Moreover, cold weather has been found to be generally favorable to the accumulation of sugar in the tissues. Observation indicates that after a season favorable for growth, a cold, protracted autumn results in exquisite autumn coloration. It is certain that nothing is more disastrous to brilliance of color than severe early frosts. In addition to enhancing pigmentation, sugar-content seems to be most important as one factor in cold-resistance. It requires a very light frost in the late summer to kill outright the leaves and young shoots of many trees, but the same shoots may be unaffected by an equal degree of cold when the conditions have been such as to bring about the normal autumn coloration.