Chapter 48 of 66 · 2656 words · ~13 min read

Chapter XV

), so that the solution is almost impossible to filter. On this account, purified solutions of this pigment are very difficult to secure, and no satisfactory analysis to indicate its composition has yet been obtained.

## Actinically, it is a complementary pigment to chlorophyll, that is, it

absorbs the blue and green rays and permits the passage of light which is of the wave length that is absorbed by chlorophyll.

=Phycophæin.=--Still less is known of the composition of this pigment than of that of phycoerythrin. It is the characteristic pigment of brown seaweeds. It is supposed to exist in the cells of algæ, chiefly as a colorless chromogen, which becomes first yellow and then brown on exposure to air. Associated with it are other pigments, which have been variously reported as carotin, phycoxanthin, etc.

THE ANTHOCYANS

These are a group of pigments of red, blue, or violet color, which occur in the flowers, fruits, or leaves of many species of plants. They are essentially ornamental pigments, and constitute a large proportion of the brilliant colors of flowers, etc. They occur not only dissolved in the cell-sap, but also as deposits of definite crystals or amorphous compounds in the cell protoplasm.

They are all glucosides. When the anthocyans are hydrolyzed, the sugar molecules are split off and the characteristic hydroxy-derivatives of the three-ring anthocyan nucleus (figured on page 83), known as "anthocyanidins," remain. These anthocyanidins are themselves pigments. They have been shown to be all derivatives of the anthocyan nucleus. The oxygen atom in this nucleus is very strongly basic and exhibits its quadrivalent property by forming stable salts by direct addition of acid radicles. The variation of color of the anthocyanins has been explained by Willstätter, as follows; the red is the acid salt, the blue is a neutral metallic salt, and the violet is the anhydride of the anthocyanidin in question, thus

Cl Cl | | O _____ O _____ HO__ / \ / \ __/ \ KO__ / \ / \ __/ \ | | | \_____/ | | | \_____/ | | | | | | \ / \ / \ / \ / C O C | \ Red | \ Blue | O _____ |/ \ / \ __/ \ | | | \_____/ | | | \ / \ / C

Violet

All of the natural anthocyanin pigments appear to contain a chlorine atom attached directly to the ring oxygen, as shown in the above partial formulas. In addition, they have four, five, or six hydroxyl (OH), or methoxy (OCH_{3}), groups attached at various points around the three rings. The following formula for _[oe]nidin_, one of the most complex of these anthocyanidins, will illustrate their structural arrangement.

Cl | _____OCH_{3} O / \ HO__ / \ / \ _____/ \OH | | | \ / | | | \_____/ \ / \ / OCH_{3} OH OH C

_Delphinidin_ is the corresponding compound without the two CH_{3} groups; while _cyanidin_ contains only five OH groups; and _pelargonidin_, only four OH groups.

The anthocyanin pigments are soluble in water, alcohol, and ether, the solutions being red or blue in color according to the acidity or alkalinity of the medium. Their presence in many species of plants is hereditable, as these plants come true to color from seed, as in the case of red beets, red cabbage, several species of blue berries, etc. In other cases, the anthocyanin development depends largely upon the conditions of growth,

## particularly those which prevail during the later stages of development: as

in the case of apples, where the amount of red color in the skin depends to a large extent upon the conditions under which the fruit ripens.

Anthocyanin pigments often make their appearance late in the season; in fruits, etc., as the result of the normal ripening process but in leaves as the result of shorter daylight illumination accentuated also by sharp frosts.

THE ANTHOXANTHINS

The yellow plant pigments, other than the carotinoids, are almost without exception glucosides having a xanthone or flavone nucleus. These typical nuclei are illustrated on page 83. In these nuclei, as in the anthocyan one, the oxygen atom is strongly basic and combines with mineral acids to form salts (the oxygen becoming quadrivalent) and the color of the pigment depending upon the nature of the combination formed in this way.

The anthoxanthin pigments are yellow, crystalline solids, which are only slightly soluble in water. They dissolve readily in dilute acids and alkalies, giving yellow or red solutions which are of the same color in either acid or alkaline media. They are extensively used as yellow dyes.

Many of the common members of this group have been mentioned in the chapter dealing with the glucosides. The characteristic pigment nucleus of several of these is as follows:

_Chrysin_, found in various species of poplar and mallows,

O _____ HO / \ / \ __/ \ | | | \_____/ | | | \ / \ / HO C ¦ O

_Apigenin_, found in parsley and celery, as the glucoside apiin,

_Campferol_, found in Java indigo, as the glucoside campferitrin,

_Fisetin_, found in quebracho wood and fiset wood,

O ____OH HO / \ / \ __/ \OH | | | \_____/ | | | \ / \ / C ¦ O

_Quercitrin_, found in oak bark, horse-chestnut flowers, and in the skin of onions,

_Morin_, found in yellow wood (_Morus tinctoria_).

_Gentisin_, found in yellow gentian (_Gentiana lutea_),

O CH_{3}O / \ / \ / \ | | | | | | | | \ / \ / \ / OH HO C ¦ O

As a rule, the most brilliant of these yellow pigments are found in the largest quantities in the bark and wood of various species of tropical plants; although they are also present, in smaller amounts, in the blossoms of species growing in temperate zones.

The anthoxanthins are easily converted into anthocyanins, and _vice versa_, by the action of oxidizing and reducing enzymes which are commonly present in the tissues of the plants which develop the pigments.

THE PRODUCTION OF ORNAMENTAL PIGMENTS IN FLOWERS, ETC.

The breeding of flowering plants having blossoms of almost any desired color has become a commercial enterprise of large importance. The results which have been obtained, in many cases, have been made the object of scientific study of the genetics of color inheritance. These studies have developed certain interesting facts with reference to the chemistry of the development of these ornamental pigments, which may be briefly mentioned here.

In many of the plants which have been studied, the color of the flowers depends upon several different factors, as follows:

_C_, a chromogen (or color-producing substance) which is generally a flavone or xanthone glucoside, and which may be either yellow or colorless.

_E_, an enzyme which acts upon _C_, to produce a red pigment.

_e_, another enzyme which acts upon the red pigment, changing it to some other anthocyanin color.

_A_, an antioxidase, or antienzyme, which prevents the action of _E_.

_R_, an enzyme which changes reds to yellows.

Thus, if a plant whose flower contains only the factor _C_ be crossed with one which contains the factor _E_, a red blossom will result, or if it contains the factor _e_ more intense pigments are developed. But if either _A_ or _R_ are present, no change in the color of the original parents will result from the crossing.

THE PHYSIOLOGICAL USES OF PIGMENTS

The vegetative pigments undoubtedly serve as agencies for regulating the rate of metabolic processes. At the same time, it is extremely difficult to determine whether the presence of a pigment in any given case is the cause or the effect of the changes in the plant's activities which result from changes in its external environment.

The chlorophylls are, of course, the regulator of photosynthesis, absorbing solar energy with which the photosynthetic process may be brought about. The simultaneous presence of carotinoids in varying amounts undoubtedly serves to modify the amount and character of the radiant energy absorbed, as these pigments absorb a different part of the spectrum of light and hence undoubtedly produce a different chemical activity or "actinic effect" of the absorbed energy. The variations in depth of color of foliage during different growing conditions, from a pale yellow when conditions are unfavorable and growth is slow to the rich dark green of more favorable conditions, is a familiar phenomenon. Whether this change in pigmentation is the result of an adjustment of the plant protoplasm, so that it can absorb a more highly actinic portion of the light, or is a direct effect of the lack of conditions favorable to chlorophyll-production and active photosynthesis, has not yet been determined.

But there must be some influence other than response to environmental conditions which controls the vegetative color in plants, since shrubs, or trees, which have green, yellow, red, and purple leaves, respectively, will grow normally, side by side, under identical external conditions of sunlight, moisture supply, etc. The hereditary influence must completely overshadow the apparent normal self-adjustment of pigment to energy-absorbing needs, in all such cases.

Again, it appears that there is some definite connection between pigment content and respiration. It is known, of course, that the gaseous exchanges involved in animal respiration are accomplished through the reversible change of hæmoglobin to oxyhæmoglobin, these being the characteristic blood pigments. The easy change of carotin, C_{40}H_{56}, to xanthophyll, C_{40}H_{56}O_{2}, and _vice versa_, and the reversible changes of the yellow anthoxanthins to the red anthocyanins, under the influence of the oxidizing and reducing enzymes which are universally present in plants, would indicate the possibility of the service of these pigments as carriers of oxygen for respiratory activities in plants in a way similar to that in which the blood pigments serve this purpose in the animal body. The fact, which has been observed in connection with the experimental studies of the development of the lycopersicin, that tomatoes which normally would become red remain yellow in the absence of oxygen, indicates that this pigmentation, at least, is definitely connected with oxygen supply; and the further fact that the development of lycopersicin in red tomatoes, red peppers, etc., is dependent upon the temperature at which the fruit ripens, may indicate a definite connection of this pigment with the need for more oxygen (or for more heat, as suggested in the following paragraph) at these lower temperatures.

Again, many investigators have concluded that at least one function of the anthocyanin pigments is to absorb heat rays and so to increase transpiration and other chemical changes. In support of this view, there may be cited the general presence of such pigments in arctic plants, their appearance in the leaves of many deciduous trees after a frost in the fall, etc. Indeed, there is much to support the view that the autumnal changes in foliage pigments have the physiological function of absorbing heat in order to hasten the metabolic processes of ripening and preparation for winter defoliation. The rapid and brilliant changes in foliage coloring after a sharp frost which kills the tissues and makes rapid translocation of the food material of the leaves to the storage organs immediately necessary, have been explained as the response of the pigmentation of the leaves to the need for increased heat-absorption. On the other hand, the red pigments of the beet-root, etc., which seem to be identical in composition with the other anthocyanin pigments, can have no such function as those which have just been described. Furthermore, the fact that the pigment often varies in color from red to yellow or brown, depending upon the temperature under which the tissue is ripening, makes it an open question whether the pigment is the regulating agency or whether its nature is the result of the environmental conditions. Or, in other words, it is a question whether these changes in color are a mechanism by which the plant cell adjusts its absorptive powers, or whether they are only the inevitable result of the changes in temperature upon a pigment material which is present in the cell for an entirely different use.

A very interesting side-light upon the color changes which many species of plants undergo when the external temperature falls has been shown by the investigations of the relation of the sugar content of the plant tissues to their pigmentation. It is a well-known fact that not only do many species of deciduous plants show the characteristic reddening of their leaves after frost in the autumn but also many evergreens (_Ligustrum_, _Hedera_, _Mahonia_, etc.) exhibit a marked reddening, or purpling, of their foliage during the winter months, with a return to the normal green color in the spring. Earlier investigations, which have been confirmed by several repetitions, showed that the red or purple leaves always contain higher percentages of sugar than do green ones of similar types. More recent studies have shown that artificial feeding of some species of plants with abnormally large portions of soluble sugars produces a reddening of the foliage tissues which is apparently identical with that which these tissues undergo as the result of low temperatures. Thus, the connection between the natural winter reddening of foliage and the development of sugar in the tissues during periods of low temperatures (see page 64) seems to be clearly demonstrated. It appears that at least a part of the seasonal changes in color of plants is either the cause of, or the effect of, variations in sugar content of the tissues of the plants, accompanying the changes in external temperatures.

Oftentimes, the anthocyanin pigments seem to be associated with sugar production, as contrasted with the chlorophylls, which seem to be more favorable to the production of starch. But in this case also, it is impossible to say whether the pigment is the direct causative agent in the type of carbohydrate production or whether it is the effect of the same external factors which determine, or modify, the character of the carbohydrate condensation.

BIOLOGICAL SIGNIFICANCE OF ORNAMENTAL PIGMENTS

The ornamental pigments undoubtedly have definite biological significance. When present in the storage roots, such as beet-roots, carrots, etc., or in the above-ground parts of plants, they may have served to protect these organs against herbivorous animals which were accustomed to consume green foods.

In flowers, the brilliant ornamental pigments undoubtedly serve to attract the insects which visit these blossoms in search of nectar, and in so doing promote cross-fertilization. Recent experiments have demonstrated that colors are much more efficient than odors in attracting insects.

Taken altogether, it is apparent that the pigments may have a variety of important rôles in plants. At the same time, some of them may be waste products, with no definite use in the plant economy.

REFERENCES

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 6, Farbstoffe der Pflanzen- und der Tierwelt," 390 pages, Berlin, 1911.

PERKIN, A. G. AND EVEREST, A. E.--"The Natural Organic Colouring Matters," 655 pages, London, 1918.

WAKEMEN, NELLIE A.--"Pigments of Flowering Plants," in _Transactions_ of the Wisconsin Academy of Sciences, Arts, and Letters, Vol. XIX, Part II, pages 767-906, Madison, Wisc., 1919.

WATSON, E. R.--"Colour in Relation to Chemical Constitution," 197 pages, 65 figs., 4 plates, London, 1918.

WHELDALE, M.--"The Anthocyan Pigments of Plants," 304 pages, Cambridge, 1916.

WILLSTÄTTER, R. AND STOLL, A.--"Untersuchung über Chlorophyllen, Methoden und Ergebnisse," 432 pages, 16 figs., Berlin, 1913.