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, C40H56, to xanthophyll, C40H56O2, 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.