FORMATION OF CAROTENOIDS AND CHLOROPHYLLS IN ETIOLATED BARLEY SEEDLINGS EXPOSED TO RED LIGHT

Abstract

For the investigation of many of the problems pertaining to the metabolism and function of the leaf pigments, it would be desirable to vary the relative proportions of the several leaf pigments in any given plant through changes in the environment. Average pigment determinations reported by Rudolph (1) have already indicated that red light, which promotes the formation of chlorophyll, does not stimulate the formation of the yellow pigments in etiolated bean leaves, but the individual determinations were subject to great variation. Moreover, etiolated bean leaves contain relatively large quantities of carotene and xanthophylls in nearly the same proportions found in green leaves, so that, for demonstration of the effect of red light on the formation of carotenoids in these seedlings, extremely precise determinations of the pigments were required. Investigations in this laboratory have shown that etiolated barley leaves, in contrast with the etiolated bean seedlings, contain very small quantities of carotene relative to the xanthophylls. These investigations also made possible the precise determination of the carotenoid pigments, because it was found that the rapid oxidation of the yellow pigments which takes place when the etiolated leaves are ground with sand and acetone during the extraction of the pigments may be prevented by treatment of the leaves with hot water (4). By the use of the improved methods of analysis, it was possible to determine accurately the effect of red light on the formation of the carotenoids in etiolated barley plants. When etiolated barley seedlings are exposed to red light, both carotene and chlorophylls increase rapidly. The xanthophylls, comparatively large quantities of which are present in the etiolated barley seedlings, increase more slowly. Investigation of the pigments of etiolated barley seedlings, which had been impregnated with sucrose and kept in the dark, demonstrated that the presence of sucrose did not promote the formation of carotene or chlorophyll. Determination of the absorption spectra of lutein acetate and of beta-carotene demonstrated that lutein and beta-carotene, the principal constituents of the leaf carotenoids, absorb considerable light in the red region of the spectrum in which chlorophyll exhibits maximum absorption. Since etiolated barley seedlings contain extremely little green