Role of Carotenoids in Protecting Chlorophyll From Photodestruction

There have been numerous suggestions in the last few years of obligatory functions of carotenoids in photosynthetic organisms. These have been functions of carotenoids other than their action as accessory pigments. Workers who are interested in the mechanism of converting radiant energy into chemical energy frequently use the carotenoid molecule as a sink, or hole, in their photodissociation hypotheses (3, 12). Kohl (9) suggested many years ago that a function of carotenoids was to protect chlorophyll. Recently, evidence for such a role of carotenoids has been accumulating. Claes (2) has described four X-ray mutants of Chlorella vulgaris Beyerinck. Three of these mutants do not form colored carotenoids and they are photosensitive. Sager and Zalokar (13) have studied a mutant of Chlacnydomonas rienhardi Dang. which contains only trace amounts of carotene and which dies when exposed to light. It responds, however, to illumination by taking up carbon dioxide and releasing oxygen. Allen (1) also has shown that a carotenoidless mutant of Chlorella pyrenoidosa Chick is unable to grow in strong light. The studies of Cohen-Bazire and Stanier (4) and Fuller and Anderson (6) have shown that colored carotenoids are required for protecting bacteriochlorophyll from photodestruction in the photosynthetic bacteria. It has been proposed that this is a unique role of carotenoids in all photoautotrophs (17). The results in the present communication support this hypothesis by extending to higher plants an experimental confirmation of the proposal through the study of an albino mutant of corn (white-3). Koski and Smith (10) have shown that this mutant forms protochlorophyll and chlorophyll; however, the chlorophyll is bleached upon continued illumination. An albino mutant of sunflower which reacts similarly has been described by Wallace and Schwarting (18).