TRICHOMES AND CANNABINOID CONTENT OF DEVELOPING LEAVES AND BRACTS OF CANNABIS SATIVA L. (CANNABACEAE)

Trichome density and type and cannabinoid content of leaves and bracts were quantitated during organ ontogeny for three clones of Cannabis sativa L. Trichome initiation and development were found to occur throughout leaf and bract ontogeny. On leaves, bulbous glands were more abundant than capitate-sessile glands for all clones, although differences in density for each gland type were evident between clones. On pistillate bracts, capitate-sessile glands were more abundant than the bulbous form on all clones, and both types decreased in relative density during bract ontogeny for each clone. The capitate-stalked gland, present on bracts but absent from vegetative leaves, increased in density during bract ontogeny. The capitate-stalked gland appeared to be initiated later than bulbous or capitate-sessile glands during bract development and on one clone it was first found midway in bract ontogeny. Nonglandular trichomes decreased in density during organ ontogeny, but the densities differed between leaves and bracts and also between clones. Specific regulatory mechanisms appear to exist to control the development of each trichome type independently. In addition, control of trichome density seems to be related to the plant organ and clone on which the gland type is located. Cannabinoid synthesis occurs throughout organ development and is selectively regulated in each organ. Typically, cannabinoid synthesis occurred at an increasing rate during bract development, whereas in developing leaves synthesis occurred at a decreasing rate. Cannabinoid content on a dry weight basis was generally greater for bracts than leaves. Analyses of leaves indicate that other tissues in addition to glands may contain cannabinoids, while for bracts the gland population can accommodate the cannabinoid content for this organ. The functional significance of trichomes and cannabinoids in relation to evolution is discussed. OUR INVESTIGATIONS indicate that the epidermal glandular trichomes present on Cannabis are components of a complex and dynamic secretory system. Three types of glandular trichomes, as well as a nonglandular trichome, are recognizable on the shoot. Vegetative leaves possess bulbous and capitate-sessile glands. Pistillate bracts and specialized leaves in association with the inflorescence possess bulbous and capitate-sessile forms as well as the more highly evolved capitate-stalked gland (Hammond and Mahlberg, 1973, 1977, 1978). Both capitate-sessile and capitate-stalked glandshave been implicated as reservoirs for cannabinoids (Fujita et al., 1967; Fairbairn, 1972; DePasquale, 1974; Malingre et al., 1975; Andre and Vercruysse, 1976). However, few definitive qualitative or quantitative data on the cannabinoid content of capitate-sessile and capitate-stalked glands on plants of the same or various strains were presented in these stud1 Received for publication 18 April 1979; revision accepted 6 July 1980. This research was supported with funds from the United States Department of Agriculture (53-32R6-922) and the National Institute on Drug Abuse (DA 00981) to PGM. DEA Registration No. P10043113 (PGM). ies. Our initial comparative studies (Turner, Hemphill, and Mahlberg, 1977) on gland density and cannabinoid concentration of various plant organs on several clones have shown that there is no simple relationship between glands and cannabinoid content. A subsequent analytical study of individual capitate-sessile and capitate-stalked glands (Turner, Hemphill, and Mahlberg, 1978) reported conspicuously higher levels of cannabinoids in capitate-stalked glands than in capitate-sessile glands. In addition, the cannabinoid content of an individual gland type varied with position on the plant organ and with gland age, although the gland retained the cannabinoid profile characteristic of the clone. Thus, data from these studies indicated the existence of a complex relationship between glands on an organ and the biosynthesis of component cannabinoids. In a related study (Hemphill, Turner, and Mahlberg, 1980), it was shown that leaves and bracts of various ages from a particular clone or strain differed quantitatively in their cannabinoid content. This suggested that the quantitative differences for these compounds were controlled directly or indirectly by factors associated with the ontogeny of a particular organ.