Cannabinoids and THC
In this study we will examine two types of the several glands on Cannabis, the capitate sessile and capitate stalked forms. In addition, there are hairs on the surface of the plant. In our commentary we will refer to cannabinoids as the broad class of specialized compounds synthesized by Cannabis. Cannabinoids are dimers in that they are formed by condensation of terpene and phenol precursors (c).
Diagram 1 (Above). Cannabinoid pathway. Cannabinoids represent a dimer consisting of a terpene and a phenol component. Cannabigerol (CBG) is the first component of the pathway. It undergoes chemical change to form either cannabichromene (CBC), or cannabidiol (CBD). Delta 9-tetrahydrocannabinol (THC) is derived from CBD.
Cannabinoids include these more abundant forms:
THC, delta 9-tetrahydrocannabinol
CBD, cannabidiol
CBC, cannabichromene
CBG, cannabigerol
Another cannabinoid, cannabinol (CBN), is formed from THC and can be detected in some plant strains. Typically, THC, CBD, CBC and CBG occur together in different ratios in the various plant strains. In fiber strains CBD/CBC are in high concentrations and THC is at a low concentration; in drug strains, THC is high and CBD/CBC are low. An important area to be studied relates to the occurrence in the plant of potential precursor for cannabinoid formation and where synthesis occurs in the cell.
Distribution and types of glands
Glands cover entire surface of the above ground portion of both pistillate (female) and staminate (male) plants, but are most abundant on bracts of female plants (Fig. 1).
Figure 1. (Above) Stalked and sessile glands on underside of a bract. Hairs also are present on bract. x35.
Figure 2. (Above). Stalked gland showing large head (star) and abscission zone at base of gland head (arrow). Sessile glands are evident in background along with a portion of a hair. X300.
Legend for all figures: C, cuticle; D, disc cell; P, plastid; S, secretory material; T, secretory cavity; V, vesicle; W, disc cell wall.
There are two types of glands active in cannabinoid secretion on female plants (Figs. 1, 2):
a. Capitate sessile - most common in that it occurs on stems, leaves and bracts.
b. Capitate stalked - develops only after flower formation, and occurs especially on the bracts subtending a flower and seed. Both types are present on the bract subtending the seed, but some factor(s) that stimulates flowering also stimulates development of the stalk related to this gland. Thus this gland has evolved from the sessile type.
An abscission zone develops at the base of the head where the stipe cells attach to the disc cells resulting in abscission of glands upon attaining maturity (Fig. 2, arrow).
Cannabinoid, including THC, composition of glands
a. Effect of position and season on contents of gland.
We examined both gland types for their cannabinoid composition. Whole glands (20 glands) were removed from bracts and leaves of the same plant to compare their contents. They were extracted for cannabinoids and analyzed (Table 1).
We also separated them as to glands over a vein as contrasted to non-vein areas. Results showed that stalked glands over a vein on the bract contained more THC content, approximately 20 times more, than the sessile glands over a leaf vein. Similarly, stalked glands over a non-vein area contained much more THC than sessile glands over the leaf non-vein area.
Surprisingly, even stalked glands on a bract varied for THC content between the vein and non-vein area. We also found that sessile glands varied for contents between vein and non-vein areas on the leaf.
This table shows two intervals during the year, October and December, during which we compared levels of THC in glands. Comparison of October with December analyses showed again that stalked glands contained considerably more THC than sessile glands. For both types, however, the vein glands now showed less than the non-vein. Interestingly, the level of THC can decrease to a non-detectable level in the glands (leaf vein glands).
In similar analyses of these glands on other strains we found the same pattern, but the cannabinoid levels in the stalked glands were not as high as for this strain. In this strain the sampling was repeated during the following March and, again, the stalked glands on the bract contained higher concentrations of THC than sessile glands on the leaf. Thus, a repeatable pattern appears to occur in the plant in which stalked glands usually contained greater quantities of THC than sessile glands.
b. Effect of gland age on cannabinoid content
We also examined cannabinoid content of stalked gland by age to measure the major cannabinoid components in both a fiber and drug strain (Table 2). Glands, viewed under a microscope, can be classified according to their secretory phases from the color of their contents. Glands most active in secretion (mature) are translucent in appearance, aged glands are yellow in appearance and senescent glands are brown in color. Mature glands possessed the highest content of their major cannabinoid in both the fiber and drug strains. Senescent glands possessed low levels of cannabinoids. The concentration of some components, as CBD in the drug strains, may be so low that is was not detectable in our analysis; similarly, for THC and CBN in the fiber strain. It is unknown where the cannabinoids go during the aging process, but we suggest that it is possible they volatilize into the atmosphere along with the terpenes in glands, as noted later in this report. Nevertheless, this phenomenon of altered content in glands during aging is one that should be studied to gain a more complete understanding of the secretory process of cannabinoids in the cell.
Table 2. (Above) Cannabinoid content of capitate-stalked glands of different ages.
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