I submit that it is to protect against ultraviolet radiation. It is found more prominently in flowering females because they have a high investment in seed production and they use it to protect against ultra violet (UV) light. This is why plants produce more THC at higher altitudes and at or near the equator.
If THC is a good UV blocker could sunblocks be formulated using these properties? I can even imagine that the molecule may have some sort of anti-aging properties.
THC and the other cannabinoids have quite a few protective qualities. They have antibiotic and anti-fungal properties as well. The physical quality of the resin and its placement in glands protruding from the leaf and flower tissue acts as flypaper, capturing some small pests in its stickiness. The cannabinoids have a profound affect on birds and mammals that might eat the substances, producing an intoxication that they may not find pleasant.
Although they are not cannabinoids, marijuana’s odor-causing substances, mono and di-terpenes, also provide marijuana with protection from predators. Some animals find its odor repulsive or learn to associate it with an unpleasant experience.
The quality that you describe ? protecting the plant and the developing seeds from UV light ? is also true, and may be helpful to the plant. In a controlled experiment, high quality marijuana plants were grown under identical conditions, except that some were provided with UV light while others didn’t receive it. The buds from plants receiving the UVB light had higher percentages of THC. This is an indication that the plant is using it as a protectant.
THC, or a derivative from it, should be studied to see if it does afford UVB protection and if it is a viable means of providing it.
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