In cannabis, there is some evidence that temperature may play a role in
cannabinoid biosynthesis. However, the few studies that exist are somewhat
conflicting. One study showed that increasing temperatures produced an increase in
cannabinoid content (Boucher et al. 1974 ) while a second study showed a decrease
in cannabinoid content (Bazzaz et al. 1975 ). It is likely that response to temperature
stress is more complex, involving multiple factors. In fact, specific strains
responded differently to thermal stress (Braut-Boucher 1980 ). Work was performed
to better understand the role of COR genes in cold acclimation in cannabis (Mayer
et al. 2015 ), but their effects on cannabinoid biosynthesis were not studied.
Therefore, before attempting to utilize thermal stress as an elicitor for cannabis, a
better understanding is needed within the genetic and environmental context.
21.6 Photo-Radiation
One environmental factor which has been more studied in cannabis is
photo-radiation. Elevated UV radiation has pleiotropic effects on plant develop-
ment, morphology, and physiology (Gorelick and Bernstein 2014 ). The most
common protective mechanism against potentially damaging irradiation is the
biosynthesis of UV absorbing compounds (Hahlbrock and Scheel 1989 ). These
secondary metabolites, mainly phenolic compounds,flavonoids, anthocyanins, and
hydroxycinnamate esters, are well known to accumulate in the plant cells and
reduce the penetration of the UV-B radiation into deeper cell layers as well as
detoxify reactive oxygen species (ROS) (Kakani et al. 2003 ; Caldwell et al. 2007 ).
ROSs increase in response to UV-B, as well as salicylic acid and jasmonic acid
which may all affect production of secondary metabolites.
It is therefore to be expected that in cannabis as well, UV radiation stimulates
secondary metabolite production. Increasing the irradiance increases the total THC
concentration in the plants (Potter and Duncombe 2012 ) in addition to increasing
the rate of photosynthesis and water use efficiency (Chandra et al. 2008 ). However,
this effect does not seem to be a response to stress but rather a consequence of the
higher proportion offloral to vegetative material. In addition, due to their UV
absorbing properties (Pate 1983 ), it is plausible that cannabinoids may play a
defensive role against UV radiation. A number of observations support this concept.
Small and Beckstead ( 1973 ) observed increased cannabinoid content in plants
originating from areas exposed to greater amounts of UV. This correlation was also
observed regarding plants found at higher altitudes (Bouquet 1950 ). There is
additional circumstantial evidence, such as a tentative correlation between seasonal
UV variations and THC content (Latta and Eaton 1975 ). However, the majority of
the mentioned studies lack adequate controls and defined parameters to be very
convincing. There were a very small number of more rigorous studies on the effects
of UV on cannabinoid content. Plants exposed to increasing intensities of UVB
radiation for 40 days contained increased concentrations of THC in bothfloral and
vegetative tissue (Lydon et al. 1985 ). Interestingly, there was no change in the
444 J. Gorelick and N. Bernstein