The effect of ultraviolet radiation on the accumulation of medicinal
compounds in plants
Wen Jing Zhang a,b, Lars Olof Björn a,c,⁎
a Lund University, Department of Cell and Organism Biology, Sölvegatan 35, SE-22362 Lund, Sweden
b QingHai Normal University, Key Laboratory of Resources and Environment in Qinghai-Tibet Plateau, Ministry of Education, Qinghai 810008, China
c Key Laboratory of Ecology and Environmental Science in Guangdong Higher Education, School of Life Science, South China Normal University, Guangzhou 510631, China
Essential oils and terpenoids
Essential oils constitute a heterogeneous collection of
chemical compounds. They have in common that they are
synthesized by plants and are volatile and mostly soluble in
ethanol. They have traditionally been obtained from plants
by extraction and distillation. Since the middle ages, they
have been widely used for insecticidal, medicinal and cosmetic
purposes. For a detailed review of their biological effects
the reader is referred to Bakkali et al. [80]. Schelz et al.
[81] have assayed antimicrobial effects of several of them.
Some essential oil compounds are terpenoids, i.e. a class of
hydrocarbons and derivatives of them. The substances from
Glycyrrhiza and yew dealt with at the end of this section are
also terpenoids.
Karousou et al. [82] studied two different chemotypes of
Mentha spicata, and found that in one of them UV-B radiation,
on a dry weight basis, caused a 50% increase in essential oil
production, while in the other chemotype the increase was
insignificant.
A more thorough investigationwas carried out by Johnson
et al. [22] on essential oil production in sweet basil (Ocimum
basilicum L.), in which the effect of ultraviolet radiation is
much greater. They separately analyzed no less than 22 different
essential oil compounds in this plant. The effect of
UV-B radiation increased with the age of the plants, and was
different for different compounds, but mostly positive. At
the 5 leaf stage the irradiated plants contained between 3
and 4 times as much of most essential oil components as did
the unirradiated control plants. In a later paper [83] by the
same group, the authors reported that UV-B radiation is
necessary for normal development of oil glands in sweet
basil. There seems to be a requirement for UV-B in the filling
of the glandular trichomes of this plant.
Cannabinoids
Pate [95] cites older literature suggesting that UV-B radiation
promotes cannaboid production in Cannabis and also
speculates about cannaboid evolution. Plots of estimated UV-B
exposure in different growth places shows an increase in Δ9-
tetrahydrocannabinol (Δ9-THC) with exposure, but a decrease
in cannabidiol. Lydon [96] and Lydon et al.[97] found that in
both leaf and floral tissues the concentration of Δ9-THC but not
of other cannabinoids increased linearly with UV-B exposure
in drug-type Cannabis sativa plants (Fig. 7), but not in fibertype
plants of the same species. Nowadays many sites on the
Internet show that the dependency of cannabinol accumulation
on UV-B radiation is common knowledge among private
entrepreneurs in the drug industry. The biosynthetic pathway
of cannabinoid synthesis is shown in Fig. 8.
It is not known which enzyme or enzymes for Δ9-
tetrahydrocannabinol biosynthesis are induced or stimulated
by UV-B radiation, but one can speculate. The gene for polyketide
synthase catalyzing the synthesis of olivetolic acid
possesses strong sequence homology with chalcone synthase
and may have evolved from this. Chalcone synthase is one of
the classic UV-B-regulated enzymes.
Conclusion
We have seen that the contents of medicinal substances of
many kinds in many plants are increased by exposure to
ultraviolet radiation, and in particular UV-B radiation. This
effect is not always regarded as a stress phenomenon, as in
many cases the increase in the secondary metabolites can be
achieved by radiation so low that they do not negatively affect
growth, and do not result in any visible damage. Thus, in order
to make the production of medicinal plant substances efficient,
it may be advisable to try exposure to ultraviolet radiation
also in cases which have not yet been investigated. I some cases UV-A or UV-C radiation may be more suitable
than UV-B.