On the occasion of the international year of light 2015

Prospekt Flora, Intresting facts

In Croatia, the hogweed (Heracleum sphondylium, Apiaceae) has many names: medvidovina, medvjeđi dlan, smrdeće zelje, trava od šapa, šapika, vučja peta and others. Bears, in particular, seem to find the plant very appetising. It is less known, however, that this plant can cause a strong photosensitivity reaction in susceptible people when it comes into contact with skin that is exposed to UV rays. We came across this phenomenon on a field trip in the Grobnik region, where this plant is called smokvina, a local name that has not been previously recorded in the professional ethnobotanical literature in Croatia (Photo by M. Randić).


There are some groups of compounds that are vital to life but cannot be produced (synthesized) by the human body. They are, however, produced in abundance by plants, and our bodies get these compounds from the food we eat. This group includes certain vitamins and other important protective compounds. Vitamin C and various antioxidants are known as essential elements of a healthy diet. The production of some of the compounds beneficial to people is facilitated by the exposure of plants to ultraviolet (UV) rays. These compounds are secondary metabolites in plants, phenols that have the ability to absorb UV rays. Plants use phenols as protection against excessive amounts of UV rays (that reach the Earth’s surface through solar radiation). In plants, the compounds also carry out numerous other useful functions (which we will probably write more about on our Website during the International Year of Light).

So, plants that grow in environments with intense solar radiation are particularly beneficial to people because of the many secondary, UV-radiation induced, metabolites that they possess. It is important to know that rich sources of phenolic compounds are, for examples, various types of medicinal plants, as well as fruits and vegetables, certain types of grain, tea plants, coffee plants, different varieties of beans, many varieties of red grapes (and fine red wine) and many other types of plants.

The hogweed (Heracleum sphondylium, Apiaceae) is an example of a herbaceous plant that reacts to UV radiation by increasing its production of phenolic compounds. The hogweed is both a wild edible plant and a medicinal plant. But alongside our mini „photo-story“, we also need to give a warning about touching and using the hogweed (because of the possibility of phototoxic dermatitis!), and about the side-effects that may occur in such cases due to exposure to the sun’s rays in people with photosensitivity. It should be noted, however, that the chemical compounds furanocoumarins are responsible for photosensitivity reactions to hogweed, rather than anthocyanins (visible as the purple colouring on the leaf stems in the below photo). This could be an interesting topic for our column „Interesting Features of Plant Life“.

Marko Randić

Category: interesting features of plant life, phytochemistry, phytobiology, International Year of Light

Key words: hogweed (Heracleum sphondylium), UV radiation, phenolic compounds, photosensitivity reaction, furanocoumarins

Posted in September 2015

The lower part of the hogweed’s leaf stalk. It is interesting to note the difference in appearance between the plant that grew in the shade (on the left) and the one that grew in full sunlight (on the right). The latter acquired a dark purple colouring, most likely from anthocyanins, compounds belonging to the phenolic group, and vital in providing plants with protection from UV radiation. In the plant growing in the shade, anthocyanins are visible only as a pinkish border at the delicate, membrane-like base of the leaf stalk. Phenols are a group of protective compounds and antioxidants, found in an unusually wide variety of shapes and types. Because phenols are, in general, beneficial for people, we should make sure that our diets include as many plants as possible that have absorbed a lot of UV rays. When plants are exposed to the sun’s rays, they considerably increase their production of phenolic compounds (as is evident in the above photo). Although the hogweed is traditionally used as both an edible and a medicinal plant, we should be careful when touching and using the plant to avoid the described photosensitivity reactions in people. We should also know that neither nature nor farmers’ markets lack other types of seasonal plants, rich in beneficial phenolic compounds, which are safe to use and are especially recommended. It is also useful to know that photosensitivity reactions in people are caused by furanocoumarin compounds that also react to UV rays, and not by anthocyanins. (Photo by M. Randić)