## The significance of secondary metabolites

In today's world, plants face even more demanding challenges than before, as in addition to the mentioned stressors, they are also threatened by increasingly intense pollution of the environment, higher levels of radiation especially UV radiation, the presence of chemicals and heavy metals in soil and water, dust particles in the air, and many other factors. How, then, does a plant respond to such high levels of stress? Plants can produce hundreds of different compounds that help it respond to one stress factor or another, tasks mainly carried out by the so-called secondary metabolism. Such metabolism is responsible for the production of specific compounds that, unlike lipids, carbohydrates, and proteins, are not essential for the survival and normal functioning of the plant. However, these secondary metabolites (compounds) benefit the plant, enabling it to survive even in adverse conditions. Interestingly, individual secondary metabolites are not found in the same proportion in all plants or even in all parts of a single plant. They are often specifically localised in specific organs, where they can perform their function. Another interesting fact is that many of these substances are not present in the plant during its entire life cycle. Some metabolites are only present during flowering, others during the germination of the plant, and some are typical of the ageing of the plant, so-called senescence. While the most important reason for plants producing these secondary metabolites is protection against adverse environmental factors, they also increase attractiveness to pollinators and protect the plant. As far as protection is concerned, substances that provide repellent effects inhibit the growth and multiplication of microorganisms but can also be helpful in fighting off other plants. This phenomenon is called allelopathy, where a plant produces metabolites that influence the germination, growth and survival of another plant (this can be positive or negative). An example of allelopathy can be seen on the leaves of a walnut. They contain compounds that block the germination of other plants, and therefore, no other plants or grass often grow under walnut trees.

A crucial function of secondary metabolites is to attract pollinating insects and other animals that help the plant produce fruits and spread through the ecosystem. This ability is provided by the presence of fragrant compounds but also pigments that attract the attention of pollinators. Other metabolites play a role in protecting plants against UV radiation, whether it is layers of cutin on the surface of leaves or various antioxidants. In order to survive, plants must also support symbiotic relationships that help them defend themselves against enemies. For example, if a parasite (often an aphid) feeds on leaves, as a response to this stress the plant will activate several defence systems immediately (Figure 11.2). The first is natural defence in the form of bitter or poisonous substances, which deters the parasite. Another possible reaction is the secretion of attractants for competitive or predatory organisms towards the parasite. Such interconnection of different defence systems allows for higher efficiency and better plant survival.