by Marjan Donko
The most important of these substances are probably the phenolic compounds. Phenolic is a broad descriptive term for aromatic molecules with at least one hydroxy group. Polyphenols are a very heterogeneous group of organic compounds. In the world of plants, they are represented by pigments, coenzymes, inhibitors, antimicrobial agens and phytoalexins. Some of them are called tannins. One of their qualities is that they can precipitate proteins from a water solution, which is why houseleeks taste bitter. Polyphenols are commonly bound to sugars, amino groups, lipids and terpenoids. It is important to mention that the concentration of compounds is different according to the time of day, the season, or the age of the plant or leaves.
Houseleeks have Crassulacean Acid Metabolism (CAM), which is one of their adaptations to arid habitats. CAM can be found in 32 families of plants and 328 genera. It was discovered accidentally when scientists researched pH in the plants during the daytime. CO2 was found to be bound to phosphoenolpyruvate at night, forming oxalacetate and then malate, so that plants could maintain their stomata closed during the day. The stomata open at night when temperatures are lower.
Arid habitat is not the only stimulus to the evolution of CAM. It is found in habitats with plenty of water. There the concentration of CO2 is the limiting factor for photosynthesis.