Heterotrophic Plants

HETEROTROPHIC PLANTS (from Greek heteros - other and trepho - I nourish), plants incapable of using CO₂ as a source of carbon for building organic matter and therefore requiring nutrition in the form of organic compounds. They are contrasted with autotrophic plants, which can build organic matter from inorganic sources and do not require organic nutrition. Heterotrophic plants include almost all chlorophyll-free plants: fungi, the majority of bacteria (with the exception of a few autotrophic ones, such as nitrifying bacteria and others), and a few flowering plants devoid of chlorophyll. Some heterotrophic plants live in nature as saprophytes, at the expense of dead organic matter, while others live as parasites, at the expense of another living organism: animal or plant. The former are often called metatrophs, and the latter paratrophs. Numerous transitions exist between them, and in many cases it is possible to grow parasitic plants in artificial culture under saprophytic conditions. The most essential factor for the nutrition of heterotrophic plants is the source from which they draw their carbon. This is always organic compounds, which vary in different cases. For the vast majority, the best source of carbon is soluble sugars. Below sugars are polyhydric alcohols and acids, with higher homologs surpassing lower ones in their nutritional value, and straight-chain compounds surpassing those with a branched carbon chain. Cyclic compounds are almost always a significantly worse source of carbon than aliphatic compounds. Finally, for many heterotrophic plants, the source of carbon can be proteins and amino acids, although sugars are usually considerably better in this respect. As for nitrogen nutrition, some heterotrophic plants, especially parasitic ones, require nitrogen in the form of proteins or amino acids; however, the vast majority well assimilate NH₃ and nitric acid, and some even elemental nitrogen. In this character of nitrogen nutrition, heterotrophic plants differ from animals, which, as is known, obligatorily require organic nitrogen (proteins and amino acids). Autotrophic (inorganic) nutrition with one element and heterotrophic (organic) nutrition with another is sometimes called mixotrophic nutrition. In this sense, the majority of heterotrophic plants are mixotrophic. More often, however, mixotrophic nutrition refers to mixed nutrition with a single element, especially carbon. The ability, alongside CO₂, to assimilate organic carbon to a certain degree is characteristic in an artificial environment of probably all green plants without exception; in some, however, which are poor in chlorophyll and therefore weakly assimilate CO₂, this ability to assimilate organic carbon has a predominant significance in nutrition (dodder, broomrape, bird's-nest orchid, etc.). Mixotrophic nutrition is especially widespread among primitive green algae, even those that contain normal amounts of chlorophyll. It is expressed differently in different forms and serves as a very important factor in their distribution in nature, while also allowing them to be used in biological analysis (see) of waters. L. Kursanov.