The vast majority of organisms belonging to the plant kingdom are autotrophs (phototrophs) . Heterotrophs include all animals, fungi, and most bacteria. Among plants, there are also optional or obligate heterotrophs that receive organic food from the external environment - saprotrophs, parasites and insectivorous plants . Saprotrophs (saprophytes) feed on the organic substances of the decomposing residues of plants and animals, parasites - on the organic substances of living organisms. Insectivorous plants are able to capture and digest small invertebrates. However, in the life of a plant there are periods when it eats only due to previously stored organic substances, i.e., heterotrophically.
Such periods include germination of seeds, organs of vegetative propagation (tubers, bulbs, etc.). the growth of shoots from rhizomes, the development of buds and flowers in deciduous woody plants, etc. Many plant organs are heterotrophic in whole or in part (roots, buds, flowers, fruits, forming seeds). Finally, all tissues and organs of the plant heterotrophically feed in the dark. That is why in culture it is possible to grow isolated plant cells and tissues without light on an organo-mineral medium.
Thus, the heterotrophic method of feeding cells and tissues is as common for plants as photosynthesis, since it is inherent in any cell. At the same time, this method of plant nutrition has been studied extremely insufficiently. Familiarity with the physiology of heterotrophically eating plants allows us to come closer to understanding the nutritional mechanisms of cells, tissues and organs of plants as a whole.
Entire plants or organs can assimilate both low molecular weight organic compounds coming from outside or from their own reserve funds, as well as high molecular weight proteins, polysaccharides, as well as fats, which must first be converted into readily available and digestible compounds.
The latter is achieved as a result of digestion, which is understood as the process of enzymatic cleavage of macromolecular organic compounds into products lacking species specificity and suitable for absorption and assimilation.
There are three types of digestion: intracellular, membrane and extracellular .
Intracellular is the oldest type of digestion. In plants, it occurs not only in the cytoplasm, but also in vacuoles, plastids, protein bodies, and spherosomes.
Membrane digestion is carried out by enzymes localized in cell membranes, which ensures maximum conjugation of digestive and transport processes. It has been well studied in the intestines of a number of animals. In plants, membrane digestion has not been studied.
Extracellular digestion occurs when the hydrolytic enzymes formed in special cells are secreted into the external environment and act outside the cells. This type of digestion is characteristic of insectivorous plants; it is carried out in other cases, in particular in the endosperm of cereal grains.
Among plants, the saprophytic diet is rather common in algae. For example, diatoms living at great depths, where the light does not reach, feed on absorbing organic matter from the environment. With a large number of soluble organic substances in water bodies, chlorococcal, euglena and some other algae easily pass to the heterotrophic method of nutrition.
In angiosperms, saprophytic nutrition is relatively rare. Such plants do not have or have little chlorophyll and are not capable of photosynthesis, although photosynthetic species are also found. To build their body, they use rotting remains of plants and animals.
As an example, Gidiophytum formicarum is a shrub whose stem forms a large tuber pierced by the numerous passages in which the ants settle. This species uses ant products for food, which has been proven using a radioactive label. Labeled fly larvae, which ants brought into the stem cavity, were digested by the plant a month later, and radioactivity was detected in the leaves and underground parts of the plant.
Some species that do not contain chlorophyll use symbiosis with mushrooms to provide themselves with organic food; these are mycotrophic plants . Especially many of these species in the orchid family. In the early stages of development, all orchids enter symbiosis with mushrooms, since the supply of nutrients in their seeds is not enough for the growth of the embryo. Mushroom hyphae that penetrate the seeds supply the growing germ with organic matter as well as mineral salts from humus. In adult orchids with mycotrophic type of nutrition, fungal hyphae are introduced into the peripheral root zone, but cannot penetrate further. Their further growth is hindered by the fungistatic action of deep root tissue cells, as well as a layer of rather large cells with large nuclei similar to phagocytes. These cells are capable of digesting fungal hyphae and assimilating released organic matter. Perhaps a direct exchange between the plant and the fungus through the outer membrane of the hyphae is also possible.
By tradition, chlorine-free plants such as Monotropa, also referred to as saprophytes. However, in this case, the saprophytic method of nutrition is not carried out directly, but in symbiosis with fungi in the form of mycorrhiza . Moreover, in many cases, these symbiotic relationships can be considered as a form of parasitism, when plant cells digest fungal hyphae that penetrate the root cells. Thus, the fungus itself is a saprotroph, and a higher plant parasitizes on it. Fungus hyphae can connect the root of the epoch with the roots of the tree, and then the epoch becomes a parasite that receives organic matter from another plant.
Most plants use mycorrhiza mainly to increase the absorption of water and mineral salts.
On the example of the ecliptic and orchids, the method of feeding higher plants by parasitism was considered. Mycorrhizal fungus also acts as a parasite (a phenomenon of mutual parasitism). The fungal hyphae form outgrowths of haustoria , tightly adjacent to the root cells or penetrating into them. Gaustoria suck nutrients (primarily carbohydrates) from the plant.
Higher parasitic plants using ready-made organic substances are usually highly specialized annuals or perennials with leaves that are reduced or completely lost during evolution, and often roots. There are species completely devoid of chlorophyll and not capable of photosynthesis.
These include, for example, broomrape (Orobanche) . parasitic on the roots of many cultivated plants. Its seeds germinate only under the influence of the root secretions of the host plant. As soon as the tip of the germinal root of the seedling touches the root of the host, it is transformed into a haustorium (sucker), which begins to secrete hydrolases, which dissolve the cell walls, and actively penetrate the root. During the period of growth and development, broomrape absorbs a large amount of nitrogenous substances, carbohydrates and mineral elements, especially phosphorus, as well as water from the roots of host plants. In tomato plants infected with broomrape, the content of, for example, protein nitrogen decreases by 3 times, and sugars by 16 times. Another example of a root parasite is the Peter Cross (Lathraea squantaria) , a parasite on the roots of trees and shrubs.
In a winding parasitic herbaceous plant of Coduta (Cuseuta), filiform stems with reduced leaf-scales twist around the stems of the host plants and are attached to them using haustoria. Dodger haustoria are transformed adventitious (subordinate) roots. They take the form of a disk that fits snugly against the bark of the host plant. A group of cells from the central part of the disk is introduced into the cortical parenchyma of the host plant and reaches the central cylinder, from where the dodder receives water, organic matter, and mineral elements. Seedling seedlings, making growth rotational movements, find the host plant, reacting to the moisture gradient and the substances secreted by it (the phenomenon of chemotropism).
Rafflesia , which feeds on the juices of the roots of tropical vines, is also a parasitic plant. It is introduced into the victim’s body with the help of haustoria that secrete cellulase and other enzymes that destroy cell walls. Rafflesia spends its entire life in the host’s body - underground. Only its flowers appear on the surface of the soil. Using a radioactive label, it was shown that parasites absorb mainly sucrose, glutamic and aspartic acids and their amides from the host’s body.
At present, over 400 species of angiosperms are known that catch small insects and other organisms, digest their prey, and use the product of its decomposition as an additional source of nutrition. Most of them are found on marshy soils poor in nitrogen; there are epiphytic and aquatic forms.
The leaves of insectivorous plants are transformed into special traps. Along with photosynthesis, they serve to capture prey. By the method of catching it, insectivorous plants can be divided into two large groups.
With passive type l o in l and production can
a) stick to leaves whose glands secrete sticky mucus containing acidic polysaccharides ( biblis, rosolist ),
b) fall into special traps in the form of jugs, ballot boxes, tubules. painted in bright colors and emitting a sweet fragrant secret ( sarracenia, heliamphora, darlingtonia) .
For active capture of insects are used:
1) gluing the prey with sticky mucus and enveloping it with a sheet or hairs ( papilla, sundew ).
2) fishing according to the trap principle - with the slamming of trapping leaves over prey ( aldandanda, venus flytrap ),
3) trapping vesicles into which insects are drawn with water due to the vacuum maintained in them (pemphigus ).
Common to all types of hunting devices is the attraction of insects with the help of polysaccharide mucus or fragrant secretion (nectar), secreted either by the hunting devices themselves or glands near the trap. The rapid movements of trapping organs are usually carried out by changing turgor in them and are triggered by the propagating action potentials in response to irritation of sensitive hairs caused by insect movements.