PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

9. HETEROTROPHIC NUTRITION IN PLANTS

Autotrophs are capable of synthesizing Organic compounds from inorganic precursors. They are classified into photosynthetic organisms (most plants and some Bacteria) and chemosynthetic ones (bacteria).

Heterotrophs—including animals, Fungi, most bacteria, and some plants—derive their Nutrition from pre-existing organic substances.

Among plants, there are facultative and obligate heterotrophs, such as Saprophytes (saprotrophs), parasites, and carnivorous plants.

Furthermore, there are stages in a plant's life cycle when nutrition relies on stored organic reserves, i.e., heterotrophically: seed germination, The formation of vegetative propagation Organs, and The Development of flowers and buds in angiosperms. Many plant organs are entirely or partially heterotrophic (roots, flowers, buds, fruits, seeds), and, ultimately, all Plant Tissues and organs undergo heterotrophic nutrition during the night.

Thus, heterotrophic nutrition is as fundamental to plant tissues as Photosynthesis. However, this mode of nutrition remains under-researched. Studying it will provide insight into the mechanisms of cellular, tissue, and organ nutrition within the whole plant.

Plants can assimilate both low-molecular-weight and high-molecular-weight organic substances. In the latter case, we refer to Digestion—The process of enzymatic breakdown of macromolecules into non-species-specific products suitable for absorption.

There are Three types of digestion:

- intracellular (the most primitive form; in plants, it occurs in the Cytoplasm, vacuoles, Plastids, and spherosomes);

- membrane (not yet studied in plants);

- extracellular (where hydrolytic Enzymes are secreted externally; this type of digestion is characteristic of carnivorous plants, the endosperm of cereal grains, etc.).

Saprophytes (Saprotrophs)

Fungi are saprophytes, and although they belong to a separate kingdom, their physiology is closely related to that of plants.

The H+-pump Functions in the fungal Plasmalemma, and the fungus secretes various acidic Hydrolases into the surrounding environment (extracellular acid digestion). The absorption mechanism is also linked to The activity of the H⁺-pump. Sugars and Amino Acids enter the cytoplasm in symport with H⁺ ions via lipoprotein carriers. The energy source is the pH gradient and the electrical Membrane Potential.

Among plants, a saprophytic mode of nutrition is quite common in Algae: specifically, in diatoms living at great depths where light is insufficient, and in chlorococcalean and euglenoid algae (when organic matter is abundant in Water bodies).

According to V.Yu. Semenenko, The ability to grow on organic media in the dark or in the light in the absence of CO₂ is characteristic of most species of blue-green, green, yellow-green, and other algal groups. Under certain conditions, these representatives transition from phototrophic nutrition to the assimilation of various organic compounds, engaging in heterotrophic or photoheterotrophic nutrition, often combining both.

It should be noted that the transition to photoheterotrophic or heterotrophic nutrition is accompanied by the induction of additional enzyme systems. Such organisms directly utilize organic sources of carbon and nitrogen. The plasticity of nutritional strategies in lower plants has allowed them to occupy broad ranges and diverse ecological niches. Research into algal nutrition has enabled the cultivation of many species in various water bodies and their Introduction into industrial culture.

Among angiosperms, this mode of nutrition is rare. These are typically achlorophyllous plants that utilize decaying PLANT AND ANIMAL remains. For example, various species of Myrmecodia, specifically Hydnophytum formicarum, are subshrubs whose stems form large tubers with numerous passages inhabited by ants. This plant utilizes the waste products and food stores of the ants for its nutrition.

Epiphytes—plants that grow on trees or shrubs—are also considered saprophytes. They are adapted to extract nutrients and water from their environment, often utilizing organic matter from decaying animal and plant remains.

Some achlorophyllous species rely on Symbiosis with fungi to secure organic food; these are known as mycotrophic plants.

This is particularly common in the orchid family. In the early Selection/3.html">Stages of development, all orchids enter into symbiosis with fungi because their seeds lack sufficient nutrient reserves for embryo development. Fungal hyphae provide the embryo with organic substances and mineral salts from the humus. In mature orchids with Mycotrophic nutrition, fungal hyphae penetrate the peripheral zone of the roots. However, the fungistatic activity of the tissues and the action of "phagocyte" Cells limit their further spread.

The ghost plant (Monotropa) is also classified as a saprophyte. However, it also maintains a symbiosis with fungi in the form of mycorrhiza. The plant acts more as a parasite, digesting the fungal hyphae, while the fungus itself acts as the saprophyte. In most cases, however, plants use mycorrhiza primarily to increase the uptake of water and mineral salts.