PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

9. HETEROTROPHIC NUTRITION IN PLANTS

Parasites

The parasitic mode of Nutrition in higher plants can be illustrated by the Examples of the ghost plant (Monotropa) and various orchids. In these cases, the mycorrhizal fungus also acts as a parasite, extracting nutrients through specialized outgrowths known as haustoria. The mechanism underlying this process is also driven by an H⁺-pump.

Higher parasitic plants are typically highly specialized species with leaves—and often roots—that have been reduced or entirely lost throughout the course of evolution. Some have lost chlorophyll altogether.

For instance, broomrape (Orobanche), which comprises about 200 species, parasitizes the roots of many crops, including sunflowers, tobacco, tomatoes, cabbage, and potatoes. These are perennial ROOT parasites; their seeds are quite small, easily penetrate the soil, and germinate only in the presence of root exudates from a host plant.

A broomrape seed produces a slender seedling that performs helical movements as it penetrates the soil. Upon contact with a host root, the broomrape radicle transforms into a haustorium, which secretes Hydrolases that dissolve Cell walls, allowing it to penetrate deep into the host's xylem. After some time, the broomrape root develops a bud, from which a thick, fleshy stem with small, chlorophyll-free leaves emerges. The parasite fuses with the host root so thoroughly that it is difficult to distinguish the boundary between them. During its growth phase, the broomrape absorbs large quantities of organic and Mineral Substances, as well as Water, from the host. In tomato plants infested with broomrape, the protein nitrogen content decreases threefold, while sugar levels drop by a factor of 16.

Another example of a root parasite is the common toothwort (Lathraea squamaria), which parasitizes the roots of trees and shrubs. Its rhizome resembles a cross and attaches to the host root via suckers. In April or May, the parasite's flowering shoots, characterized by pink scales instead of leaves, appear on the soil surface. After cross-pollination and seed set, the aerial shoots die off.

In dodder (Cuscuta), thread-like stems with reduced scale-leaves twine around the stems of host plants and attach themselves using haustoria (modified adventitious roots acting as suckers). From these disc-shaped haustoria, a group of Cells from the central part penetrates the host's cortical parenchyma and reaches the vascular cylinder, from which it draws all necessary nutrients. This plant causes significant damage to agricultural crops, particularly clover and flax.

In late spring, a thin yellow thread grows from the dodder seed, performing circular growth movements to locate a host plant through chemotropism. Upon contact, the seedling twines around the host, and at the points of contact, wart-like structures form, whose suckers grow into the victim's stem. Dodder lacks a root system, which is replaced by these suckers—modified adventitious roots through which it obtains essential organic matter. After a certain period, the dodder loses its connection to the soil and Functions entirely at the expense of the host plant. It contaminates soil and seed stocks and is considered a noxious quarantine weed.

The parasitic plant Rafflesia feeds on the sap of tropical vine roots, which it penetrates using haustoria and hydrolytic Enzymes. Rafflesia spends its entire life underground, with only its flowers emerging On the surface.

Mistletoe (Viscum L.), which is widespread in Ukraine, belongs to the category of hemiparasites. These are predominantly evergreen plants with highly branched stems. Instead of roots, they form suckers containing Vascular Tissues that connect to the host plant's transport system. While mistletoe species in temperate zones cause only minor damage to their hosts, in the tropics, they can deplete the host's sap so intensely that the plant eventually dies.