PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

8. LONG-DISTANCE TRANSPORT OF SUBSTANCES IN PLANTS

Long-distance transport refers to the movement of substances between Organs within the whole plant.

Short-distance transport refers to the movement of ions, metabolites, and Water between Cells and Tissues (as opposed to membrane transport within individual cells).

Substances can be transported throughout the plant via various tissues or specialized vascular bundles.

The Movement of water and dissolved solutes can occur through:

a) vascular bundles;

b) Cell walls (the apoplast);

c) Cell Cytoplasm via plasmodesmata (the symplast);

d) potentially, the ER via plasmodesmata.

The movement of solutes via vascular bundles occurs through:

a) the xylem (upward flow);

b) the phloem (downward flow from leaves to consumption zones, or varying directions during the mobilization of stored reserves).

In multicellular Algae, metabolites move through the symplast (with the exception of Laminaria, which possesses sieve tubes). Symplastic and apoplastic water conduction is also characteristic of mosses. All higher terrestrial plants possess vascular bundles consisting of xylem tracheids and vessels, phloem sieve tubes and companion cells, and other specialized cell types.

Vascular bundles connect all plant tissues, enabling the Transport of substances over distances ranging from tens of centimeters to tens of meters (in woody plants). Transport through cell walls and cytoplasm occurs over short distances, measured in millimeters, such as radial transport in roots or stems, or the movement of substances within leaf mesophyll.

Xylem Transport

The Structure of the xylem and the mechanisms of xylem transport were partially covered in the chapter "Plant Water Relations."

Composition of Xylem Sap

Xylem sap is an aqueous solution of inorganic substances. However, various nitrogenous compounds (Amino Acids, amides, Alkaloids, etc.), organic acids, certain sugars, polyhydric alcohols, and phytohormones can be detected in the sap exuded from a cut stump.

The organic components of xylem sap vary depending on the plant species and The Nature of ions in the soil solution. The composition of xylem sap differs significantly from that of vacuolar sap. Its pH ≈ 5,9.

Class="center">MECHANISMS OF XYLEM TRANSPORT

Xylem loading is most intensive in the ROOT Hair zone. This process relies on several pumps that consume metabolic energy. The primary pump is located in The Plasma Membrane of rhizodermis and cortical parenchyma cells. It is driven by H⁺-pumps, which are H⁺-ATPases and, possibly, proton-transporting redox chains. Water and mineral salts move through endodermal cells with Casparian strips exclusively via the symplast. In these regions, cations and anions move from The Cell walls into the cytoplasm.

In the parenchyma Cells of the bundles (transfer cells), another pump may operate, transporting Mineral Substances through the pores of tracheids into their cavities. This increases the osmotic potential and suction force within the vessels. Water enters the tracheids and vessels According to the laws of osmosis, causing an increase in hydrostatic pressure, which drives the delivery of xylem sap to the aerial PARTS OF THE plant (the lower end-driver). All these processes require ATP energy. The portion of the upward water flow driven by Transpiration (the upper end-driver) occurs passively according to physicochemical laws and does not require metabolic energy.

Thus, the upward flow of xylem sap is driven by the upper end-driver (solar energy), the lower end-driver (ATP energy), and capillary phenomena. Water and dissolved substances move through xylem vessels at high velocities of ≈ 1-20 m/h.

Xylem unloading

The content of organic and inorganic substances in the xylem depends on the plant species and mineral Nutrition conditions.

As the ascending sap moves through the xylem, its Qualitative and quantitative composition changes. In maize and bean plants, the Na⁺ content can decrease by 2–10 times from the root hairs to the root collar. Sodium and potassium ions also exit the xylem sap within the stems.

In squash seedlings, the concentration of potassium, calcium, phosphorus, and nitrate ions in the xylem sap from leaf petioles is higher than in the sap from the stem base. The largest quantity of ions is absorbed by leaf cells. This is evidenced by the composition of guttation fluid, which is released from xylem endings through hydathodes.

Xylem unloading, i.e., the movement of water and ions out of the vessels, is driven by hydrostatic pressure within the vessels, transpiration forces, and the attracting action of surrounding cells. It also depends on the selective permeability of membranes and the Nutritional Requirements of the cells. Xylem sap enters the cytoplasm via cell walls and the Plasmalemma, where H⁺-pumps are active. Developing leaves require various inorganic substances; therefore, young leaves serve as powerful sink regions for xylem sap. Most of the water evaporates, which can lead to the supersaturation of the chlorenchyma with salts. Cells employ at least three mechanisms to eliminate excess ions:

- formation of poorly soluble salts (in cell walls or vacuoles);

- efflux of salts via the phloem;

- excretion of salts through salt glands and trichomes.

Regulation of xylem transport is achieved by modulating The activity of H⁺-pumps and other ion pumps, which is linked to respiratory energetics and the supply of assimilates and oxygen to The Root System. Another powerful regulator of xylem transport is transpiration, the intensity of which depends on the state of the stomatal apparatus—regulated by the plant—and environmental conditions, which the plant largely cannot influence.