Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

3. Water Exchange in Plants. Diffusion, Osmosis, Chemical and Water Potentials, Osmotic Pressure
3.5 Mechanisms of Water Transport in the Plant. Lower Terminal Engine. Water Absorption by the Root

Terrestrial plants primarily absorb Water from the soil. However, some water can also be taken up by leaves from the atmosphere. There are even plants for which the atmosphere serves as the main source of moisture. These are epiphytes—plants that grow On the surface of other vegetation without being parasitic. They possess aerial roots with hollow, thin-walled Cells that absorb water vapor and precipitation much like a sponge. In some epiphytes, rainwater is collected by the leaves and subsequently absorbed through specialized trichomes (leaf hairs).

The ROOT system is the primary organ for water absorption from the soil. A developed root system is a complex Structure with a highly differentiated architecture. Research shows that the total surface area of The Root System can exceed that of the above-ground Organs by approximately 150 times. Root growth and branching continue throughout the entire lifespan of the plant.

The uptake of water and nutrients is mediated by the root hairs of the rhizodermis. The rhizodermis is a uniseriate (single-layered) tissue covering the outer surface of the root. In some plant species, every epidermal Cell produces a root Hair, whereas in others, the tissue consists of two distinct cell types: trichoblasts, which form root hairs, and atrichoblasts, which are incapable of hair formation.

From the rhizodermis, water enters the cortical cells. In herbaceous plants, the root cortex typically comprises several layers of living parenchymal cells. Large intercellular spaces between the cells facilitate root aeration. Water and mineral salt solutions can travel across the cortical cells via two distinct pathways: the symplast and the apoplast. Water transport is faster via the apoplast, as water in the Cytoplasm is continuously consumed to meet cellular metabolic demands.

Subsequently, water reaches the Cells of the endodermis. The endodermis is the innermost layer of the cortex, bordering the central cylinder. Their cell walls are rendered impermeable to water due to the deposition of suberin and Lignin (forming Casparian strips). Consequently, water and mineral ions must cross the endodermal cells via the symplastic pathway, which slows down water transport through the endodermis (Fig. 3.2). This regulation is essential because the diameter of the stele (central cylinder), which receives the water from the endodermis, is smaller than the absorbing surface of the root.

The central cylinder of the root contains the pericycle along with two vascular tissue systems: the xylem and the phloem. The pericycle consists of a single- or multi-layered sheath surrounding the vascular elements. Its cells regulate the Transport of substances both from the outer layers into the xylem and from the phloem into the cortex. Furthermore, the pericycle Functions as a meristematic tissue capable of initiating lateral roots. Parenchymal cells of the pericycle actively transport ions into the conducting elements of the xylem. This contact occurs via pits in the secondary cell walls of the vessels and cells, with no plasmodesmata present between them. Water and dissolved solutes then diffuse into the vessel lumen through the primary Cell wall.

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Figure 3.2 - Diagram of radial water and ion transport pathways through the root to the xylem vessels (after J. Moorby, 1981; cited in V. V. Polovyi)

Certain parenchymal cells of the vascular bundle feature characteristic wall ingrowths—transfer cell walls—lined with the Plasmalemma (Plasma Membrane), which significantly increases its surface area. These cells actively participate in The transport of substances into and out of the vessels and are referred to as transfer cells. They may border both xylem vessels and phloem sieve tubes simultaneously. Organic compounds are transported from the above-ground PARTS OF THE plant to the roots via the phloem vessels.

Water passively diffuses into the xylem vessels via an osmotic mechanism. The osmotically active substances within the vessels include mineral ions and metabolites secreted by The Plasma Membrane pumps of the surrounding parenchymal cells. The suction pressure of the vessels is higher than that of the adjacent cells due to the increasing concentration of the xylem sap and the absence of significant

counter-pressure from the relatively inelastic cell walls. As water enters the xylem vessels, a hydrostatic pressure known as root pressure develops. This pressure contributes to driving the xylem sap upward through the xylem vessels from the roots to the aerial Organs of the plant. The upward Movement of water driven by this generated root pressure is referred to as the lower terminal driver.

A classic example of the lower terminal driver in action is bleeding in plants—the exudation of sap (xylem fluid) from a cut or damaged stem driven by root pressure. In woody plants, this occurs during spring sap flow (e.g., birch sap exudation), whereas in herbaceous plants, it can be observed throughout the entire vegetative period. Plant bleeding typically peaks at midday and reaches its minimum in the pre-dawn hours, lasting anywhere from a few days to several months. The intensity of plant bleeding serves as an indicator of root physiological activity. In early spring, intense upward fluid flow can be observed through damaged trunks or branches, during which root pressure at the Base of the trunk may reach up to 10 atm.

Another manifestation of the lower terminal driver is guttation. Guttation (from Latin gutta - drop) is the exudation of excess water droplets by plant leaves through specialized structures called hydathodes. Guttation can be observed in plants under high air humidity when Transpiration is impeded, while water uptake by the root system remains active. In our climate zone, this phenomenon is commonly seen in the morning and before rain. Guttation is particularly characteristic of tropical plants growing in high-humidity environments—under such trees in a rainforest, it practically feels like a constant drizzle. Plants that exhibit active guttation include fuchsias, potatoes, primroses, lady's mantles, strawberries, ground-elders, and common houseplants such as Arum, Philodendron, and Monstera. In aquatic plants, guttation occurs continuously and represents the sole pathway for water excretion. The Biological Significance of guttation likely lies in ridding the plant of excess water and salts.