PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

4. WATER RELATIONS

Mechanisms of Water Transport in Plants

Terrestrial plants are capable of maintaining a continuous upward flow of Water. This process is supported by a series of mechanisms that ensure plant water exchange. Plant water relations consist of three stages:

1) water absorption by the roots;

2) transport through The Vascular System;

3) Transpiration (water evaporation from leaves).

Class="center">The ROOT System AS A WATER-ABSORBING ORGAN

Soil Water Status

Plants absorb water from the soil via the root system. Soil is a multiphase medium consisting of four primary components: solid mineral matter, humus, soil solution, and soil air. A portion of soil water is unavailable to plants and is referred to as bound water. This includes chemically bound water, water in fine capillaries, and water bound by silicates, clay minerals, and humus (colloids). Forces acting at the interfaces of different phases retain water in the soil. Various terms are used to describe soil moisture availability. Field capacity refers to the maximum soil saturation level at which downward water movement ceases. The permanent wilting point serves as an indicator of the minimum moisture level at which plants remain wilted until additional water is supplied to the soil.

Available soil moisture is defined as The amount of water stored in the soil between the permanent wilting point and field capacity.

Root Structure

All morphological and Anatomical Features of the root are adapted to the necessity of absorbing water and mineral nutrients from the soil.

The growing region of the root typically does not exceed 1 cm in length and consists of the meristem (1-2 mm), protected by the root cap, and the zone of elongation.

The primary Functions of the root cap are:

a) Protection of the meristem;

b) facilitation of soil penetration through the secretion of polysaccharide mucilage by secretory Cells;

c) perception of geotropic stimuli by statocyte cells (containing statoliths) to ensure proper root orientation in space.

Meristematic cells undergo continuous division—each Cell dividing 6-7 times—followed immediately by differentiation. After division ceases, root cells enter the zone of elongation, where phloem differentiation is completed and protoxylem elements are formed. Tissue differentiation in the root concludes in the root Hair zone. At this stage, The formation of the primary root Tissues is finalized: the rhizodermis, primary cortex, endodermis, and the central cylinder tissue system.

The rhizodermis is a single-layered tissue covering the root surface. In some plant species, every rhizodermal cell has the potential to form root hairs; in others, it consists of two cell types: trichoblasts, which produce root hairs, and atrichoblasts. The primary function of the rhizodermis is the absorption of water and mineral nutrients. With age, the rhizodermis is replaced by the exodermis and periderm (cork tissue). In some plants, it persists while undergoing structural changes. In herbaceous plants, the root cortex typically consists of several layers of living parenchyma cells situated between the rhizodermis and the endodermis. The endodermis borders the central cylinder. Its Cell walls contain Lignin and suberin, making them impermeable to water. Through this layer, water moves exclusively via the symplast.

The central cylinder comprises a complex of tissues: the pericycle, phloem, and xylem. Pericycle cells form the outermost single-layered Sheath of the central cylinder, located just beneath the endodermis. They regulate the Transport of substances to the xylem and phloem. Furthermore, they function as meristematic tissue, producing lateral roots, and in dicots, they contribute to the Formation of the cambium and ray parenchyma.

In the root hair zone, the xylem is represented by metaxylem. In gymnosperms, transport is performed by tracheids—long (0.1-12 cm) tapered cells connected by bordered pits.

In angiosperms, vessels—hollow tubes formed from cell walls (0.1-2 m)—predominate. Xylem vessels communicate with each other and with parenchyma cells via bordered pits. There are no plasmodesmata between them. Water and dissolved substances diffuse from parenchyma cells into the vessel lumen through the primary Cell wall.

Root Growth and Motility

To perform its functions, the root possesses The ability to orient itself in space and respond to gradients of vital environmental factors, while also maximizing its contact surface area with the soil.

During germination, the root orients itself within the gravitational field (Ch. Darwin, N. Kholodny). This stimulus is perceived by statoliths. The growing distal end of the root is highly sensitive to mechanical pressure and penetrates only into loose soil areas. Variations in the content and localization of ABA lead to targeted root curvature and differential growth rates. The movement of the root tip is facilitated by the root cap, which periodically undergoes mucilage secretion and sloughing.

During dry periods, the root begins to respond primarily to the moisture gradient (J. Sachs), and only subsequently to the gravitational field.

In conditions of water scarcity, the absorptive surface area increases dramatically due to the intensive growth of root hairs. The root hair zone shifts through the soil following the distal zone.

Water uptake by the root and its radial transport

Water is absorbed most intensively in the elongation and root hair zones. The primary function of root hairs is to increase the absorptive surface area. A four-month-old rye plant has about 14 million rootlets (S_surface = 230 m) with 14 billion root hairs (S_surface = 400 m²). The absorptive surface area of the root system is tens of times larger than the area of the above-ground parts where transpiration occurs.

The gradient of osmotic pressures between the soil solution and the vacuolar sap of root cells determines the direction of water flow from the soil into the plant. Water always moves in the direction of a more negative water potential, i.e., from a system with higher energy to one with lower energy. Roots absorb water across their entire surface; however, above the root hair zone, The rate of absorption decreases sharply due to the suberization of cells.

Upon being absorbed by the rhizodermis (epidermis), water passes through the cortical parenchyma cells via two pathways (symplastic and apoplastic) in accordance with the laws of osmosis (following The water potential gradient). Transport through cell walls occurs significantly faster.

Subsequently, at the level of the endodermis, the Presence of water-impermeable Casparian strips causes rapid apoplastic transport to shift to slower symplastic transport. This does not occur in growth zones where Casparian strips have not yet formed, or in areas where lateral roots are initiated, where the endodermis is interrupted. Further along the path to the xylem vessels through the pericycle, water movement encounters minimal resistance and likely proceeds via the apoplastic pathway.

Thus, the root system is capable of actively moving water within root cells in a specific direction—through root hairs, cortical parenchyma cells, the endodermis, and the pericycle to the xylem Vessels of the root's central cylinder.

ROOT PRESSURE

Mechanisms of root pressure

Water moves from the soil into rhizodermal cells As a result of the difference in osmotic pressure between root Hair cells and the soil solution. Water moves passively toward a higher concentration of osmotically active substances (salts, sugars, etc.), i.e., toward a lower water potential. For water to move into the rhizodermal cells rather than the reverse, the concentration of osmotically active substances (and thus the osmotic pressure) in the root hair Cells must be higher than that of the soil solution. Otherwise, water would flow out of the root cells into the soil.

There are several mechanisms for maintaining the osmotic pressure and suction force of root cells at an appropriate level. One of these is The activity of ion pumps in the Plasmalemma of root hairs. These pumps utilize ATP energy to transport ions from the soil solution across the membrane into the Cytoplasm. In this way, a higher salt concentration is created within the cells than in the soil solution.

High osmotic pressure and suction force in root cells can also be maintained by the influx of sugars, organic acids, and other solutes.

Water also enters the xylem vessels due to osmotic phenomena. This has been experimentally confirmed. The osmotically active substances in the vessels consist of mineral compounds and metabolites secreted by active ion pumps in the plasmalemma of the parenchyma cells surrounding the vessels. This creates a suction force greater than that in the surrounding cells, facilitating water transport to the xylem. Furthermore, there is no counter-pressure in the xylem cell walls because they are lignified and inelastic.

Two pumps can function simultaneously in the root: the collective ion pumps of the root hair plasmalemma and the collective ion pumps of the xylem parenchyma plasmalemma.

Thus, as a result of the active work of ion pumps in the root and the osmotic influx of water into the xylem vessels, a hydrostatic pressure is generated, known as root pressure. It ensures the upward movement of the xylem solution through the vessels from the root to the above-ground parts. This entire mechanism is called the lower end-motor. Its operation requires energy, which is supplied by products of Photosynthesis (CARBOHYDRATES). ATP is produced through their oxidation during Respiration. ATP energy is consumed to create a specific concentration of osmotically active substances, which must be higher than the concentration in the soil solution and must increase from the root hair cells toward the xylem vessels.

Plant bleeding. Guttation. An example of the lower end-motor in action is plant bleeding. In early spring, one can observe an intensive flow of liquid from bottom to top through damaged trunks or branches. During this period, root pressure at the Base of the trunk can reach 10 atm. When a trunk is cut, a prolonged release of xylem sap occurs. By attaching a manometer to the stump, one can measure the root pressure.

Another example of the lower end-motor is guttation. As a result of its activity, sap is forced into the shoots, and under conditions of high air humidity, droplets of moisture are released at the tips of the leaves. This phenomenon is particularly characteristic of tropical plants.

TRANSPIRATION AND THE UPPER END-MOTOR

Transpiration is the physiological process of water evaporation by plants. The primary organ of transpiration is the leaf. Transpiration can be cuticular (through the cuticle), stomatal (through the Stomata), or lenticular (through the lenticels).

Plants possess a large leaf surface area, which facilitates CO₂ absorption, light capture, and evaporation. Water evaporates through the leaf surface and through the stomata. As a result of water loss by cells, their water potential decreases, meaning their suction force increases. This leads to an increased uptake of water by leaf cells from the xylem Veins and the Movement of water through the xylem from the roots to the leaves.

This is how the upper end-motor (UEM) is formed. The force of the UEM is greater when transpiration is more active, thereby increasing the suction force of the parenchyma cells. This ensures the upward movement of water through the plant. The UEM can function even if the lower end-motor is completely removed. Its operation does not rely on metabolic energy, but rather on environmental energy—Temperature and air movement.

The Leaf as an organ of transpiration

Water travels from the stem to the leaf through the petiole and into the veins. As these veins branch out, the number of conducting elements decreases, and the finest veinlets consist of individual tracheids, forming a dense network. In C₄ plants, a bundle sheath is present. Tracheids terminate between mesophyll cells. Externally, a single-layered epidermis covered by a cuticle (sometimes accompanied by hairs and scales) creates a barrier to water movement.

The internal surface area of a leaf is typically an order of magnitude greater than its external surface area. Water within the intercellular spaces evaporates from all exposed areas of the mesophyll.

Transpiration consists of two processes:

1) the movement of water through the veins to The Cell walls of the mesophyll;

2) the evaporation of water into the intercellular spaces, followed by diffusion through the stomata, or evaporation directly from the cell walls into the atmosphere via cuticular transpiration. Water moves toward the evaporative surfaces along the cell walls with less resistance than through the symplast. Water molecules exit the plant by moving—as they do within the plant—toward a region of lower water potential (which decreases as relative humidity decreases).

Types of transpiration

Stomatal transpiration. Stomata are the primary pathway for water vapor, CO₂, and O₂. Their number and distribution vary among different plants. On average, the number of stomata ranges from 50 to 500 per 1 mm². Transpiration through stomata occurs at a rate nearly equal to that of a free water surface (according to Stefan's law, the rate of transpiration depends on the diameter of the pores).

Cuticular transpiration. The intensity of cuticular transpiration varies significantly between species, ranging from negligible losses to up to 50%. Conifers and magnolias possess a thick cuticle and lose very little water through the epidermis. In young leaves with thin cuticles, cuticular transpiration accounts for half of the total transpiration, whereas in mature leaves, it represents only 1/10 of the total.

A portion of water is lost through buds and reproductive Organs. Occasionally, these losses can be substantial (e.g., in sunflower heads, poppy capsules, or pepper fruits). As a result of branch transpiration during winter, water deficits often occur, leading to plant death from dehydration.

Regulation of stomatal transpiration. Stomatal opening is regulated by several interacting mechanisms. The driving force behind changes in pore width is The change in the turgor pressure of the guard cells. As a guard cell osmotically absorbs water, it curves outward, causing the pore to widen. Both external and internal environmental factors influence this process. Among external factors, air humidity, water availability, light, and temperature have the greatest impact on stomatal movement. Internal factors include the partial pressure of CO₂ in the intercellular system, the plant's Hydration status, ion balance, and phytohormones, where cytokinin promotes stomatal opening and Abscisic acid (ABA) promotes closure. Stomatal status is also influenced by leaf age, the plant's developmental stage, and endogenous circadian rhythms.

The degree of cellular water supply has a profound effect on stomatal movement. We distinguish between hydroactive and hydropassive stomatal responses. Hydropassive closure is linked to the compressive action of neighboring epidermal cells under conditions of full turgor. Hydropassive opening occurs when this compression is reduced.

Hydroactive closure of stomata occurs as soon as transpiration exceeds water uptake by the roots and the turgor in the guard cells reaches a critical low. This reaction is triggered by an increase in ABA concentration within the leaf tissues. ABA inhibits the activity of H⁺-pumps on The Plasma Membrane of guard cells, resulting in a drop in turgor and subsequent stomatal closure.

Unlike other epidermal cells, guard cells contain chlorophyll. Given adequate water supply in the light, stomata open wider as light intensity increases (the active factor being blue light). Photosynthesis also influences stomatal movement. During intense carbohydrate production, the suction force of the cells increases, causing the stomata to open. The reversible conversion of starch into sugar is also essential for changes in suction force and turgor. Morning stomatal opening is partially triggered by light (photoactive opening).

Stomatal status also depends on CO₂. If the CO₂ concentration in the substomatal cavity falls below 0.03%, the turgor of the guard cells increases, and the stomata open. This is partly linked to morning stomatal opening (as CO₂ levels decrease due to photosynthesis). At night, CO₂ concentration in the intercellular spaces rises due to respiration in the absence of photosynthesis, causing the stomata to close.

In succulents with a specific circadian rhythm of organic acid METABOLISM, stomata open at night when the partial pressure of CO₂ in the intercellular spaces decreases due to intensive malate formation, and they close during the day when CO₂ is released during malate decarboxylation. Thus, a multitude of factors regulate stomatal activity through direct and feedback loops. The simultaneous action of these loops results in minor oscillations in stomatal aperture.

Diurnal fluctuations in transpiration

In trees, shade-tolerant plants, many grasses, and other species with sophisticated regulation of stomatal transpiration (homoiohydric plants), water evaporation reaches its maximum before the peak daily temperature is reached. Transpiration then declines and may increase again in the late afternoon. This pattern of transpiration causes minor diurnal changes in osmotic pressure and leaf water content. In species capable of tolerating sharp changes in cellular water content throughout the day (hydrolabile species), the diurnal transpiration pattern shows a single peak at midday. In both cases, transpiration is minimal at night.

Transpiration drives the movement of vast quantities of water through the plant and holds adaptive significance, closely linked not only to water exchange but also to other metabolic processes, including photosynthesis, respiration, and mineral Nutrition. Therefore, when investigating the water regime of various plants, it is crucial to study transpiration metrics such as intensity, productivity, etc.

Transpiration intensity is a value representing the amount of water in grams evaporated by a plant per unit of S per hour (during the day: 15-250 g/m per hour; at night: 1-20 g/m per hour).

Transpiration productivity is a value defining the amount of dry matter produced per 1000 g of water lost (≈ 1-8 g per 1000 g of water).

Transpiration coefficient is the amount of water in grams expended to produce one gram of dry matter (≈ 120-150 g per 1 g of dry matter).

From this, we can conclude that only 0.2% of H₂O is utilized for synthesis; the remainder is lost to transpiration.

Water movement through THE VASCULAR SYSTEM

The upward flow of water occurs primarily through the xylem—a distribution system that supplies all Plant Tissues and organs with water. As mentioned, the xylem is formed from procambial Cells of the root and stem; initially, it contains cytoplasm, but in its mature state, it consists only of lignified cell walls. 1-10% of the upward water flow travels along the cell walls of living cells and is also maintained by the water potential gradient created by transpiration. Simultaneously with the upward movement of water, there is an exchange of water molecules with all stem cells. Several theories explain The Mechanism of the upward water flow.

Cohesion theory

The driving force behind the upward movement of water through xylem conducting elements is the water potential gradient across the plant, extending from the soil to the atmosphere. This gradient is maintained by:

1) the osmotic potential gradient within root cells (from the soil to the xylem vessels), resulting from The Active Transport of ions by living cells, and

2) transpiration.

Maintaining the first gradient requires metabolic Energy Expenditure, while transpiration utilizes the energy of solar radiation.

The former ensures water uptake by the roots; the latter serves as the primary driving force for the upward water flow. This force generates a significant negative hydrostatic pressure gradient within the xylem, which manifests as tension within the water columns in the vessels.

According to the cohesion-tension theory (19th century), water rises through the capillary tubes of xylem vessels in response to the suction force of transpiration, driven by the cohesive forces (cohesion) between water molecules and the adhesive forces (adhesion) of the water Column to the hydrophilic vessel walls. These forces also prevent the formation of air-filled cavities that could block the vessels. In the event of a blockage (embolism), there is always a sufficient number of intact water columns in adjacent vessels. Furthermore, mechanisms exist to restore the continuity of these water columns. It has been demonstrated that the tensile strength of water is 30 MPa, which is sufficient to support water columns reaching heights of 120-130 meters.

The absolute velocity of water movement through the xylem is relatively low: 20 cm³/h per 1 cm² for deciduous species, and 5 cm³/h per 1 cm² for conifers (compared to 40-50 cm³/s for Blood FLOW IN Arteries). However, this velocity is sufficient to minimize resistance to water transport.

Water exchange between xylem and phloem in plants

The upward flow supplies water and mineral nutrients to every cell in the aerial PARTS OF THE plant. While the majority of this water evaporates, a small fraction (~ 0.2 %) is utilized for:

1) metabolic reactions;

2) maintaining cell turgor;

3) The transport of Organic compounds via the phloem.

The downward phloem flow of solutes originates in the leaf mesophyll cells, where substances move from the cell walls into the phloem terminals. Water moves from leaf cells and xylem vessels into the phloem along an osmotic gradient created by the accumulation of sugars and other photosynthetic products within the phloem cells. This downward phloem flow delivers organic nutrients to the root cells. Here, the phloem bundle terminals lie adjacent to xylem elements, and water moves back into the xylem along an osmotic gradient. This process facilitates water exchange within the vascular system.

Significance of the upward flow for plants

1. The upward flow from the root system to the aerial parts serves as a mechanism for transporting and accumulating mineral nutrients and chemical compounds produced in the roots within the aerial organs.

2. The upward flow is dependent on transpiration, which is intrinsically linked to the uptake of carbon dioxide required for photosynthesis. To obtain CO₂, the plant must release water; conversely, reducing H₂O loss (by closing stomata) restricts CO₂ intake. In agriculture and forestry, understanding the balance between photosynthetic productivity and water loss is crucial for optimizing crop yields through effective regulation.

3. The upward flow is essential for maintaining adequate water supply to all cells and sustaining turgor pressure. Water deficiency leads to various physiological disruptions in cells. Consequently, developing irrigation systems is vital for achieving high yields in arid regions.

4. Transpiration acts as a mechanism to protect plants from overheating.