Agriculture for Engineers 4 (3+1)

Lesson 24. Role of Water in Plant and Its Absorption

24.2 ROLE OF WATER IN PLANT DEVELOPMENT

Water plays a vital role in plant life. It is essential to plants in the following ways:

1. Water is a structural constituent of plant cells and it maintains the cell from through turgor pressure. When plenty of water is available, cells are turgid and plants retain their normal structural from. Water accounts for the larges part of the body weight of an activity growing plant and it constitutes 85 to 90 per cent of the body weight of young plants and 20 to 50 per cent of older or mature plants.

2. Water is a source of two essential elements, oxygen and hydrogen required for synthesis of carbohydrate during photosynthesis.

3. Water serves as a solvent of substances and a medium in plants allowing metabolic reactions to occur.

4. Water acts as a solvent of plant nutrients and helps in uptake of nutrients from soils. Further, plants also absorb nutrients through leaves from nutrient sprays. These nutrients are carried in soluble form to different parts of the plant for use.

5. Food manufactured in green parts is distributed to various parts of the plant in soluble form and water acts as a carrier of food materials.

6. Transpiration is a vital process in plant and it occurs at a potential rate as long as water is available in adequate amount. If there occurs soil water deficit, transpiration process is curtailed down seriously affecting plant growth and yield.

7. Adequate supply of water maintains the turgor pressure of guard cells helping stomata to open fully. Water deficit, on the other hand, closes stomata partially or completely reducing water loss through transpiration.

8. Cells and tissues are formed and growth of plant occurs when an adequate amount of soil water is available. Water deficit shows down the growth processes.

9. Leaves get heated up with solar radiation. Plants dissipate heat by increased transpiration. Water acts as a buffer against high or low temperature injury as it has high heat of vaporization and high specific heat.

10. Water encourages good growth, development and yield of plants and quality of plant produce when it is available in plenty. Conversely, plants die when water supply is curtailed down.

24.3 WATER ABSORPTION BY PLANTS

Plants absorb water from soil, rain and from water sprays. Various parts of the plant are involved in water absorption processes. The details on water absorption processes involve have explained as below:

24.3.1 Water Absorbing Plant Structures

Absorption of water occurs mainly through roots. An insignificant amount of water is, however, absorbed through aerial structures. Young roots offer largely the water absorbing surface in actively growing annual plants, while they offer relatively a small fraction of the total absorbing surface in old perennial plants and trees.

A young growing root tip consists of a root cap, a zone of maximum meristematic activity, a region of rapid cell elongation and a region of quick cell differentiation and maturation. A rapid absorption of water occurs through younger part of the root immediately basal to the meristematic region. It is usually the area where root hairs grow extensively. Root hairs are thin walled protuberances of the epidermal cells. They present relatively large absorbing surface. The xylem elements develop to conduct water up the plant system. Suberization of cell walls also begins, but the same is not well advanced. Suberization of cell walls reduces the permeability to water. But a considerable volume of water is absorbed, though slowly, through suberized roots in order plants. The role of such roots in water absorption is very important as they comprise the largest portion of a root system in older plants and tress and offer relatively large water absorbing surface.

A typical young root has three distinct regions transversely viz., epidermis, cortex and stele or vascular cylinder. The epidermis consists of closely of closely packed thin-walled elongated cells. When young, these cells develop root hairs. Water and nutrients absorbed by root pass through this layer of cells having no intercellular spaces. Maturation of roots leads no suberization or cutinization of these cells. The cortex consists of a layer of cells in the outermost region called, exoderm, a layer of cells in the innermost region called, endodermis and parenchyma cells between these two layers. The cortical cells have intercellular spaces. As the root matures, the exodermal cells may become suberized and the endodermal cells develop a casparian strip. This is a strip of thickened and suberized or cutinized cell walls. The innermost region of the root is the stele or vascular cylinder that has pericycle, the outermost tissue of this region. Vascular system consists of phloem strands and the xylem. Phloem tissues consist of sieve tubes and companion cells that serve as the conducting system of organic substances manufactured in leaves do the root meristematic region. The xylem has tracheid and vessels for conducting water and mineral solutes upwards to the various parts of plants from the absorbing regions of roots.

24.3.2 Radial Movement of Water through Root

In the absorption process, water travels from the soil medium into epidermal cells. It then moves from cell to cell through cell walls and protoplasm and through intercellular spaces to the xylem. The xylem system constitutes the conducting system of water from roots to leaves. So the path is from epidermal cells of the root through the cortex to the xylem of vascular bundles.

24.3.3 Root Growth and Water Absorption

The volume of water absorbed by a plant depends largely on the growth of root system. Movements of water in soils are too slow to make sufficient water reach the roots. It is the root system that grows and meets water in new region of soil. Upper soil layers dry up quickly owing to continuous evapotranspiration, while lower soil layers maintain a better water balance. Plants with deep root systems are therefore capable of drawing greater quantity of water for their survival and growth. Plants with expanded root system continue to grow with water available from the extended region of the root zone, while plants with restricted root system experience water stress in the same field under the same condition of limited water supply.

24.3.4 Energy Concept of Water Absorption

The energy status of water in plant cell and of the water supply medium in soils may explain the mechanism of water absorption by plants.

Pure water has zero water potential y., when solutes are present in water, the y decreases below zero. A cell has therefore negative water potential. When a cell is placed in pure water, water moves into the cell due to a gradient of decreasingy. This movement produces turgor pressure or pressure potential, yp inside the cell and reduces the osmotic potential, ys by diluting the concentration of cell sap. The turgor pressure acts against the forces responsible for movement of water into the cell and is considered positive. The cell water potential has several components that may be expressed as,

           ΨCell = Ψs + Ψm + Ψp………..         (24.1)

          Where,

           ΨCell = cell water potential

           Ψs = osmotic potential (influence of solutes like sugars, acids, inorganic ions etc.)

           Ψm = matric potential (effect of matric substances like proteins, polysaccharides etc.)

          Ψp = pressure potential (influence or turgor pressure)

Values of Ψs and ym are negative, while the value of Ψp is positive. The ycell is usually negative, unless the cell becomes fully turgid. The cell water potential becomes zero when the cell is fully turgid since the combined magnitude (negative) of osmotic potential and matric potential becomes equal to the magnitude (positive) of the pressure potential under this situation making the cell water potential zero according to Equation 24.1.

With entry of water into the cell owing to osmotic and matric potentials (Ψs + Ψm), the pressure potential (Ψp) increases as the volume of the cell increases. The elasticity of cell wall puts a limit to this increase in the cell volume. When the cell becomes fully turgid, yp becomes equal to the Ψs + Ψm and there is no more entry of water into the cell. The y of pure water and that of cell sap becomes equal in magnitude.

A cell inside the plant system is surrounded by other cells, while epidermal cells of root are surrounded by soil water outside and cortical cells inside. The movement of water from epidermal cells to the adjoining cells occurs exactly in the same manner when a cell is immersed in pure water as described earlier. The gradient of decreasing water potential from epidermal cells to xylem results in the radial movement of water in the root.

24.3.5 Water in Soil-Plant-Atmosphere System

When the soil-plant-atmosphere system is considered, differences in magnitude of the water potential at different points in the system create the driving force of atmosphere (Ψair) decides the flow of water from soil to the atmosphere through plant. This movement occurs so long yair ­is less than the soil water potential (Ψsoil). Plant and Moreshet (1973) provided an approximate magnitude of water potential in the soil-plant-atmosphere system (Table 24.1).

Table 24.1: Approximate magnitude of water potential in the soil-plant- atmosphere system

Component	Water potential (bar)
Soil Leaf Atmosphere	-0.1 to -20 -5.0 to -50 -100 to -2000

 

24.3.6 Pathway of Water in Soil-Plant-Atmosphere System

Water moving from soil to air through the plant takes the path along

(i) epidermal cells

(ii) cortical cells and intercellular spaces in the cortex

(iii) conductive system of xylem

(iv) leaf cells

(v) intercellular spaces in the leaf

(vi) stomatal cavities and stomata and

(vii) air layer in the immediate vicinity of the leaf.

In this system water takes the path of least resistance and moves as a continuous cohesive liquid from epidermal cells of the root to leaf cells. Water from leaf cells moves to air in vapour from through the intercellular spaces, stomatal cavity and stomata. The most satisfactory theory of water conduction in liquid form through the plant body is cohesive theory (Kramer et al. 1967 and Kramer, 1969). The evaporation from leaves sets up imbibition forces in the cell walls that are transmitted to epidermal cells of roots through the hydrodynamic system and causes the water absorption and then its ascent through the plant body.

Movement of flow of water from soil to air through the plant is a function of the difference in yair and ysoil with resistance offered by the plant system. The water flow rate in the system in stages may be considered by the following expression:

Where, Ψ = water potential at various sites of the system and

                     r = resistance at the corresponding sites.

 24.4 WATER ABSORPTION PROCESSES

Water absorption by plants occurs by two processes namely, active absorption and passive absorption. Renner (1915) suggested these two terms. In active absorption plants play an active part. In passive absorption water is absorbed mechanically through roots without plants playing an active role and plants present simply the absorbing surfaces.

24.4.1 Active Absorption

A well-watered slowly transpiring plant absorbs water by active absorption under the tension developed in the root xylem due to matric effect of solids and osmotic effects of solutes present in it. This tension is usually called the root pressure caused by metabolically active roots acting as an osmometer. Salts are accumulated by the active transport mechanism in living roots. Root pressure causes guttation or exudation of liquid from tips and margins of leaves and occasionally from lenticels of twigs. The exudation from wounds and cut stumps of plants is an evidence of root pressure. Root pressure is detectable only during periods of low transpiration. The amount of water absorbed by the active absorption is very negligible and is usually less than 5 per cent of the total water required by a rapidly transpiring plant (Kramer et al. 1967 and Kramer, 1969).

24.4.2 Passive Absorption

In rapidly transpiring plant water loss from leaves exceeds the volume of water that the plants can absorb by active absorption. Evidently, a tension or diffusion pressure deficit is created in the mesophyll tissues of leaves. In other words, the water potential in mesophyll cells gets reduced. The tension thus created is transmitted through the hydrodynamic system into the xylem system in roots and then to the root surface. A gradient of decreasing water potential from soil to atmosphere through the plant is created and this results in movement of greater volume of water through the plant. Under conditions of rapid transpiration and high diffusion pressure deficit in the xylem system, water is literally pulled into the roots from the soil by mass flow (Levitt, 1956). Root tissues offer considerable resistance to this movement and the water absorption tends to lag behind the transpiration. This absorption lag causes development of considerable water deficit and tension in the hydrodynamic system. The water deficit may sometimes be so great that plants show sign of water stress even when the water supply in the soil is adequate. This condition is often observed during mid day in summer months.

The diffusion pressure deficit (DPD) of a cell or solution is the amount by which its diffusion pressure is less than that of pure water under atmospheric pressure and at the same temperature. It gives a measure of the force with which water diffuses into a cell immersed in pure water. Diffusion pressure deficit in a cell is the difference of the osmotic pressure (OP) and the turgor pressure (TP) and the value is positive. The DPD may be expressed as,

DPD = OP – TP

It increases with decrease in water content of the cell until it equals the osmotic pressure. Whenever a negative wall pressure develops, DPD exceeds the osmotic pressure. It becomes zero under turgid condition of cells.