Absorption of Water and Minerals, Biology tutorial

Introduction:

Water and mineral nutrients are two necessary needs for plants healthy growth. Former forms very important component of protoplasm in plants, and it accounts for approximately 80-90% of fresh weight of plant. Water participates in many vital activities of plants. It plays important roles as universal solvent in translocation of both organic and inorganic materials inside plants. Mineral nutrients needed by plants exist in dissolved form ions of salts. While in solution, these ions are capable of dissociating and moving freely. Soil comprises main source of mineral salts for plants and their uptake is performed via subterminal meristematic region of root. This uptake is generally greatest in the region of root hair. Additionally, mineral salts are present with soil particles and water in colloidal form.

Importance of Water to Plants:

Water, as the universal solvent comprises a essential requirement for healthy growth and survival in plants. It is essential constituent of protoplasm, serves as significant medium for number of metabolic activities and translocation of minerals in plants. Water is necessary for germination of seeds. All enzymes in plant can only achieve their optimum condition in presence of water. Additionally, water participates in condensation and hydrolysis of water activities that are going in cells of plant. It compensates loss of water in transpiration.

Water Absorption by Roots:

Water is absorbed either by whole surface of roots or by rhizoids. Though, in higher plants like pteridophytes and spermatophytes, absorption of water occurs through root. In higher plants, root is divided in four morphologically distinct parts that participate in absorption of water.

Root cap region:

The thin, small and smooth cap-like structure is present at apex of root known as root cap.

Meristematic region:

This region lies just below root cap. Cells of this region divide constantly. Rate of ion absorption is highest in the region.

Elongation region:

This is 3-4 mm thick region which lies behind the meristematic region in which new cells grow in length is called elongation region. Absorption of water is very slow from this region.

Maturation region (root hair region):

This region lies behind elongation region in which separation of cell occurs to perform different functions. Root hairs are only found in this region. Maximum absorption of water occurs from this region. Path of water absorption is best explained in direction of osmotic gradient. Water situated in soil is has to reach up to xylem of root. Root hairs remain in contact with water.

Osmotic pressure is greatest in Soil solution → Root hairs (Epiblema) → Cortex → Endodermis (passage cells) → Pericycle cells → Protoxylem → Metaxylem.

Water enters in epidermis of root hairs. From here water reaches up to endodermis through cortex. Walls of endodermis are suberised. But cells lies in front of protoxylem are thin walled called as passage cells. Such cells transfer water to xylem. From here water reaches xylem from endodermal cells by thin walled pericycle cells.

Mechanism of water absorption:

Water is absorbed by two various methods:

1) Active water absorption.

2) Passive water absorption.

Active water absorption:

This procedure happens consequently of activities in root. This procedure takes place at that time when transpiration is going slowly and water is enough in soil. In active absorption, water may be absorbed through expenditure of energy that is produced by metabolic activities. This absorption is of two kinds.

1. Osmotic active water absorption: Cell wall acts as permeable membrane and plasma membrane acts as semi-permeable membrane in root hairs. Water is absorbed by osmosis when osmotic concentration of soil water is less than that of cell sap. Outermost pectin layer of root hair absorbs water from soil solution (Endosmosis). At the moment DPD of soil solution is less than that of DPD of root hair. Thus water comes in root hair from soil. Absorption of soil solution root hairs becomes turgid and their osmotic pressure decreases DPD of 'B' cell increases because of decreasing of OP and increasing T.P. and water comes in 'A' cell from 'B' cell of root hair. Magnitude DPD of 'B' cell decreases because of entrance of water and DPD of adjacent 'C' cell increases. So that water enters in 'C' cell from 'B' cell because of higher DPD. Thus water reaches up to xylem via 'C' cell that is cortex, endodermis and pericycle.

2. Non-Osmotic Active Water Absorption: Water is absorbed in the conditions where osmotic pressure of soil solution is greater than that of osmotic pressure of cell sap of root hair. In this condition water isn't absorbed by osmotic process. Energy is utilized in the situation which is made by metabolic activities of living cells of root. Cells of root generated energy in form of ATP by which water is absorbed from soil against osmotic gradient. The positive pressure is developed in xylem of roots because of activity of living cells of roots. This is known as root pressure. This root pressure assists in absorption of water. This pushes water from xylem of root towards xylem of stem. Root pressure is estimated by manometer.

Passive water absorption:

Passive absorption occurs through fast transpiration in aerial parts and few forces develop in shoot. Root stays inactive (passive) in passive absorption. It signifies absorption of water by root rather than by root. Amount of water decreases in the mesophyll cells of leaves because of transpiration. This loss of water increases DPD. For compensation of the loss, such cell pulls water from neighbouring cells. In this way water is pulled from one cell to another cell and through endodermis, pericycle and finally from xylem. Fast transpiration causes higher DPD in xylem. Resulting the negative tension developed. This tension is called as suction force or transpiration pull. Unbroken water column established from xylem of leaves to xylem of roots is because of transpiration pull.

Factors affecting water absorption:

Available soil water:

Plant absorbs capillary water that is present in soil. Absorption of water depends on amount of capillary water present in soil. Absorption rises by increasing amount of capillary water. If, water is present in higher amount in soil then soil is said to be water logged.

Soil temperature:

Soil temperature influences chemical potential of water, permeability of cell membrane, activity of enzymes and viscosity of capillary water. Increasing or decreasing soil temperature lowers down rate of absorption of water. If temperature of soil is higher, then enzymes begin degeneration and absorption of water decreases. Rate of absorption decreases with decreasing soil temperature. This is due to at low temperature, movement of capillary water decreases and viscosity of water increases.

Soil air:

Absorption of water carries on more rapidly in well aerated soil. Deficiency of oxygen in soil results in inappropriate respiration in roots. Additionally of that bacteria produced toxicity by formation of CO2 and organic acids in anaerobic environment. Deficiency of oxygen influences process of osmosis. It decreases rate of absorption.

Soil salt:

Rate of water absorption in plants is inversely proportional to concentration of mineral salts present in soil. Water absorption only occurs in suitable soil solution. When concentration of soil minerals is more rate of water absorption, thus saline soil is physiologically dry. Halophytes grow only in this type of soil. Therefore, for proper absorption, concentration of soil solution must be normally less than that of root hairs.

Transpiration:

Rate of water absorption is directly proportional to transpiration. Rate of absorption increases because of increase in transpiration; therefore, passive water absorption increases because of transpiration. 99% of absorbed water is transpired by plants; only 1% is available to carry out different vital activities of plants. Rate of absorption is maximum in summer noon but water absorption decreases. In this situation plant illustrates temporary wilting. This is due to cells contract because of decreasing turgidity of cells; therefore, plants droop. But, rate of absorption increases during the evening as compared to transpiration which raise the turgidity of cells and drooping of plant stops.

Mechanism of Absorption of Mineral Salts:

Dissolved mineral salts and water are performed by root of plants by 2 mechanisms. These are passive absorption and active absorption.

1) Passive Absorption: This happens when ions move by mass flow and diffusion by apoplast. Examples of passive absorption include:

Diffusion: Mineral salts are diffused from higher concentration towards lower concentration.

Carbonic acid exchange theory: CO2 is made by respiration in roots with water to create carbonic acid. Acid formed dissociates in H+ and HCO3- ions. Exchange of negative (-ve) and positive (+ve) ions of solution occur by positive and negative ions present in root.

2) Active Absorption: This type of uptake is selective and dependent on respiration. Mineral salts enter in higher concentrated cell sap from lower concentrated soil solution by expenditure of energy in form of ATP produced from respiration. Certain carriers are there in plasma membrane which is composed of proteins for negative and positive ions that combines with ions to form ion- carrier complex.

Transpiration:

All terrestrial plant utilized absorbed water by roots in different metabolic activities. Though, large amount of water is absorbed by plants from soil, but whole of water is not utilized by plants for growth and development. The small fraction of total water is utilized up by plants for its development and remaining amount of water is lost from aerial part in vapor form and goes in external atmosphere. Therefore, loss of water in vapor forms from aerial parts (organs) of living plants is called as Transpiration. Only little percentage [1%] of absorbed water is utilized by plants while remaining [99%] of water lost in atmosphere.

Kinds of Transpiration:

There are 3 kinds of transpiration.

i) Cuticular transpiration:

Loss of water occurs through cuticle that is present on herbaceous stem and leaves with very thin. The cutinised wax like thin layer is there on epidermis. Function of the layer is to decrease or to stop transpiration. But fraction of water lost in vapor forms through thin cuticle.

ii) Lenticular transpiration:

Minute pore-like structures found in stem of some woody plants and epidermis of some fruits is known as lenticels. Process of water loss through lenticels is called as lenticular transpiration. Though, it is about 0.1% to 1% of total water lost.

iii) Stomatal transpiration:

Transpiration that occurs through stomata that are present on leaves of plant and green organs is known as stomatal transpiration. Maximum amount of total water delicate is lost by the transpiration. Of the total amount of transpiration from plants approximately 80% to 90% transpiration is through stomata. Some amount of transpiration is also occurs through bark of plants.

Factors influencing rate of transpiration:

Factors affecting rate of transpiration are separated in two kinds:

i) External factors

ii) Internal factors

i) External factors are related to atmosphere. They comprise:

Light:

It is affected movement of stomata. Stomata open in presence of light and phenomenon of transpiration occurs. Opening of stomata is less in dim light that decreases rate of transpiration. Action spectrum of transpiration is blue and red. Rate of transpiration is quicker in blue light than that of red light. This is due to stomata are entirely opened in blue light.

Temperature:

Value of quotient of transpiration is - 2. It signifies by the increase 10ºC temperature rate of 10 transpiration is roughly doubled. Water vapor holding capacity is influenced by temperature. Water vapor holding capacity of air is increased at high temperature, resulting in increased rate of transpiration increased.

Wind velocity:

Transpiration is less in constant air but if wind velocity is high rate of transpiration also rises. This is due to wind eliminates humid air (saturated air) around stomata and is replaced by unsaturated air. So that transpiration increases. Transpiration increases in starting at high wind velocity [30 - 35 km/hour]. But later on it cause closure of stomata because of mechanical effect and transpiration is decreased.

Atmospheric pressure:

Speed of air increases at low atmospheric pressure, because of this rate of diffusion increase that is term increases rate of transpiration. Rate of transpiration is at maximum in high intensity of light at high attitude. Rate of transpiration reduces at high atmospheric pressure.

ii) Internal factors are those which deal with structure of plants. They comprise:

Structure of Stomata:

Stomata are situated on aerial delicate organs and outer surface (epidermis) of leaves in form of minute pores. Stomata are enclosed by two epidermal cells known as guard cell. They are kidney shaped. Number of guard cells of the leaf stomata is two. Guard cells are epidermal cells. But because of presence of chloroplast they are dissimilar from that of epidermal cells. Outer wall of guard cells are thin and elastic whereas inner wall is thick and non elastic. Guard cells have one nucleus, cytoplasm and indefinite chloroplast. Guard cells are enclosed by some specialized epidermal cells are known as subsidiary cells or accessory cells. Mesophyll cells lies between both epidermises. Stomata are joined with air chambers and which forms the cavity that is known as sub stomatal cavity.

Water absorbed by root hairs, reaches xylem from cortical passage (path) by passage cells of endodermis. Water reaches up to upper part of plant from xylem by procedure of ascent of sap. All cells of mesophyll remain in turgid situation because of presence of water. This water vapor collects in substomatal cavities from intercellular spaces and diffuses in atmosphere because of less water vapor present in atmosphere.

Mechanism of opening and closing of stomata:

Opening and closing of stomata rely on osmotic pressure of guard cells. When guard cells are in turgid state stomatal aperture opens and when guard cells are in flaccid state stomatal aperture closes. When osmotic concentration of guard cells increases then they absorb water from accessory cells and turn out to be turgid by endosmosis. Thin wall of guard cell is pushed out side because of this turgor pressure therefore tension is created on inner thicker wall thus, pulling inner wall towards periphery. This occurs in both guard cells and stoma and gap is formed between them and that leads to opening of stomata.

Factors which affect stomata in opening and closing

Light: In most plants, stomata open in day except succulent xerophytic plants and close in dark. Opening and closing of stomata is completed in presence of blue and red light. Blue light is most effective and causing stomatal opening.

Temperature: Stomata opening and closing happens at specific temperature, below which it may not happen.

CO2 concentration: Stomata are sensitive towards internal CO2 in deficiency of water in cells of leaves. Stomata open at low concentration of CO2 as it is close at high concentration of CO2.

Guttation:

Loss of water from undamaged part or leaves of plant in form of water droplets is known as guttation. Procedure of guttation is found in both shrubs and woody kind of plants. Exuded liquid of guttation excluding water also has some organic and inorganic (dissolved) substances. Guttation happens from margins of leaves through special pore, known as Hydathodes. Hydathode is created by colorless epidermal cells. Parenchymatous and loose tissue lies under hydathode that is called as Epithem. Cells of epithem are soft and with no chloroplast. In anterior part of epithem, cavity is there that is known as water cavity. It opens out through hydathode. Procedure of guttation of occurs because of root pressure. Procedure of guttation happens in special situations when higher rate of absorption and transpiration is low.

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