TRANSPORT IN PLANTS

Transport of water and minerals by plant root

Objectives

This  blog post provides readers with the following objectives. The reader will be able to:

o   Describe the process of uptake and movement of water and mineral salts in plants.
o   Explain the concept of translocation.
o   Demonstrate that transport of synthesized organic nutrients occurs through the phloem.
o   Explain the term transpiration.
o   Distinguish between the types of transpiration.
o   Determine the rate of transpiration.
o   Identify the environmental factors that affect transpiration.
o   Explain the concept of guttation
o   Explain the concept of excretion in plants. 

 TRANSPORT IN PLANTS

Absorption of Water and Mineral salts

Water is absorbed by the root tips and root hairs of the root. The solutes concentration in the cell sap of the root hair is more than the soil water solution. Water therefore enters the root hairs by osmosis. The root hair sap thus becomes less concentrated than the sap in the surrounding cells. Water then moves from the root hair into the surrounding cells following the concentration gradient until it reaches the xylem vessels. The water moves through the root cortex from cell to cell by three pathways before reaching the xylem vessels:

1.     Apoplastic pathway through the cell wall. More rapid - less resistance to the flow of water

2.   Symplastic pathway through cytoplasm via plasmodesmata conceptions

3.   Vacuolar pathway through the vacuole

Before water enters the xylem, it must cross the endodermis. Cells of the endodermis possess cell walls that are ringed by the Casparian Strip, a waxy layer (composed of suberin). The Casparian Strip prevents the apoplastic flow of water. Water crosses the endodermis by symplastic pathway. The Casparian Strip prevent the backflow of water out of the vascular cylinder into the root cortex. Once the water passes under the Casparian Strip in the endodermal cells, it is free to enter the apoplast again on its way to the xylem

The uptake of minerals by the plant is achieved by diffusion and active transport. Mineral salts are found in ionic form, dissolved in the soil water surrounding the roots. Depending on the mineral salt required by the plant, the roots hairs absorb the salts by diffusion along concentration gradient or against concentration gradient by active transport. Active transport is needed because the concentration of some minerals may be greater in the root hairs than in the growth medium. As a result, energy is needed to make minerals travel against their concentration gradients.


Adaptations of root hairs for absorption of water

Root hairs are specialized structures that play a crucial role in the absorption of water and nutrients from the soil. These microscopic extensions of root epidermal cells are highly adapted to maximize their efficiency in taking up water and essential minerals, which are vital for plant growth and development. Here are the key adaptations of root hairs that facilitate their function in water absorption:

1. Increased Surface Area

Root hairs significantly increase the surface area of roots, enhancing their ability to absorb water and nutrients. The more extensive the surface area, the greater the volume of soil that can be explored and utilized for resource uptake.

2. Thin Cell Walls

The cell walls of root hairs are thin and semi-permeable, allowing for easier and quicker absorption of water. The thin walls minimize resistance to water movement, facilitating the flow of water into the cells.

  • Thin Cell Walls: Enhance the efficiency of water uptake by reducing barriers.

3. High Density and Distribution

Root hairs are densely packed along the root surface and are distributed over a large area, ensuring that the root system can access a broad volume of soil. This dense distribution pattern maximizes the root's contact with soil water and nutrients.

  • Dense Root Hairs: Increase the root's capacity to explore the soil.

4. Proximity to the Root Tip

Root hairs are typically found just behind the root tip, where new cells are continually being produced. This positioning ensures that root hairs are always in fresh soil, which is likely to contain more available water and nutrients than soil further away from the growing root tip.

5. Efficient Water Channels

Root hairs possess aquaporins, which are specialized water channels in their cell membranes. These channels facilitate the rapid movement of water into the root hair cells, enhancing water uptake efficiency.

6. Active Transport Mechanisms

Root hairs are equipped with active transport mechanisms that allow them to absorb essential nutrients against a concentration gradient. This process often involves the use of energy in the form of ATP to move ions and molecules into the root hair cells, which can then drive water uptake through osmosis.

7. Symbiotic Relationships

Root hairs often engage in symbiotic relationships with soil microorganisms, such as mycorrhizal fungi, which enhance water and nutrient absorption. These fungi extend the root system's reach into the soil, providing additional pathways for water uptake.


Understanding the Transport of Water Up the Stem

Water transport in plants is a vital process that ensures the delivery of essential nutrients and maintains turgor pressure for structural support. This article delves into the mechanisms and pathways through which water is transported from the roots to the rest of the plant, focusing on the journey up the stem.


The Pathway of Water Transport

Water transport in plants primarily occurs through specialized tissues known as xylem. The process involves several key steps:

1. Water Absorption by Roots

Roots absorb water from the soil through root hairs, which are tiny extensions of root epidermal cells. This water then travels to the root cortex and eventually reaches the xylem vessels.

  • Root Hairs: Increase the surface area for water absorption.

2. Movement through the Xylem

Water moves upward through the xylem vessels in the stem, a process driven by capillary action, root pressure, and transpiration pull.

  • Xylem Vessels: Hollow tubes made of dead cells that facilitate water transport.
    • Discover the structure of Xylem on Britannica.


Mechanisms of Water Transport

From the roots, water is transported up the stem through the xylem vessels. The transport involved the following mechanisms; osmosis, root pressure, capillary action, cohesion and adhesion, transpiration and transpiration pull.

transpiration stream


1. Capillary Action

Capillary action occurs due to the adhesive and cohesive properties of water molecules. Adhesion allows water to stick to the walls of xylem vessels, while cohesion keeps the water molecules together, enabling them to move upward. The long, narrow tubes of the xylem vessels enable water to move up stem, a process known as capillary action.

  • Cohesion and Adhesion: The movement of the water in the xylem vessels is due to cohesion (attraction between water molecules) and adhesion (attraction of water molecules to the vessel walls).
  • Understand Capillary Action on ScienceDirect.

2. Root Pressure

Root pressure is generated when ions are actively transported into the xylem, creating an osmotic gradient that draws water into the xylem from the surrounding root cells. As the water is continuously absorbed into the xylem vessels in the root, it builds up a hydrostatic pressure, (called root pressure) that pushes the water up the root. This pressure helps push water up the stem, especially during times when transpiration rates are low. This is illustrated by exudation of water from stumps of the stem after a tree has been cut.

  • Osmosis: The movement of water from areas of low solute concentration to high solute concentration.

3. Transpiration Pull

Transpiration pull is the primary mechanism driving water transport in plants. As water evaporates from the leaf surfaces during transpiration, it creates a negative pressure (tension) that pulls water upward from the roots through the xylem vessels.

  • Transpiration: The process of water vapor loss from plant leaves and stems.

Factors Affecting Water Transport

1. Environmental Conditions

  • Temperature: Higher temperatures increase the rate of transpiration, enhancing water transport.

  • Humidity: Low humidity levels increase transpiration rates, whereas high humidity reduces transpiration.

  • Wind: Wind removes the water vapor around the stomata, increasing transpiration rates.

2. Plant Factors

  • Leaf Area: Larger leaves have more stomata, leading to higher transpiration rates.

  • Stomatal Density: More stomata per unit area can increase the rate of transpiration.

  • Xylem Structure: The diameter and length of xylem vessels can affect the efficiency of water transport.


Importance of Water Transport

  • Nutrient Distribution: Water carries essential nutrients from the soil to various parts of the plant.

  • Photosynthesis: Water is a key reactant in the photosynthesis process.

  • Cell Turgor: Maintains turgor pressure in plant cells, providing structural support and enabling growth.

  • Temperature Regulation: Transpiration helps in cooling the plant by releasing excess heat.



Experiment to Show Water Uptake from the Soil into the Plants

Aim: To show that roots absorb water.

Material: Test-tube, Young Leafy shoot with intact root, mustered oil, test-tube stand

Procedure

1.    Fill two test tubes with water

2.   Insert leafy shoot with roots intact into one of the test tubes.

3.  Pour few drops of mustered oil in both test-tubes to prevent loss of water by evaporation.

4.  Observe the level of water in both test tubes for 24 hours

Observation: At the end of the experiment, there will be drastic fall in level of water in test tube with plant indicating that roots absorb water which helps the entire plant.

There will be little fall in level of water in the control, indicating there wasn’t absorption of water in absence of root.

Inference:  Roots of the plant absorbs water and helps the entire plant.


Experiment to Show the Path Taken by Absorbed Water

Aim: To show water moves through the xylem tissues   

Material: beaker, Young herbaceous plant, eosin solution or colored liquid, razor blade

Procedure

1. Fill a beaker with dilute eosin solution.□

2.   Place the young herbaceous plant with the root washed inside a beaker containing the eosin solution.

3.  Allow the set-up to stand for 1-6 hours.

4.  Cut longitudinal or cross sections of a root, stem, petiole and leaf.

5. Examine and observe it under microscope or with hand lens.

Observation: The color of the solution will stain the xylem vessels.

Conclusion: It is claimed that xylem is responsible for the conduction of water.


Experiment to Show Root Pressure Affect the Rise of Water in the Xylem of a plant

Aim: To show that root pressure affects the ascent of water in the xylem.    

Material: potted plant, glass tube, eosin solution or colored liquid

Procedure

1. Cut across the stem of a well-watered potted plant, just above the soil level and observe the sap exuding from the cut surface. 

2. Attach a glass tube to the cut end of the stem using a rubber tube. 

3.  Attach the glass tube to a retort stand.

4. Pour a colored solution such as eosin solution into the glass tube and note the level.

Observation: The level of eosin solution in glass tube rises above that of the original level

Conclusion: The rise in the level of eosin solution is due to root pressure resulting in the continuous absorption of water by the root hairs.

Root pressure experiment



Translocation of Organic Food

Translocation is the movement of organic food, mainly sucrose through the phloem from regions of production to regions of storage or utilization. The glucose formed during photosynthesis in mesophyll cells, is used in respiration and the excess is converted into sucrose.  

The phloem contains a very concentrated solution of dissolved solutes, mainly sucrose. This solution is called the sap. It transports through the phloem from source (regions of production) to sink (regions of storage or utilization e.g. roots, tubers, fruits, leaves, growing regions).


Mechanisms of Translocation

Pressure or mass flow theory

The main mechanism is thought to be the mass flow of fluid up the xylem and down the phloem, carrying dissolved solutes. The mass flow is driven by a combination of active transport and evaporation. This is called the pressure or mass flow theory, and it works like this:

1.  Sucrose produced by photosynthesis is actively pumped into the phloem vessels by the companion cells. 

This decreases the water potential in the leaf phloem, so water diffuses from the xylem vessels by osmosis.

2. This increases the hydrostatic pressure in the phloem, so water and dissolved solutes are forced downwards to relieve the pressure. This is mass flow (the flow of water together with its dissolved solutes due to a force).

3. In the roots the solutes are removed from the phloem by active transport into the cells of the root. The mass-flow explains the fast speed of solute translocation.


Cytoplasmic streaming

There is a circular movement of cell protoplasm when viewed under the microscope. This is known as cyclosis. The cytoplasmic streaming theory states that;

1.  as cyclosis occurs it stirs up the sap of the vacuoles and by so doing the sap drips through the sieve pores along the concentration gradient into the next sieve element

2.  Therefore, translocation of solutes in opposite direction due to concentration gradient.


translocation

Translocation Experiments

1.      Puncture Experiments

If the phloem is punctured with a hollow tube then the sap oozes out, showing that there is high pressure (compression) inside the phloem (this is how maple syrup is tapped). If the xylem is punctured then air is sucked in, showing that there is low pressure (tension) inside the xylem. Water is pulled up in the xylem, sap is pushed down in the phloem.


2.      Ringing Experiments

Since the phloem vessels are outside the xylem vessels, they can be selectively removed by cutting a ring in a stem just deep enough to cut the phloem but not the xylem. After a week there is a swelling above the ring, reduced growth below the ring and the leaves are unaffected. This was early evidence that sugars were transported downwards in the phloem.

translocation of food in plants


3.      Radioactive Tracer Experiments

Radioactive isotopes can be used to trace transported compounds. They can be traced using autoradiograph or a GM tube. This technique can be used to trace sugars or ions or even.

A plant is grown in the lab and one leaf is exposed to carbon dioxide containing the radioactive isotope 14C. This 14CO2 is taken up by photosynthesis and the 14C is fixed into organic products (glucose) of photosynthesis which are then translocated to other parts of the plant.

The autoradiograph analysis shows that organic compounds (presumably sugars) are transported downwards from the leaf to the roots and is found entirely in the phloem.


4. Aphid Stylet Experiments

Aphids, such as greenfly, have specialized mouthparts called stylets, which they use to penetrate phloem tubes and sup of the sugary sap therein. If the aphids are anaesthetized with carbon dioxide and cut off, the stylet remains in the phloem so pure phloem sap can be collected through the stylet for analysis. 


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