TRANSPORT IN PLANTS Summarized Biology Notes

LEARNING OBJECTIVES

• identify and describe the structure of a dicotyledonous root and stem limited to, epidermis with root hairs, cortex, phloem and xylem from photomicrographs, and as seen under the light microscope
• explain the mechanisms by which water enters a plant, crosses the root, moves up through xylem vessels, enters leaf cells, and leaves the plant through stomata
• define transpiration as loss of water vapour from plant leaves by evaporation of water at the surfaces of mesophyll cells followed by diffusion of water through stomata
• describe transpiration, and explain how environmental conditions (temperature, wind speed, humidity and light intensity) affect the rate at which water vapour diftuses out of the stomata, and hence affect the rate of water uptake
• describe how and why wilting occurs
• describe the ways in which xerophytes can reduce water loss, With reterence to two locally occurring examples (e.g. Aloe; Euphorbia; quiver tree)
• define translocation in terms of the movement of sucrose and amino acids from region of production or of storage to region of utilisation or demand in respiration or growth
• describe the translocation of applied systemic pesticides in phloem throughout the plant.

Plants do not have a pumping organ like animals, but instead internal transport within plants occurs by mass flow. Two separate tissues take part in transport in plants. Water and mineral ions travel in the xylem. The xylem is made up of vessels connected end to end to form non-living tubes, originally made from living cells. The products of photosynthesis, mainly glucose in the form of sucrose and amino acids, are translocated in a living tissue, the phloem. Phloem consists of cells called sieve tubes and companion cells. The xylem and phloem are collectively called vascular tissue, and they occur together in the vascular bundles that branch throughout the plant body and serve roots, stems, leaves and growing points.

Structure Of A Dicotyledonous Root
To understand how water is transported in a plant, we need to look at the structure of the root of a flowering plant to see how water is taken in by the roots. We will look at root hairs and xylem vessels in detail. Water and mineral salts are taken into plants from the soil through root hairs. In Figure below, you can see the root hairs just behind the tip of the root. There are millions of microscopic root hair cells in every root. The roots push in between the soil particles and absorb water by osmosis.

The root hairs are very delicate and easily damaged, so new hairs are being produced all the time. The root hairs grow out of the cells of the epidermis, the outer layer of the root. Figure above shows a root hair cell. As shown in Figure above, the root hair cell has a long, thin extension arowing out into the soil. This gives each cell a large surface area for the absorption of water and mineral salts.
Root systems are responsible for the:
• absorption of water and inorganic compounds (mineral salts)
• anchoring of the plant body to the ground
• storage of food and nutrients
• production of new cells
• absorption of oxygen from the soil
• support of the plant.

The root of a plant develops when a seed germinates; the first structure to appear is the root or the radicle. This becomes the primary root. Roots that branch out of the primary root are called secondary roots (Figure below).

 The growing root tip is protected by the root cap as it moves through the Soil. The root cap is slimy to allow for easy movement. Above the root cap is the apical meristem. In this meristematic region, cells divide continuously by mitosis to produce new cells. In addition to mitosis, newly divided cells undergo elongation in the same direction of root lengthening. Thousands of tiny root hairs are found in the root hair region above the region of cell elongation. The function of the root hairs is to absorb water and dissolved mineral salts from the soil. As the root grows, it thickens and may produce lateral roots in the mature region, as shown in Figure below.

There are two major types of root systems: the taproot system and tibrous root system.

• Taproot system: This is a root system comprising one primary root and many secondary roots branching off the primary root. Taproots are foundin dicotyledonous plants and examples of taproots include carrots and beetroots.

• Fibrous root system: This is a root system with no dominant primary root, but many secondary roots of similar size. Fibrous or adventitious root systems are Common in monocotyledons and examples include coconuts and grasses.

 Tissue Distribution In Root
The different tissues in the root have a distribution that is common to all dicotyledonous plants and is shovwn in Figure below.

The epidermis is a single layer of cells on the outside that protects the inner tissues. The epidermal layer of the root has no waterproof cuticle, as this would prevent the absorption of water Structurally, the cells of the root hair (shown in figure listed earlier) have large central vacuoles and cover a large surface area, which allows water to enter these cells readily via osmosis. The cortex consists of parenchyma cells. These cells are large which enables them to store water and food. They also facilitate the movement of water from the root hair cell on the outside of the plant to the xylem on the inside of the plant. The endodermis forms.the innermost layer of the cortex. It is a layer of tightly packed, modified parenchyma cells. The radial and transverse cell walls are thickened with a water-impermeable, waxy suberin layer, knOwn as the Casparian strip. This layer helps to regulate the flow of water from the cortex into the stele, rather than allowing the water to spread to all the root cells. The stele, or vascular cylinder (responsible for transporting water and minerals) consists of the pericycle, phloem, cambium and xylem. The pericycle is the outermost layer of the stele, and consists of one or more rows of thin-walled parenchyma.cells. It is in close contact with the xylem and phloem tissues of the root. Its functions are the formation of lateral roots and the formation of a lateral meristem to allow seconday growth (thickening) to occur in the root. The phloem tissue is responsible for translocating sucrose from the leaves of the plant to the cells of the root. The cambium separates the xylem and phloem tissues from each other. This is the area where secondary growth of xylem and phloem tissues occur. Xylem tissue is responsible for transporting water and dissolved mineral salts to the xylem tissue of the stem and leaves. These cells are strengthened with lignin for support. The pits in the cell walls allow for the lateral movement of water.

 Structure Of A Dicotyledonous Stem
Stems usually grow above the soil surface and towards the light from the sun. Depending on the hardness ot the stem, we can distinguish  between herbaceous stems, which are leafy non-woody structures, and woody stems. Woody stems are harder than herbaceous stems.
Stems have four main functions:
• support for the plant as it holds leaves, flowers and fruits upright above the ground, stems keep the leaves in the light and provide a place for the plant to keep It's flowers and fruits
• transport of fluids between roots and shoots in the xylem and phloem
• storage of nutrients
• production of new living tissue; stems contain meristematic tissue that generates new tissue.

The main stem develops from the plumule of the embryo and the lateral branches develop from the buds. Nodes and internodes are regions m found on the stem. Nodes are the regions from which leaves and lateral branches develop, and the regions between nodes are known as internodes (shown in Figure below). Stomata, or pores, can be found in stems of younger plants. We will subsequently discuss the tissues present in the dicot stem. The trunk of a tree (shown in Figure below) is the stem.

 Internal Structure Of The Dicotyledonous Stem
Figure below shows the arrangement of tissues in a dicotyledonous stem. Details of each tissue type are below.

• Epidermis: A single layer of cells that covers the stem, and is in turn covered by a waxy cuticle. The waterproof cuticle helps prevent water loSs and thus prevents the inner tissues from drying out. Since the function of the epidermis is to protect underlying tissues, epidermal cells are tightly packed and have thickened walls. The epidermis may contain hair-like outgrowths known as trichomes, and stomata with guard cells. Stomata present in the epidermis allow for gaseous exchange for respiration and photosynthesis
• Cortex: A region that consists of collenchyma, parenchyma and the endodermis
• Collenchyma: A few layers of living cells that lie under the epidermis. These cells are not lignified, but do have thickened cell walls, which serve to strengthen the stem. The collenchyma cells contain chloroplasts, which produce food for the plant during photosynthesis.
• Parenchyma: Found beneath the collenchyma cells and makes up the bulk of the cortex, The cells are thin-walled, and there are intercellular spaces that are important in gaseous exchange. Parenchyma stores synthesised organic food (mostly starch) that the collenchyma produced.
• Endodermis: A single layer of tightly packed rectangular cells that forms the innermost layer of the cortex. The endodermis also stores starch and, as the border between the stele and cortex, regulates the passage of solutions from the vascular bundles to the cortex
• Vascular cylinder or stele: Made up of the pericycle, vascular bundles and pith.

• Pericycle: Made up mainly of lignified, dead, fibrous cells Known as scIerenchyma cells. Slerenchyma cells have end-to-end  onnections (tapering enas), and have extremely thick walls that consist of lignin and/or cellulose. These thickened, woody cell walIs are very hard and play an important role in strengthening the stem, and providing protection for the vascular bundles.
• vascular bundles: Characteristically organised in a ring inside the pericycle of the dicot plant. Mature vascular bundles are made up of water-conducting xylem, cambium, and food-conducting phloem. The phloem is located on the outside of tne bundle and the xylem towards the centre (see Figure above). Ine phloem and xylem are separated by meristematic tissue known as cambium, which is responsible for secondary thickening. Xylem has lignified cell walls that helps it fulfil its two important roles: strengthening and supporting the stem, and transporting water and minerals from the root system to the leaves. The function of phloem Is to translocate synthesised food (Sucrose and amino acids) from the leaves to other parts of the plant.
• Pith (or medulla): Occupies the large, central part of the stem. The pith is made up of thin-walled parenchyma cells containing intercellular spaces. Where the parenchyma extends between vascular bundles as thin bands, it is known as medullary rays, and can be continuous with the pith and cortex of the parenchyma. The cells of the pith store water and starch, while the intercellular spaces allow for gaseous exchange. The medullary rays facilitate transport of substances from xylem and phloem to the inner and outer parts of the stem.

Xylem vessels run from the roots to the leaves and flowers. They contain a continuous column of water in which inorganic salts are dissolved. They form a continuous transport system inside a plant and are the main water-conducting cells in the plant.
They have two functions:
• the movement of water and mineral salts
• the support of the plant.
A xylem vessel is made up of many dead, hollow cells arranged end to end to form long tubes. The walls of the cells are strengthened with a complex organic compound called lignin. The lignin gives the xylem vessels great strength to support the plant. The stems of trees contain so many xylem vessels that the entire tree is used extensively for building and furniture. Xylem vessels run from the roots to the leaves and flowers. They Contain a continuous column of water in which inorganic salts are dissolved. They form a continuous transport system inside a plant and are the main water-conducting cells plant.

For easy distinguishing we numbered the figures below, Figure 10 a)  

and b) 

shows a transverse section of a stem. It shows how these xylem vessels fit together to look like a mass of tubes for carrying water. Figure 10 c) is a longitudinal section along the length of the xylem vessels. Figures 10 a), 10 b) and 10 c)

 show how the dead xylem vessels are interspersed with other living cells.

Figure below is a light micrograph of mature xylem vesse in a plant stem. You can clearly see the patterns of thickening produced by the lignin. This makes it strong enough to support the plant. The fact that xylem vessels are long hollow tubes make them adapted for carrying water, minerals and ions throughout the plant.

How Water Is Absorbed By A Plant
The root hair absorbs water and minerals from the soil. Water enters the root hair by osmosis. Water diffuses down its concentration gradient, into the root hair, through the partially permeable cell membrane. This happens because the water in the soil is a dilute solution; the cytoplasm and cell sap inside the cell are concentrated solutions. Once inside the root hair cell, water travels by osmosis across the cortex cells to the xylem vessels
1. Water enters
2. Water passes across the root, from cell to cell, by osmosis (symplast). It also moves via cell walls (apoplast).
3. Water is drawn up the xylem vessels, because transpiration is constantly removing water trom the top of them.

The path taken by the water is shown as arrows in Figure above. The arrows that show water movement via the cell walls indicate apoplast water movement, and the arrows that show water movement via cytoplasm or vacuoles indicate symplast water movement. Water travels through three kinds of pathways:
• apoplast pathway -from cell to cell through the cell walls
• symplast pathway - from cell to cell through the cytoplasm
• vacuolar pathway -from cell to cell through the vacuoles.

These xylem vessels then transport the water through the root to the xylem in the stem from where the water passes to all other parts of the plant. The xylem vessels are made up of dead, hollow cells that are placed end to end to form a continuous tube that runs up through the stem and into the leaves.

Why does water move up the xylem vessels? To answer this question, think about why water moves up a drinking straw when you suck the top of the straw. You reduce the pressure at the top of the straw when you suck the top of the straw. The liquid in the glass is at a higher pressure, so it flows up the straw into your mouth. In the same way, the pressure at the top of the xylem vessels is reduced by the loss of water through a type of evaporation known as transpiration. The pressure in the roots stays high so that water flows up the xylem.

Water Potential And Water Uptake Into Root Hairs
To understand the uptake and movement of water in plants, you need to have an
understanding of:
• Osmosis and diffusion
• Water potential and water potential gradients.

Water tends to move from a region where there is more water to a region where there is less water; it moves from a high water potential to a low water potential. The symbol for water potential is in the figure below

The reason why water moves from the soil into the root is as follows:
• The root hair is in close contact with water and dissolves minerals in the soil.
• There is more water in the soil than in the plant; we can say there is a high water potential in the soil and a low water potential in the plant.
• Water moves through the cell membrane of the root hair into the vacuole by
• Osmosis; this means that water moves from the soil into the root hair, down a water potential gradient.

Water is constantly being taken from the top of the xylem vessels, to supply the cells in the leaves. This reduces the effective pressure at the top of the xylem vessels, so that water flows up them. In effect, the upward movement of water can be thought of as a 'pull' from above, which maintains a water potential gradient through the plant. This process is known as the transpiration stream, which you will learn about in this unit. Water moves through the leaf by osmosis from the xylem in the vascular bundies of the leaf vein to the mesophyll cells of the leaf. From there the water moves into air spaces, between the mesophyll cells of the leaf. Then water diffuses into the air spaces, between the mesophyll cells, from a high water potential to a low water potential. Water changes from a liquid state in the mesophyll cells to a vapour in the air spaces.

Transpiration
Water moves out of the leaves through the stomatal pore, and evaporates into the air as water vapour. Transpiration is the loss of water vapour from a plant's leaves by Evaporation of water at the surfaces mesophyll cells, followed by diffusion of water vapour through the stomata. It occurs mainly from the leaves. Water evaporates from the cell walls, and the vapour accumulates in the air spaces and diffuses out through tiny holes called stomata. As water evaporates, the leaf cells are cooled. Water is drawn by osmosis from the xylem vessels of the leaf veins to replace the lost water. A stream of water is drawn up through the plant. Most leaves have more stomata on the lower surface than the upper surface to reduce the loss of water vapour. Some leaves have a waxy covering, called the cuticle, especially on the upper surface, to reduce transpiration. To understand how transpiration comes about, you have to knowwhat the internal structure of the leaf looks like.

The cells of the spongy mesophyll layers are not tightly packed; they have many intercellular between them. Therefore water moves by osmosis from the cell contents down the water potential gradient into the cellulose fibres of the cell wall. As the water evaporates from the moist cell walls the air spaces become saturated with water vapour. The air in the internal spaces of the leaf has direct contact with the air outside the leaf, through tiny pores or stomata. If there is a water potential gradient between the air inside the leaf and the air outside, then the water vapour will dittuse out of the leaf down this gradient. Most of the water vapour is lost through stomata; 10% of water vapour is lost through the cuticle, which is not completely impermeable to gases. The thicker the Cuticle, the less water vapour is lost and a very small amount of water vapour is lost through lenticels (small areas of loosely packed cells that protrude through the cork layer of woody stems).

 Water Movement In The Leaves And Transpiration
- Transportation up a water potential gradient in the leaves. This gradient is responsible for the movement of water out of the xylem by osmosis and into the leaf to replace the water lost through transportation.

 Water Potential Gradients In The Leaf
How does evaporation of water from the leaf by transpiration result in a water potential gradient

 Let us look again at the cells labelled A, B, and C in figure above:
• When water evaporates trom cell A, the loss of water causes its water potential to become lower than that of its neighbour, cell B.
• Cell B now has a higher water potential than cell A and water moves from cell B to cell A.
• This movement of water out of cell B now lowers it's water potential compared with cell C
• Cell C now has a higher water potential than cell B and water now moves from cell C to cell B.
• This lowers the water potential of cell C.
• The eventual result is that the xylem.has a higher water potential than the leaf cells and water moves out of the xylem by osmosis.

Upward Movement Of Water Through The Plant
There are several theories to explain how water moves upwards from the roots, in the xylem of the stem. One theory suggests that evaporation from the leaf and transpiration are responsible for the upward movement of water through the plant. With transpiration, water molecules evaporate from the mesophyll cell walls (in the leaf), and move into the air space above the stomata in the form of water vapour. The water vapour diffuses out of the stomata. This water loss causes movement of water up the xylem, and this movement is called transpiration pull. The columns of water stay vertical due to  adhesion and cohesion forces. Capillarity also ensures that water moves up the xylem.

Look at Figure above , which summarises how water moves through the plant from the roots to the leaves.
The mass flow of water in a column, rather than as individual molecules, is possible because of the strong forces of attraction:
• between water molecules (cohesion)
• between water molecules and the walls of the xylem vessels (adhesion).

 Factors Affecting  The Rate Of Transpiration

When you hang wet clothes on a washing line, they sometimes take a long time to dry. Wet clothes dry quicker on a hot, dry, windy day and slower on a cool, humid, still day. The same factors that affect how long it takes to get your washing dry also affect transpiration. Both processes involve loss of water by evaporation. The transpiration rate is fastest on a hot, dry, windy day and slowest on a cool, humid, still day. Four environmental conditions affect the rate at which water vapour diffuses out of the stomata and affects how much water is taken up by plants. The four conditions are:
• light internsity.
• temperature
• humidity
• wind speed

The hotter  the day, the higher the temperature, the faster the rate of transpiration. How can we explain this? Higher temperature will increase the transpiration rate because molecules move faster when heated and therefore evaporation of water around the mesophyll into the air spaces of the leaf occur more frequently. An increase in
temperature increases kinetic energy of water molecules, causing them to diffuse quicker from inside the leaf to the atmosphere. This increases the rate of transpiration. Higher temperature will also Iower tne humidity of the air around the leaf, which will increase the water potential gradient speeding up transpiration. Humidity is the amount of water vapour in the air. The more water vapour in the air, the more humid the day, the slower the transpiration rate. This happens because the increased water vapour decreases the water potential gradient between the inside and outside of the leaf. Low humidity speeds up the diffusion rate of water vapour molecules from the leaf to the air because of dry air on the outside. Thus, the rate of transpiration will increase  ecause there is less water vapour on the outside of the plant.

On a windy day, air movement increases, and water vapour around the stomata is blown away from the leaf surface faster. In this way, a difference in water potential is maintained between the leaf and the air, and therefore transpiration rate will increase. Wind carries water vapour. The stomata open during the day in response to light intensity. Water vapour diffuses out of the leaf through the stomata. Carbon dioxide, the gas needed for photosynthesis, diffuses in through the stomata. The opening of the stomata during the hottest part of the day means that the rate of transpiration is increased while the plant is photosynthesising at its maximum. If the plant is not able to take up enough water to replace the water lost by transpiration, its cells lose their turgidity and the plant wilts, as seen in Figure below.

One of the advantages of transpiration is that it keeps the water moving up through the plant. However, if the plant loses more water than what the root can take up, the plant wilts. Observe Figure above and Figure below

 Advantages Of Transpiration:
• It causes transpiration stream in plants.
• It carries water and mineral ions into plants.
• It has a cooling effect on the plant.
# Disadvantages Of Transpiration:
• The rate of water loss can exceed the rate of water absorption.
• This will lead to wilting, desiccation.and often death of the plant.

Measuring And Comparing The Rate Of Transpiration
The rate of transpiration is the volume of water a plant loses in a given time. This is difficult to measure directly, so the rate of transpiration is measured indirectly. The amount of water taken up by the plant is measured using a potometer. The main use of the potometer is to compare transpiration rates of one species of plant under different environmental conditions. Potometers are sometimes used to compare transpiration rates of different species of plants. One type of potometer used in a laboratory is shown in Figure below.

It is assumed that the amount of water taken up by the plant equals the amount of water lost however, not all the water a plant takes up is transpired; some is used for photosynthesis and other processes in the cells. The volume of water taken up is only approximately equal to the transpiration rate. There are a number of different designs of potometer, but they are all based on the same principle: the time it takes for an air bubble to move a given distance along the capillary tube indicates the rate of water uptake. By recording how fast the air bubble or water meniscus moves along the capillary tube, you can compare how fast the plant takes up water in different conditions. A potometer can be used to look at the effects of temperature, wind speed, humidity and light intensity on the rate of transpiration.

Adaptations To Living In Dry Conditions
The Surface of leaves of many plants are adapted to reduce water loss by transpiration.
Two of these adaptions are:
• a waxy waterproof cuticle

• the distribution of stomata, mainly on the lower surface of the leaf, away from direct sunlight.

Plants that are adapted for growing in dry places, and to withstand periods where water is unavailable, are called xerophytes. Examples of xerophytes are Welwitschia, quiver tree, camel thorn tree and the Aloe. A major problem for xerophytes is that they lose Water by transpiration and very little water is available where they grow. They have adapt to conditions in which the loss of water exceeds the amount of water taken in.

The leaves and roots of xerophytes are adapted to reduce transpiration rate in some or all
of the following ways:
• a thick waxy cuticle that is impermeable to water and reduces water loss
• relatively few stomata that are often sunken; sunken stomata are buried deeper than the surface of the epidermis; this means that water vapour has further to diffuse and so slows down the rate of water loss by increasing the length of the diffusion pathway
• leaves reduced in surface area, sometimes to small spines or thorns; these reduced leaves do not transpire as they have no stomata and photosynthesis is carried out by the stems

• a hairy epidermis that traps water vapour and in this way reduces the water potential gradient

• leaves that fall off in the dry season
• leaves that curl up during the heat of the day

 
• deep taproots

• additional adventitious roots.

In southern Africa, many places have a low or unpredictable raintall, which explains why many plants in the region are xerophytes. Xerophytes that store water in their stems or leaves are called succulents. You can recognise them by their fleshy leaves or stems. Figures below shows two succulents commonly found in Namibia.

Translocation Of Sugars And Amino Acids

Translocation is the movement of manufactured food (sugars and amino acids) in the phloem. Sugars are made in the leaves by photosynthesis and translocated as sucrose to sites of growth, such as the roots and stems, seeds and fruits. The transport system is a series of tubes called phloem sieve tubes. Glucose is made in the leaves and translocated as sucrose, a more complex sugar. Glucose is used by other parts of the plant for respiration. Amino acids are mostly made in the root tips, where nitrates are absorbed. They are also transported and used as the building blocks for protein. The transport of sucrose and amino acids in phloem tubes is called translocation. Phloem Consists ot sieve tube elements and companion cells. Sieve tubes are narrow, elongated elements, connected end to end to form tubes. The end walls of the tubes cells are known as sieve plates and are perforated by pores. The cytoplasm of a mature Sieve tube has no nucleus. However, each sieve tube is connected to a companion cell via plasmodesmata. The companion cells are believed to service and maintain the cytoplasm of the sieve tube in some way.

 Translocation Of Systemic Pesticidess
As we have seen, the phloem tubes of a plant provide a means of connecting together different parts of the plant. As there is usually a high proportion of sugars and amino acids in the phloem, these are areas of great interest to many insects who derive their food from sucking the juices found inside the phloem of plants. Plant growers use this knowledge to provide a means of attacking and destroying these plant pests with a chemical pesticide. Systemic pesticides will kill pests when the pest takes in the pesticide with its food. Systemic pesticides can be sprayed onto plants. These will be absorbed by the plants and translocated in the phloem vessels. Any pest sucking the juices of a plant that has been contaminated with systemic pesticide will ingest the poison and die.

A PHLOEM TISSUE

 SUMMARY

● A Dicotyledonous root nas root hairs, cortex, phloem and xylem.
● Water and mineral salts are transported by the xylem vessels from the roots to the leaves.
● Xylem vessels are tubes made of dead, holloN lignified cells.
● The structure of xylem is adapted to its two functions of transport and support.
● Water is absorbed by the root hairs in a process called osmosis. The water moves up through the xylem vessels, enters leaf cells andleaves the plant through the stomata.
● Transpiration, the loss of water vapour from the plant, establishes a water potential gradient in the leaf; this is responsible for the movement of water from the roots to the stem and to the leaves.
● Environmental conditions, such as temperature, wind speed, humidity and light intensity, affect the rate of transpiration and the rate of water untake
● A potometer can be used to Compare the rate at which water is taken up by a plant under different environmental conditions.
● Xerophytes, plants living in dry conditions, have adaptatioons hat enable them to survive periods without water.
● Plants have two separate transport systemns: Xylem tubes that are adapted to transport water and mineral salts.
● Phloem sieve tubes that are adapted to transport sucrose and amino acids, in solution.
● Phloem sieve tubes are living, but do not contain a nucleus. They have a companion cell that provides energy to the sieve tube.
● Translocation is the transport of sucrose and amino acids from where they are made in the leaves to other organs where they are used for respiration and qrowth or stored.
● Pesticides are used to kill insects and pests that can damage plant crops.
● Systemic pesticides are carried in the plant phloem.

THE END SEE YOU NEXT POST, posted by Mr DeHaan Ahil.