TRANSPORT IN PLANTS

Learning Objectives
After studying or reading this, you should be able to:

1. Describe the process of uptake and movement of water and mineral salts in plants.

2. Explain the concept of translocation.

3. Demonstrate that transport of synthesized organic nutrients occurs in the phloem.

4. Explain the term translocation.

5. Distinguish between the types of translocation

6. Explain how water is able to move to the apex of trees and herbs.

7. identify the environmental factors that affect transpiration.

8. Determine the rate of transpiration.

9. Explain the concept of guttation.






WATER AND MINERAL SALTS UPTAKE FROM THE SOIL INTO THE PLANT

Water is absorbed mainly by root hairs of plants. Root hairs are tubular extensions of epidermal cells of the root. Slender and flexible, they penetrate between the soils particles and absorb water from the intervening spaces. They are numerous and greatly increase the surface area of the root available for absorption of water and mineral salts. To aid this they lack cuticle and have thin cell walls. All these characteristics are adaptations of roots for absorption of water from the soil. A considerable amount of water is also absorbed beyond the root hair zone towards the apex of the root. Here the surface cells have no cuticle and water is absorbed directly from the surrounding soil water.





Water is drawn into the root hairs mainly by osmosis. On account of sugars and other metabolites, the water potential of the sap is lower than that of the surrounding soil water. Water molecules therefore pass across the cellulose cell wall and protoplast into the vacuole. The low water potential of the root hair or the cytoplasmic content is due to the presence of sugar and other metabolites like protein, carbohydrates and also salts. Water potential gradient therefore exists between the root hairs and the soil water. Water is therefore drawn into the root hairs down the concentration gradient mainly by osmosis. The water potential gradient existing is maintained in two ways: the transpiration pull and presence of carbohydrates in the root.

Movement of water in the root


From the root hair, water passes to the vascular tissues in the centre of the root via the intervening parenchyma cells of the cortex. endodermis, pericycle and finally the xylem cells or vessels and tracheids.
There are three ways water moves in roots:

1. Vacuolar pathway.

2. Symplastic pathway through cytoplasm.

3. Apoplastic pathway through the cell wall.




The vacuolar pathway In this water moves from the vacuole to another through the neighboring cells by osmosis. The water moves down a water potential gradient. Movement along the vacuolar pathway is negligible because of high resistance.


Symplastic pathway through cytoplasm

       This is the system of interconnected protoplasts in the plant. The cytoplasm of neighbouring protoplasts is linked by small pores called plasmodesmata. ln symplAston pathway, water moves from one cell to the other through the plasmodesmata strands and from the cytoplasm of one cell to another.


Apoplastic pathway

        Water moves from one cell to another through the dead cell walls of the plant (except for the casparian strips). This pathway is mainly by diffusion down the concentration gradient. It is a pathway of great importance since it offers the least resistance to water movement.



Generally, movement of water through the symplast and apoplast would be possible if the water potential was lower in the centre of the root than .flirther out. Such a gradient is believed to exist; the water potential being comparatively low in the vascular tissues because of the high concentration of solutes there. 0f the two alternatives, the apoplast pathway is considered the most likely because it would offer the least resistance to the flow of water.



Mechanism of movement of water into the xylem

      The mechanism of movement of water into the xylem is still not clear but there are two possibilities to explain. First, the cortical cell and the endodermal cells may actively secrete salts into the xylem. This will produce a local region of high osmotic concentration (i.e. lower water potential). The water in the endodermis or cortex will therefore move into the xylem by osmosis.



Second, water is possibly pulled into the xylem by cohesive forces of the transpiration pull. Transpiration is the process by which plants lose water as water vapour moves into the atmosphere. Most of this 1055 takes place through the leaves but evaporation also occurs from the stem and flowers. Evaporation of water from the leaf cells causes their tugor to fall and the concentration of their cell sap to rise and consequently produce a decrease in osmotic potential. Cells in this condition will absorb water from their neighbouring cells and eventually from the xylem vessels in the leave. Withdrawal of water by osmosis from the xylem vessels produces a tension, i.e. the water is submitted to the pressures below atmospheric. This tension draws water up the vessels of the stem from the roots. This flow of water is called the transpiration stream and is dependent on the rate of evaporation from the leaves.

The cohesion theory outlined above, however, supposes that the cohesive forces between water molecules in very thin columns of water not so easily overcome. This theory, then, offers an explanation of the movement of water up the stem of plants; including trees nearly 100 metres high. Water also tends to stick to the vessel walls. a force called adhesion. The high cohesion of the water molecules results in a high tension in the column of water. This tension in the xylem vessels built up to a force capable of pulling the whole column of water upwards by means of flow. and water enters the base of the column in the root.

From the root. the water enters the cortex, endodermis, xylem of the stem and finally through the stomata to the atmosphere. The lignifled cellulose cell wall of the xylem provides strength against the high tension of the water in the vessels. This prevents buckling inwards of the xylem vessels.

Water also moves up the plant by capillarity actions. Capillarity is the rise of liquids in small tubes. it occurs because water wets the side of the tube (by adhesion) and it is pulled up by the same when it is believed that water rise up the xylem elements in plant by capillarity.

The Vital Hypothesis also accounts for the upwards movement of water in plants. This hypothesis suggests that living cells like xylem parenchyma are involved in the upwards movement of water. The hypothesis says that these living cells secrete solute (ions) into the xylem vessels. As they do that the water potential would be low and water will move upwards the gradient. The fact that translocation of water had been observed to take place in dead xylem vessels and tracheid does not support this hypothesis.



MINERAL SALTS UPTAKE FROM THE SOIL INTO THE PLANT
        Water is not the only substance that enters the roots by passing into the cells of the root hairs. Members of root hair cells contain a variety of ions transport channels that actively pump specific ion into the plant; even against large concentration gradient. These ions, many of which are plant nutrients, are then transported throughout the plant as a component of the water flowing through the xylem; usually by active transport. Active transport involves the use of energy in the form of ATP to move mineral ions into the root hair cells against a concentration gradient. Once absorbed, the mineral ions diffuse through the cortex cells and enters the xylem with the water. Both water and mineral salts are transported via the xylem vessels to all other parts of the plant. Virtually all of the absorption of water and mineral salts from the soil takes place through the root hairs. These root hairs greatly increase the surface area and therefore the absorptive powers of the roots.





EXPERIMENT TO SHOW WATER UPTAKE FROM THE SOIL INTO THE PLANT
     This experiment can be done using a suitable plant such as balsam. beaker, water, and dye such as eosin. razor blade, microscope and microscope slide.



Procedure

1. Obtain a suitable plant. e.g. balsam, and remove it carefully from the soil, keeping the roots intact

2. Place the plant in a beaker of water stained with a dye such as eosin, so that the mots are completely immersed.

3. Leave the set up for 24 hours. 

4. Using a razor blade carefully cut a thin transverse section of the stem.

5. Mount the section in drop of water on a slide, cover with slide cover and observe under microscope.



Observation
         The xylem vessels become stained indicating that the coloured water moves from the root through the xylem vessel to parts of the plant.


Translocatlon of Organic Solute
         Most of the carbohydrates manufactured in plant leaves and other green parts are moved through the phloem to other parts of the plant. This process. known as translocation, makes suitable mrbohydrate building blocks available at the plant's actively growing regions. The carbohydrates concentrated in storage organs such as tubers. often in the form of starch, are also converted into transportable molecules; such as sucrose and moved through the phloem.

Organic solutes translocated include sucrose, hormones, growth regulators (auxins, gibberellins) and cofactors (NAD, ATP). Generally. substances are translocated from the region where they are manufactured or plentiful to regions where they are scarce. For example, in vascular plants, carbohydrates are translocated from the leaves where they are synthesized to all parts of the plant. Substances are also transported from storage organs to parts of the plant where they are needed. The Places where the substances are manufactured known as source; e.g. leaves and the place Where they are needed as the sinks. e.g flowers, seeds, roots, fruits and storage organs.


In vascular plants, the phloem is the tissue that carries organic solutes from leaves to all parts of the plant. About 90% of the total solutes ' carried in the phloem are the carbohydrate, sucrose. Sucrose is relatively inert and highly soluble. it therefore does not play a direct role in metabolism and so making it an ideal transport sugar, unlikely to be used in transit.





Basic Theories Underlying Translocatlon
          The basic theories that explain the process of translocation in the phloem vessels include pressure flow hypothesis and cytoplasmic streaming.

The pressure flow hypothesis views phloem transport of organic assimilates as a mass or pressure ilow. The theory asserts that during photosynthesis, carbohydrates are produced in the leaves in the translocatable form. The carbohydrate exists as sucrose. Water which has ascends the stem in the xylem vessels is absorbed by the cells of the leaves containing high concentration of the sucrose as a result of osmotic forces.

The absorption of water by leaf cells brings about an increased hydrostatic in these cells. At the same time, there is lowering of concentration of sucrose regions where the assimilates are utilized for growth, storage and respiration. This decrease of sucrose in the sinks results in decrease in hydrostatic pressure. Thus there is a gradient of hydrostatic pressure between the source where the assimilates are produced (green leaves) and sinks where assimilates are used. There is therefore a mass flow or bulk flow from the source to the sinks along the hydrostatic gradient established via the phloem.





PHYSICAL DEMONSTRATION 0F MASS FLOW THEORY
       Two cells, A and B, have membranes which are permeable only through water and are connected by glass tube C. A contains a solution of high osmotic pressure such as sucrose solution and cell B contains only water.

When the two cells are placed in water in a vessel D, water will enter cell A as a result of osmosis. This will create a hydrostatic pressure in cell A and cause its solution to move out of the cell along tube C. This, in turn, will force water out of the second cell B. The process will continue until the concentrations of sucrose in both cells are equal; the flow will therefore cease. Cell A can be likened to a source and B sink. If by some means one could replenish the source in A and at the same time continually remove it from B, then the process will persist as a mass flow from A to B via tube C.



The above system in nature can be likened to a plant where A will represent the supplying cells of the leaves (source) and B the receiving cell of roots or other organs (sinks). The connection between the two cells will represent the phloem.



EVIDENCE IN SUPPORT OF THE MASS FLOW THEORY
   
The Aphid Experiment.
          The exudation which occurs from attached aphid’s mouthparts inserted into the phloem; frequently persists at a flow rate of about 1mm3/hour for two to three days. Weatherly and Peel (1959) asserted that the exudation observed stylets of the aphid known as Tuberdachus saliquus on winnow plant as a result of longitudinal movement of flow of solution known as the phloem sieve tube. When cuts were made into phloem directly above the point of insertion of the stylets, the flow of the exudates ceased. However, if the cuts were made 15cm below the stylets on either sides of the point of attachment then they had no effect on exudation.




The Cytoplasmic Streaming Theory                          

       There is circular movement of the cell protoplasm when viewed under the microscope. This is known ascyclosis. The theory states that ‘As cyclosis occurs it stirs up the sap of the vacuoles and by so doing the sap drips through the sieve pores along a concentration gradient into the next sieve element. There is therefore translocation of solutes in opposite direction due to concentration gradient.



Evidence In Support Of The Cytoplasmic Theory

     Cytoplasmic streaming requires metabolic energy. It has been found that under different conditions of temperature, translocation is affected. The theory gives support to bidirectional flow in the same sieve tube but this occurs in young plants.

Translocation is very important because, obviously, all the cells that are themselves unable to photosynthesize need a share of the products of photosynthesis. Also, the apices of the stem and side branches where cell division and growth are taking place are particularly in needy of nutrients. Transport of food materials to these growing points is greatest in the spring and summer when growth is most prolific. Later in the year many plants form perennating organs (tubers, bulbs, corms, etc.) to which food materials are transported for storage until the following season. When the next season arrives, the stored food is transported in soluble form to the growing points of the new plant.



Translocation of Synthesized Organic Nutrients

       Evidence to show that food passes through the phloem vessel in plants. Evidence that the phloem is responsible for food translocation is obtained from the results of ringing experiments carried out on trees. This involves the removal of a ring of bark and associated phloem from a tree. After a few hours, the sugar content of the tissues above the ring increases. Below the ring, the sugar concentration decreases. This shows that sugars are not able to move through the xylem. See diagrammatic representation of the experiment below.

RING EXPERIMENT TO SHOW TRANSLOCATION OF FOOD IN PLANTS

Translocation Of Food In Phloem







TRANSPIRATION
  Transpiration is a biological process in which water evaporates from a plant; especially through tiny openings called stomata on the surface of leaves. Through transpiration, water is pulled in a continuous stream through the plant, from root to leaf, by capillary action: a wick or suction effect, known as transpiration tension. Much of the water passes through the plant and into the air without being taken into the plant’s cells. Tugor pressure in the mesophyll cells forces water outwards through the cell walls from the outer surface of the cell walls. The water then evaporates into the intercellular spaces and diffuses out of the stomata into the atmosphere.





Advantages of transpiration
    
      All living things need continuing supplies of water to survive. A plant needs water to keep up the internal pressure or turgidity in its cells and tissues (which maintains the plant's shape), to bring in dissolved minerals and raw materials from the soil and for photosynthesis. Transpiration ensures continuous flow of water in the plant for these activities. It also provides the pathway through which mineral  elements are transported in the plant.

For adequate photosynthesis to take place, a large surface area of the leaves must be exposed to atmosphere to absorb sunlight and carbon dioxide. A leaf which is permeable to carbon dioxide will also be permeable to water Vapour. It seems, therefore, that evaporation of water must inevitably accompany photosynthesis. Nevertheless. transpiration produces effects which may be regarded as benencial to the plant and humans. For example, transpiration tends to cool the leaves, an important effect particularly in hot conditions. To humans, the process of transpiration within the body provides a cooling effect. On a hot day, for example, the leaves of a large oak can lose 200 litres (53 gallons) in an hour through transpiration. One hectare of tropical forest releases 200,000 litres (53,000 gallons) of water vapour daily into the atmosphere. This is 20 times the rate of direct evaporation from a lake or sea. in this way, transpiration returns massive volumes of water from the ground to the atmosphere and is therefore a very important part of the general water cycle on Earth.




Disadvantages of transpiration

Transpiration is the inevitable result of the necessity for the inside of a leaf to be opened to the atmosphere. This phenomenon results in the loss of water from the plant through excessive evaporation. When a plant loses more water through transpiration than it can take up into its roots, it wilts and is said to suffer from water stress.







TYPES OF TRANSPIRATION

The stomata, as well as permitting the entry of carbon dioxide, allow evaporation of water from the plant, a phenomenon known as transpiration. Transpiration is by no means restricted to the leaves for there are generally a number of stomata in the stem epidermis as Well. However, the leaves with their large surface area and abundant stomata represent the main source of water loss. This is known as stomata] transpiration. To a small extent evaporation also takes place through the cuticle of the epidermal cells, a process known as cuticular transpiration, but this rarely exceeds five percent of the total water loss. Plants, living in humid conditions such as a tropical jungle have numerous stomata. In such plants the humidity of the atmosphere may be so high that more water accumulates in the leaves than can be lost by evaporation. Under this circumstance water may drip from the leaves. This is known as guttation; which even in temperate regions may be observed on warm, humid days. The water either oozes out of the stomata or, more usually, it is excreted by special glandular structures called hydathodes situated at the edges of the leaves.




EXPERIMENT TO SHOW THAT TRANSPlRATlON OCCURS THROUGH THE LEAVES OF PLANTS

The shoot of a recently watered potted plant, or a plant in the garden, is completely enclosed in a transparent, polythene bag which is tied around the base of the stem. The plant is allowed to remain for an hour or two in direct sunlight. The water vapour transpired by the plant will soon saturate the atmosphere inside the bag and drops of water will condense on the inside. The bag is removed and all the condensed water shaken into a corner so that it can be tested with anhydrous copper sulphate. A controlled experiment is set up using a shoot in a similar situation but from which all the leaves and flowers have been detached. The results will show that the water collected in the polythene bag changes the anhydrous copper sulphate blue, indicating presence of water. In the controlled experiment, no water will be collected in the polythene bag since the leaves have been detached or removed. Hence, transpiration occurs through the leaves.







PHYSIOLOGICAL FACTORS AFFECTING THE RISE OF WATER IN XYLEM


Many of the leaves of a large tree may be more than ten stories off the ground. How does water manage to rise so high? Factors responsible for this phenomenon include cohesion-adhesiontension mechanism, transpiration pull, root pressure and water potential gradient.



Coheslon-adhesion-tension mechanism

    The stream Of relatively dry air causes water molecules to evaporate from water surface in a tube. The water level in the tube does not fall because as water molecules are drawn from the top, they are replenished by new water molecules forced up from the bottom. This, in essence, is what happens in plants. The passage of air across leaf surface results in the lost of water from the leaf by evaporation, creating a 'pull' at the open upper end of the ‘tube’. Meanwhile, new water molecules entering through the roots of the plant are pushed up by atmospheric pressure. in addition to these pushing and pulling forces, the adhesion of water molecules to the walls of the very narrow tubes that occur in plants also helps to maintain water flow to the top of plant.

The column of water in a tall tree does not collapse simply because of its weight but because water molecules have an inherent strength that arises from their tendency to form hydrogen bonds with one another. These hydrogen bonds cause cohesion of the water molecules; in other words, called tensile strength. It varies inversely with the diameter of the column; that is the smaller the diameter of the column, the greater the tensile strength. Therefore, plants must have very narrow transporting vessels to take advantage of tensile strength.

How the combination of the forces of gravity, tensile strength and cohesion affect water movement in plants is called the cohesionadhesion-tension theory.






TRANSPIRATION PULL


The cohesion-adhesion-tension theory explains the process by which water leaves a plant; a process known as transpiration. More than 90% of water taken in by plant roots is ultimately lost to the atmosphere and almost all of it from the leaves. It passes primarily through thestomata in the form of water vapour. On its journey from the plant’s interior to the outside, a molecule of water first passes into the pockets of air within the leaf by evaporating from the walls of the spongy mesophyll that lines the intercellular spaces.

These intercellular spaces open to the outside of the leaf by way of the stomata. The water that evaporates from these surface of the spongy mesophyll cells is continuously replenished from the tips of the veins in the leaves; because the strands of xylem conduct water within the plant in an unbroken stream all the way from the roots to the leaves, when a portion of the water vapour in the intercellular spaces passes out through the stomata, the supply of water vapour in these Spaces is continually removed.


Stomatal and cutlcular transpiration of a leaf





Root Pressure and Water Potential Gradient

         This is observed and measured when a freshly cut wood stump continues to exude sap from its vessels and tracheids. It is a hydrostatic force which develops in the root as a result of osmotic potential. The mechanism depends on active creation of salt and other solutes in the xylem sap; thus lowering water potential.

Water then moves into the xylem by osmosis from the neighbouring root cells in the roots. Root pressure of about 5 to 6 bars has been recorded, although in most species, values do not exceed one bar. Root pressure is inhibited by respiratory inhibitors like cyanide, lack of oxygen and low temperature. Root pressure appears in most plants at night when ample water is present in the soil and the humidity is high; that is when transpiration is very low or virtually absent. In such conditions, root pressure can be sufficient to cause guttation. Guttation is the loss of water as drops of liquids at the tip of leaves of plants. This is mostly observed in rainforest species and also tops of young grass seedlings. Root pressure and water potential gradient can therefore be responsible for the rise of water in the xylem of a plant.


Experiment to demonstrate physlological factors that affect ascent of water In the xylem





ENVIRONMENTAL FACTORS AFFECTING RATE OF TRANSPIRATION
   The environmental factors that affect transpiration rate include light intensity, relative humidity, temperature, air movement, atmospheric pressure and water supply. When light intensity increases the stomata Open and allow more rapid evaporation. When the atmosphere is saturated with water vapour, little more can be absorbed from the plant and transpiration will be reduced. In a dry atmosphere, transpiration will be rapid due to the transpiration gradient existing between the atmosphere and the leaves of plants.

Air movements also affect transpiration. in still air, the region round a transpiration leaf will become saturated with water vapour so that no more can be absorbed from the leaf; as a result, transpiration is much reduced. In moving air (wind) the water vapour will be swept away from the leaf as fast as it diffuses out, so that transpiration continues rapidly. A high temperature provides latent heat of vapourization and therefore encourages evaporation from the mesophyll cells.

Atmospheric pressure also influences the rate of transpiration in plants. The lower the atmospheric pressure, the greater the rate of evaporation. Plants, such as alpine, living in high altitudes where the atmospheric pressure is comparatively low are likely to have a high rate of transpiration, and many of them therefore have adaptations for preventing excessive water loss.


Another factor which affects rate of transpiration is water supply. Transpiration depends on the walls of the mesophyl cells being thoroughly wet. For this to be so the plant must have an adequate water supply from the soil. if for some reason the plant cannot take up water from the soil (for example if it is too dry), sooner or later the stomata close; thus reducing the rate of transpiration.



EXPERIMENT TO DETERMINE THE RATE OF TRANSPIRATION IN PLANTS

     The cobalt thiocyanate is useful as a rough demonstration of transpiration but it gives no quantitative information of the rate. This can be estimated by measuring either the rate at which the plants lose mass or the rate at which they take up water. In the first case it assumed that loss in mass is due only to evaporation of water. In the second case it is assumed that water up take is equal to water loss. The first experiment can be done by placing a potted plant on an automatic balance and measure the changes in mass over a given period of time. The whole of the plant must be enveloped in a polythene bag to prevent water from evaporating from the soil. The soil must be well watered before so that the plant has a plentiful supply of water for the duration of the experiment. Changes in the rate of transpiration are determined by recording the mass at interval and plotting the results on a graph. The total leave area is then estimated by tracing the outline of the leaf on a squared paper and counting the number of squares within each leaf. In this way all the data necessary to express the rate of transpiration as loss in mass per unit time per leaf area would be available. As might be expected, the results depend on numerous factors such as the type of plant used and the environmental conditions such as temperature, humidity and so on.

It is sometimes desirable to measure the evaporating power of the atmosphere to see how this correlates with water loss from a plant. For this, an atmometer may be used. This is similar to a photometer but instead of a plant, a porous pot is used as the evaporating surface. By running a photometer and an atmometer side by side, one can compare changes in the rate of evaporation from a plant with that of a purely physical system.


Experiment to determine the rate of transpiration in plants




Control of transpiration.
         As water is often in short supply, most plants control transpiration. A leaf's surface layer, the epidermis, has a waxy, waterproof coating (the cuticle) to minimize evaporation through it. inside the leaf is the parenchyma; a spongy layer of cells and air spaces. Water loss actually occurs here, as water evaporates from cell surface into the air spaces and then passes (as water vapour) through microscopic leaf pores or stomata in the epidermis to the outside. Each stoma is flanked by two specialized guard cells. A guard cell's inner (stoma-side) wall is thicker and stiffer than its outer wall. When a plant begins to wilt through too much transpiration, the guard cells become floppy or flaccid with lack of water. A flaccid guard cell is straight so the stoma between them is closed to a thin slit.

When thousands of stomata shut on a leaf, the rate of transpiration is drastically reduced. The plant continues to absorb water through the roots. This replenishes the plant cells which swell and become turgid. As the guard cells swell, the thin, outer wall stretches more easily than the thick, inner wall so they become curved, making the stoma between them wider and rounder. More water vapour passes through the Open stomata and transpiration increases until the plant begins to wilt again. This self-adjusting system regulates the plant's water loss by transpiration. The concentration of the plant hormone abscisic acid also increases when the leaves of a plant wilt as it causes the guard cells to close the stomata.
Stomata allow gas exchange: water vapour out and atmospheric carbon dioxide in for photosynthesis. Closed stomata reduce transpiration and limit photosynthesis and so a plant must balance these two processes. Most plants have stomata only on the underside leaf surfaces, which lose less water than the sunny upper surfaces. Also, stomata open to allow gas exchange late at night and during the morning, becoming smaller at midday and during the afternoon.


Leaf fall is another mechanism used by plants to reduce transpiration. In temperate climates, deciduous trees shed their leaves in winter; those in the tropics may shed theirs in the dry season. If the leaves were retained, transpiration will tend to go on even though the supply of water will be limited by low temperature or drought respectively.

Leaf shape and cuticle also affect the rate of transpiration in plants. Leaves with small surface area will transpire less rapidly than the broad, flat deciduous leaves. Waxy cuticles and stomata sunk below the epidermis level, e.g. Oleander, are also modifications thought to be associated with reduced transpiration. They are often found in plants which grow in dry or cold conditions or in situations where water is difficult to obtain. Most evergreen plants have one or more of these characteristics and this probably plays a part in their retention of leaves during the winter months in temperate climates and dry period in the tropics.






BIOLOGICAL PRINCIPLE UNDERLYING GUTTATION.
          Active transport of ions into the root results in an increasingly high ion concentration within the cells. This causes water to be drawn into the root hair cells by osmosis. In terms of water potential active transport increases the solute potential of the roots. This results in the movement of water into the plant and up the xylem column. This phenomenon is called root pressure.

Root pressure, which is primarily active at night, is caused by the continued, active accumulation of ions by the roots of a plant at times when transpiration from the leaves is very low or absent. When root pressure is very high, it may force water up to the leaves; from which the water may be lost in a form through a process known as guttation. Guttation is the loss of water as drops of liquids at the tip of leaves of plants. Guttation takes place not through the stomata but through special groups of cells that are located near the ends of small veins that function only in this process. Root pressure is not sufficient to push water up great distances and guttation thus takes place only in relatively short plants.






Did you understand what you have just read or studied?

To make sure you really UNDERSTOOD, ANSWER the QUESTIONS below:

1. Explain how either active transport or diffusion may be responsible for the uptake of mineral ions by root under different circumstances.



2. Define the following terms:
a. Transportation
b. Transpiration stream
c. Transpiration pull
d. Root pressure
e. Capillarity


5. Explain how water is able to reach the topmost branches of very tall trees.

6. What effect would the following environmental factors have on the rate of transpiration:
a. High air humidity
b. High air temperature
c. High light intensity

7. What are the advantages of transpiration to:
a. plants?
b. animals?

8. Name three environmental factors which can affect loss of water in plants.

9. a. Which two gases are used by a leaf in sunlight.
b. For each gas, state one way in which it is used.

10. Explain why ‘ringing’ a shoot provides evidence for what translocation of solutes takes place in the phloem.

11. If you make a tiny hole in a xylem vessel, would you expect water to come out or air to go in? Explain your answer.

12. Design an experiment to show that organic nutrients are transported through the phloem tissues.

13. Describe an experiment to show that:
a. transpiration occurs through the leaf of a plant.
b. root pressure could be responsible for the rise of water in the xylem of a plant.

14. Distinguish between transpiration and guttation.

Comments