Form 6 Biology – TRANSPORTATION

TRANSPIRATION

Definition:

Transpiration is a process whereby a plant loses water from the epidermal cells of the leaves in the vapour form.

TYPES OF TRANSPIRATION

• There are three types of transpiration:-
  

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(a) Stomatal transpiration

• This is a major way by which water evaporates from the plant leaves. It is a type of transpiration where by the plant loses water through the stomatal pores.
  

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(b)Cuticular transpiration

• This involves loss of water through the cuticle. In this way a very little amount of water is lost from the plant because the cuticle among other functions restricts water loss from the plant.
  

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(c)Lenticular transpiration

• This involves loss of water by the lenticels.
  
• The latter are small slits in the stems and bark of trees for gas exchange.
  

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Significance of transpiration in plants:

Transpiration is considered to be “Necessary evil”. This is because it is an inevitable but potentially harzadous process. It thus, has beneficial and harzadous effects.

1. Beneficial effects
   

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Transpiration is necessary in that;

1. It is a means of transportation of water and dissolved mineral salts through a plant.
   
2. It is a means of cooling the plant ie: evaporation of water from the surface of the plant eg: leaves has a cooling effect.
   
3. It is a means of removal of excess water as waste product.
   
4. Aids uptake of water and mineral ions.
   
5. Hazardous effects
   

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• Transpiration is an “evil” process because excessive transpiration (loss of water) leads to dehydration of cells.
  
• It also interferes with the process of photosynthesis, excretion, respiration etc. all of which require water.
  
• As a consequence of excessive water loss the plant wilts and finally dies.
  

  STRUCTURE OF A STOMATA
  

• Structurally, the stomata pore is bordered by two sausage shaped guard cells. The latter have their inner walls being thick and less elastic whereas, the outer wall is thin and elastic (extensible).
  
• The guard cells have chloroplast capable of photosynthesis. Around the guard cells are the epidermal and subsidiary cells.
  

  Diagram:
  

  
  

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Mechanism of stomata closure and opening:

The closure and opening of the stomatal pore is caused by change in turgor pressure of the guard cells. If water is drawn into catalyses the hydrolysis of ATP into ADP and Pi and energy is released.

• The released energy used to pump K⁺ ions into the guard cell and H⁺ ions out of the guard cells. This also causes the inside of the guard cell to be alkaline.
  
• The accumulation of K⁺ ion and glucose into the guard cells results into increased osmotic pressure in there.
  
• The result of this increased osmotic pressure is the osmotic movement of water in to the guard cells from the epidermal cells.
  
• Turgidity of the guard cells result into opening of the stomata aperture.
  
• On contrary during the night, K⁺ ions are pumped out of the guard cells and H⁺ ions are pumped in. There is also an accumulation of CO₂ in the intercellular spaces.
  
• This result into increased acidity of the guard cells ie: Fall in the pH value. This fall in pH value favours the association of glucose forming starch in the guard cells while in the surrounding epidermal cell K⁺ ions (allophone) causes the accumulation of glucose.
  
• The net effect is the osmotic movement of water from the guard cells to the epidermal cells. Thus loss of water from the guard cells causes them flaccid and hence closure of the stomatal pore.
  

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Illustration:

• Stoma is closed in the dark, but in the presence of light ATPase is stimulated to convert ATP to ADP and so provide the energy to pump out H⁺ from the guard cells. These protons return on a carrier, which also bring Cl^– with it. At the same time K⁺ enter guard cells.
  
• As a result of this influx of ions, the water potential of the guard cells becomes more negative (lower) causing H₂O to pass in by osmosis. The resultant increase in pressure potential causes the stoma to open.
  
• In the dark, the movement of ions and H₂O is reserved.
  

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Question:

Describe the mechanism of stomatal closures and stomatal opening based on the osmotic pressure (Pressure flow) hypothesis.

• The guard cells, the latter become turgid and stomatal pore opens. And when the cells are flaccid, the stomatal pore closes. The guard cells, the latter become turgid and stomatal pore opens. And when the cells are flaccid, the stomatal pore closes.
  
• The guard cells have thicker inner inelastic walls and thinner elastic outer walls. During expansion they do not expand uniformly in all directions.
  
• The thick and less elastic inner walls are less pulled out wards leaving an open between them.
  

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How is the mechanism explained?

1. A traditional hypothesis; the starch-sugar hypothesis suggested that an increase in sugar concentration in guard cells during the day led to their solute potential becoming more negative, resulting in entry of water by osmosis.

However, sugar has never been shown to build up in guard cells to the extent necessary to cause the observed changes in solute potential.

K⁺ ion and osmotic pressure theories:-

It has now been shown that potassium ions and associated negative ions accumulate in the guard cell during the day in response to light and are sufficient to account for the observed changes.

In darkness, potassium (K⁺) ions move out of the guard cell into surrounding epidermal cells. The water potential of the guard cells increases as a result and water moves out of the cells. The loss of pressure makes the guard cells change shape and stoma closes.

What causes K⁺ to enter the guard cells in the light?

Ans: K⁺ may enter in response to the switching on of an ATPase which is located into the cell surface membrane which pumps out H⁺ and K⁺ may then enter to balance the charge.

More explanations:-

• During the day, the plant photosynthesizes by consuming CO₂.
  
• This reduces the concentration of CO₂ in the intercellular spaces of the leaf.
  
• This lowers the level of Carbonic acid and hence a rise in pH value ie: The cells become more alkaline.
  
• This favours the conversion of starch into glucose which accumulates in the guard cells. At the same time the enzyme ATPase.
  

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2. Using carbon – 14 isotope

• If a plant with a ringed stem is supplied with CO₂ containing C-14 isotope ie: ¹⁴CO₂, the food substances accumulated above the ring appear to contain C-14. This suggested that the synthesized food is translocated through the phloem. 
  

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3. Using mouth parts of a feeding aphid

• An aphid is an insect that uses its tubular needle – like mouth part to feed on the sugary solutions from the phloem sieve tubes.
  
• If the feeding insect is anaesthesized with CO₂ and the mouth parts are carefully cut so that the tube remains inserted into the phloem vessel, the food substances continue to move through the tubular needle of aphid.
  
• Analysis of this solution reveals the presence of sugary substances and amino acids all of which are the products of photosynthesis.
  

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1. There are diunal variations in the concentrations of the glucose which are in turn reflected in the phloem sieve tubes.
   

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Mechanism of Translocation by the phloem:

There is no one agreed mechanism by which food substances are translocated through the phloem. However there are various hypotheses that try to describe the mechanism of phloem translocation. They include:

A. Mass flow hypothesis (Münch 1930)                                    

This is also called Münch’s hypothesis or pressure flow hypothesis. According to this hypothesis, food substances are translocated through the phloem in a mass flow mechanism.

Consider the munch model bellow:

 

• Could the ions reach the xylem entirely by means of the apoplast pathway?
  

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Ans: No, the endodermis is a barrier to the movement of water and solutes through the apoplast pathway. This is due to the presence of casparian strips which prevents further progress

• To cross the endodermis, ions must pass by diffusion or active transport through the cell surface membranes of endodermal cells, entering their cytoplasm and possibly there vacuoles. Thus the plant monitors and controls which type of ions eventually reach the xylem.
  

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Ions can also move through the symplast pathway. The final stage in the movement of mineral salts across the root is the release of ions into the xylem.

Once in the xylem, they move by mass flow throughout the plant in the transpiration stream.

The chief sinks, ie: Regions of use, for mineral elements are the growing regions of the plant, such as the apical and lateral meristerms, young leaves, developing fruits and flowers and storage organs.

Translocation of the manufactured food:

• In higher vascular plants, food substances are translocated through the phloem.
  

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Evidence to show that phloem translocates food:

(i) Ringing experiment.

A ring of tissue containing phloem was removed from the outer region of the stem, leaving the xylem intact. It was found that the leaves did not wilt, but growth below the ring was greatly reduced. This is because, movement of sugars down the plant were stopped without affecting passage of water upwards.

Description of the model

• In the model, there is an initial tendency of water passing by osmosis into A and C. However, the tendency is greater for A than for C because the solution in A is more concentrated than that in C.
  
• As water passes into A, a pressure potential (hydrostatic pressure) builds up in the closed system A-B-C forcing water out of C.
  
• Mass flow of solution occurs through B along the hydrostatic pressure so generated.
  
• There is also an osmotic gradient from A to C and eventually the system comes into equilibrium as water dilutes the contents of A and solutes accumulate at C.
  

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Application of the model to the living plant

• The leaves which make sugars during photosynthesis are represented by A. The synthesized sugars, lower the water potential of the leaf cells and consequently this fuses the flow of water into the leaves by osmosis through the xylem (D).
  
• Due to hydrostatic pressure generated into the phloem (B), food from the source (A) to the sinks such as roots and storage organs (C) are transported in a mass flow system.
  
• In the plants, equilibrium is not reached because sugars are constantly being made at sources (A) and constantly being used at sinks (C).
  

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Critique
(weakness)
to the hypothesis

1. It is purely physical explanation and so does not explain why sieve tubes must be living and metabolically active.
   
2. It does not explain the observation that the leaf cells are capable of loading sieve tube against the concentration gradient. ie: The fact that the Ψ_s of sieve tubes is more negative than that of the leaf cells. The hypothesis
   

   has therefore been modified to include an active loading mechanism of solutes into the sieve tubes. The osmotic and hydrostatic pressure gradient therefore starts in the tubes rather than in the photosynthetic cells. It is
   

   also believed that unloading at the sinks is an active process.
   

3. It ignores the membrane barriers between the sieve tubes and the plastids.
   
4. It assumes an empty sieve lumen and fully open sieve plate pores. 
   

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(B) Transcellular strands hypothesis (THAINE).

• The hypothesis was described by Thaine. It explains the role of phloem proteins in the translocation of food.
  
• According to Thaine, the protein fibrils that run from one end of the sieve tube to the other are the ones that carry food substances.
  
• The food substances pass along these fibrils due to the peristatic action of the protein sheath in a manner resembling cytoplasmic streaming.
  
• This is an active transport and it accounts for transportation of materials in both directions in the same sieve tube.  
  

  Ideas of the hypothesis are summarized as;
  

  (i) Food is transported by phloem protein, due to peristatytic action the food flows along the fibres.
  

  (ii) Food is transported in both directions.
  

  (iii) Food is transported actively.
  

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(C) ELECTRO – OSMOSIS HYPOTHEIS (SPANNER)

• According to Spanner, the flow of food is produced and maintained by electro-osmotic force set up across the sieve plates.
  
• According to this hypothesis, K⁺ ions are actively transported across the sieve plates, carrying with them water and dissolved mineral salts.
  
• This means that K⁺ ions create an electric potential gradient as a result of which water molecules flow through the sieve plates carrying the sucrose molecules with them.
  

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(D) SURFACE SPREADING HYPOTHESIS

In this hypothesis, the idea is that the solute molecules might spread over the interface between two cytoplasmic materials as oil spreads over an air-water interface the form of bands called Casparian strips. Therefore water and solutes particularly salts in the form of ions, must pass through the cell surface membrane and into the living part (cytoplasm) of the cells of endodermis. In this way the cells of the endodermis can control and regulate the movement of solutes through the xylem. Such control is necessary as a protective measure against the entry of toxic substances, harmful disease-causing bacteria, fungi etc.

As roots get older, the extent of suberin in the endodermis often increases. This blocks the normal exit of water and mineral salts from the cell     

Uptake of mineral salts and their transport across roots.

In plants, minerals are taken up from the soil or surrounding water by roots. Uptake is greatest in the region of the root hairs.

Note:

• Mineral elements exist in the form of salts which are made up of ions, and in solution the ions can separate (dissociate) and move freely.
  
• Ions can cross membranes in a number of ways, including:-
  

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1. Active uptake (transport)- In which ions are taken up into cells against a concentration gradient using energy from respiration.
   
2. Passive uptake– Where ions move by mass flow and diffusion through the apoplast.
   

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The figure below shows the uptake of K⁺ ions by young cereal roots which had previously been thoroughly washed in pure water. After 90 minutes respiratory inhibitor potassium cyanide was added to the solutions.

                                                                                         

Figure above shows that two distinct phases of uptake. The 1^st phase lasts for about 10 -20 minutes. Uptake during this phase is relatively rapid. K⁺ ions come into contact with the epidermis of the root and start to move through the cell walls of the apoplast pathway, it is shown that this phase is more or less independent of temperature since it occurs just rapidly at 0^oC. It is passive process.

• The 2^nd phase is temperature dependent and does not occur at 0^oC when the rate of metabolism and respiration is very low. Its inhibition by KCN shows that it is dependent on respiration and the uptake at this time
  

  probably by active transport across all membrane into cells.
  

• Why the roots were thoroughly washed before placing them in a solution containing K⁺ ions?
  

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Answer:

To flush out any existing K⁺ ions from the root

• It is shown that if the carrot discs are transferred from pure water to KCl solution, the rate of respiration increases. Why?
  

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Answer:

Rise in respiratory rate is accompanied by a rise in KCl uptake. Once KCl is available, it is therefore apparently taken up by active transport, the energy being supplied by an increased respiratory rate.

• If KCN is added, the rise in KCl stops, this is because KCN inhibits respiration and therefore inhibits active transport of KCl into the carrot discs.
  

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GUTTATION

• Guttation is a physiological process of the plants in which water is lost in form of liquid / droplets.
  
• The process occurs in members of the grass family, and in species found at the leaf margin and apex.
  
• Guttation is favoured by those factors that favour low rate of transpiration eg: High humidity, low temperature, absence of light etc.
  

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Question. Summarize differences between guttation and transipiration.

Ans:

Evidence to show that xylem transports water

The evidence for water transportation in the xylem comes from the following observations:-

1. If a red dye such as cosin is dissolved in water and a cut end of the stem is immersed in that water, the plant takes water. After a time lag, the red dye is traced in the xylem vessels. That shows that xylem transports water.

2. If molten fats are added into water having a plant example; Potted plant, as the plant absorbs water, it takes some fats too. The latter block the xylem vessels resulting into wilting of the plant.

3. Ringing experiment

If a part of stem is ringed to remove the phloem, the plant does not wilt. However, if the tissues beneath the phloem are removed, the plant wilts showing that the removed tissues are xylem.

Question. Summarize the properties of xylem which make it suitable for the long distance transport of water and solutes.

Answer

1. Long tubes formed by fusion of neighboring cells, with breakdown of cross walls between them.
   
2. No living contents, so less sensitive to flow.
   
3. Tubes are rigid, so do not collapse.
   
4. Fine tubes are necessary to prevent water columns from collapsing.
   

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Uptake of water by roots

Water moves across the root by pathways similar to those in the leaf namely apoplast, symplast and vacuolar pathways.

Symplast and vacuolar pathways

As water moves up the xylem in the root, it is replaced by water from neighboring parenchyma cells. As water leaves cell A, the water potential of cell A decreases and water enter it from cell B by osmosis or through the symplast. Similarly, the water potential of cell B then decreases and water enters it from cell C and so on across the root to the epidermis.

The soil solution has a higher water potential than cells of the epidermis including the root hairs. Water therefore enters the root from the soil by osmosis.

Apoplast pathway

The apoplast pathway operates in much the same way as in the leaf. However, there is one important difference. When water moving through spaces in the cell walls reaches the endodermis, its progress is stopped by a water

proof substance called Suberin which is deposited in.

Question. Why does transpiration occur mainly through leaves and not so much through the cuticle and lenticels?

Answer:

• Leaves contain a very large number of stomata for gaseous exchange and there is little resistance to movement of water vapour through these pores.
  
• Leaves have a large surface area (for trapping sunlight and exchanging gases). The greater the surface area, the greater will be the loss of water by transpiration.
  

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FACTORS AFFECTING THE RATE OF TRANSPIRATION

• The factors that affect the rate of transpiration are of two main categories;
  

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1. External (Environmental) factors.
   
2. Internal (plant) factors.
   

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A: External factors

(i) Light

The rate of transpiration is high during the day. This is because the stomata pores get open due to turgidity of the guard cells. Thus, to night when the stomatal pores are closed, only lenticular and cuticular transpiration occur.

(ii) Temperature

High temperature favours the rate of water loss from the mesophyll cells. High temperatures do also lower humidity of air around the leaf. All these favour loss of water from the leaf to the surrounding area.

(iii) Humidity and vapour pressure

Low humidity around the leaf favours transpiration, because it results into a steeper diffusion gradient of water from the leaf atmospherer to external atmosphere.

(iv) Wind (Air currents)

If the air is still, the rate of transpiration becomes low. This is because the humidity of the atmosphere is high. If air is in motion (in windy situation) the rate of transpiration is high. This is because the blowing wind sweeps away the water vapour concentrated around the leaf surface thereby lowering the humidity and hence favoring high rate of transpiration.

(v) Availability of soil water

The rate at which the plant loses water by transpiration depends in the amount of water available in the soil. If the soil has insufficient amount of water, the rate of transpiration gets reduced as in decidious trees that shed their leaves in the dry season.

B: Internal factors

The plant factors include the following;

1. Surface areas to volume ratio – The greater the surface area to volume ratio, the greater is the rate of transpiration, since broad leaves have high transpiration rate than narrow leaves.
   
2.  Cuticle (water proof material)-The thinner the cuticle, the higher the rate of transpiration and vice versa.
   
3. Stomata
   

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(a) Size of the stomatal pore – The larger the size of the stomatal pore, the higher the rate of transpiration and vice versa.

(b) Number of the stomatal pore – The greater the number of stomata, the higher the rate of transpiration.

(c) Density of stomata

• The higher rate of transpiration occurs at the upper side of the leaf because it is at this side where stomata are directly exposed to light energy.
  

  Question: Describe the factors that affect the rate of transpiration.
  

   The molecular film so formed could be kept moving by molecules being added at one end removed at the other.
  

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(E) ACTIVE TRANSPORT HYPOTHESIS

• This suggests that, the translocation of food through the phloem involves some sort of active mechanisms. This is supported by the facts that;
  

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1. The phloem tissue has a high rate of respiration and there is a close correlation between the speed of transduction and metabolic rate.
   
2. Lowering temperature and treating the phloem with metabolic poisons, also lower the rate of translocation. This means that the enzymes involved in the production of energy are affected.
   

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Question:Describe the various hypotheses of the phloem translocation.

Xerophytic adaptations

• Xerophytes are plants which have adapted to conditions of unfavourable water balance. This is the condition where the rate of loss is potentially greater than the availability of water.
  
• Mesophytes are plants which have adapted to conditions where water is available.
  
• Halophytes are plants that live in salt marshes where the concentration of salts in the soil makes it difficult to obtain water. Halophytes also exhibit Xeromophic features.
  
• Xerophytes plants have evolved a wide range of features designed to reduce the rate of transpiration. These are known as Xeromophic features.
  

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Xeromophic adaptations take three general forms:

1. Reduction in the transpiration rate – clearly anything which lowers the rate of transpiration helps to conserve water when in short supply.

2. Storage of water – Plants living in areas where water supply is intermitted, store water for use during periods of drought. Plants which store water are termed Succulents.

3. Resistance to desiccation – Some species exhibit a remarkable tolerance to water loss and resistance to wilting.

Xeromophic adaptations of plants

1. Features for reduction of the transpiration rate:-

1. Thick cuticle – Reduces cuticular transpiration by forming a waxy barrier preventing water loss.
   
2. Rolling of leaves – Preventing water diffusing out through stomata which are confined to the inner surface.
   
3. Layer of protective hairs on leaf – Moist air is trapped in the hair layer, so reduce transpiration rate.
   
4. Absence of leaves – Reduces the rate of transpiration.
   
5. Orientation of leaves – The positions of leaves are constantly changed so that the sun strikes them obliquely. This reduce their temperature hence rate of transpiration.
   
6. More negative water – This makes it more difficult for water to potential of the cell sap, be drawn from them.
   

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2. Futures for succulence (water storage)

1. They have succulent leaves which stores water.
   
2. They have succulent stems which stores water.
   
3. Closing of stomata during day light, so reducing transpiration rate.
   
4. Shallow but extensive root systems – This allows efficient absorption of water over a wide area.
   

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3. Features for resistance to desication

(a) Reduction of transpiration surface through loss or adaption of leaves.

(b) Lignification of leaves – Preventing it from wilting in times of drought.

(c) Reduction in cell size – Making the plants less liable to wilt.

Hydrophytes adaptations

• Plants living in wholly or partly submerged in water are called hydrophytes.
  
• The greatest problem for hydrophytes is to obtain oxygen for respiration.
  

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Adaptations

(i)Plants have aeration tissue (Aerenchyma) which comprises large air spaces called Lucunae between the cells of the stem and leaves. These stores oxygen produced by photosynthesis which can be used for respiration.
          
           (ii) Plants can tolerate high level of ethanol which is a product of anaerobic respiration.   
     
          (iii) Aerating tissue confers buoyancy, raising leaves to the surface where they can take  maximum advantage of the light.

          (iv) They lack supporting tissue (water provides support) which would make the plant more rigid, rendering it liable to breakage by water currents.

Evidence supporting the role of xylem in transporting minerals

1. The presence of mineral ions in xylem sap.
   
2. A similarity between the rate of mineral transport and the rate of transpiration.
   
3. Experiments using radioactive tracers show that where lateral transfer of minerals can take place, minerals pass from the xylem to the phloem.
   

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NOTE:

• The xylem transports water and dissolved mineral salts from the roots to the leaves, and phloem transports sugars and other products of photosynthesis from leaves to other parts of the plants.
  
• The fascinating thing is that two systems employ quite different principle. Xylem transport is essentially a passive process, depending mainly on water potential gradients within the plant. Indeed, the xylem tissue in which it takes place is composed of dead cells. Phloem transport on the other hand is an active energy requiring process which takes place in living tissue.
  

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Factors affecting the rate of translocation

1. Temperature – Increase in temperature up to a maximum of about 35^o C, increases the rate of translocation probably by affecting the enzymes involved in the secretion and removal of sucrose from the tubes.
   
2. Light– Translocation to the roots is greatly enhanced in the dark.
   
3. Metabolic inhibitors– Hydrogen cyanide and dimtrophenol inhibit carbohydrate translocation.
   
4. Concentration gradients – Carbohydrate seems to move from regions of higher concentration to regions of lower concentration.
   
5. Mineral deficiencies- Boron seems to be important in forming an ionisable complex with sucrose which then passes more easily through the cell membranes. It also appears to slow down the enzymic conversion of glucose-1-phosphate to starch thus keeping more sugar available for translocation.
   
6. Hormones – Cytokinins, IAA and gibberellins appear to at best control translocation, probably by their effects on metabolic rates at the source and sink.
   

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