Translocation And Transpiration (Control of Transpiration)



 TRANSLOCATION is the movement of dissolved substances through a plant. Transpiration is the evaporation of water from the leaves and the subsequent movement of water through the xylem.


In very general terms, water and dissolved salts from the soil travel upwards through the xylem vessels while food made in the leaves passes downwards or upwards in the sieve tubes of the phloem.

  The sucrose produced by photosynthesis in the leaves is carried in the phloem out of the leaf and into the stem. It may then travel up the stem to actively growing regions or maturing fruits and seeds or downwards to the roots and underground storage organs. It is quite possible for substances to be travelling both upwards and downwards at the same time in the phloem.

Mechanism of translocation in the phloem. The mechanism, in fact, is not known though it does depend on the fact that the Sieve tubes, unlike the xylem vessels, contain living cells. Anything which kills the phloem cells, severely interferes with the movement of food. This is illustrated by the experiment in. The leaf makes sucrose from the radio-active carbon dioxide. When the phloem of the stem below the leaf is killed by a jet of steam the substances containing radio-active carbon are found to move up the stem. When the phloem above the leaf is killed, conduction is down the stem. If the phloem above and below the leaf is killed, the radio-active substances do not appear anywhere in the stem. Similarly if the oxygen supply to the phloem is cut off, translocation of sugars ceases.

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  Xylem vessels consist of dead cells which are unaffected by heat or oxygen shortage. If transport of sucrose took place in the vessels, heat treatment of the stem would not be expected to interfere with the movement of sugar.

One of the most widely held theories to account for the movement of solutes in the phloem is the mass flow hypothesis depicted diagrammatically in. The "root cell" could, in practice, be any cell in which sucrose is being used up in respiration or converted to starch and removed from solution.

There are several objections to the mass flow hypothesis. For example, how can it account for the simultaneous movement of substances both up and down the phloem and why must the sieve tube cells be alive for the process to go on?

Movement of salts in the xylem. It can be shown that if a ring of bark and phloem is removed from a stem, the upward movement of salts is little affected. Ifa core of xylem 1S removed, however, the upward movement of salts is arrested. Similarly, killing the phloem by heat treatment does not Significantly affect the upward movement of salts.

The forces moving the salts through the xylem in the trans- piration stream are described below under "Transpiration.

Uptake of salts by the roots. There is, as yet, no wholly Convincing explanation of the uptake of mineral salts from the sOil by r0ots. It may be that diffusion from a relatively high concentration in the soil to a lower concentration in the root cells accounts for some uptake of salts, but it has been shown (a) that salts can be taken from the soil even when their concentration is below that in the roots and (b) that anything which interferes with respiration impairs the uptake of salts. It looks, therefore, as if active transport" plays an important part in the uptake of salts.

Active transport" is itself only a hypothetical process. By expenditure of energy in respiration it is thought that enzyme like substances, carriers, might combine with the salts, carry them across the cytoplasm and release them into the vacuole.


Transpiration is the process by which plants lose water as water vapour into the atmosphere. Most of this loss takes place through the leaves but evaporation also occurs from the stem and flowers.

Turgor pressure in the mesophyll cells forces water outwards through the cells walls. From the outer surface of the cell walls, the water evaporates into the intercellular spaces and diffuses out of the stomata into the atmosphere. Closure of the stomata greatly reduces, but does not entirely prevent, evaporation from the leaf.

Significance of transpiration. The transpiration is probably an inevitable consequence of photosynthesis. For adequate photosynthesis to take place, a large surface area must be exposed to the 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 beneficial to the plant.

(a) Transpiration stream. Evaporation of water from the leaf cells causes their turgor to fall and the concentration of their cell sap to rise and consequently produces a decrease in osmotic potential. Cells in this condition will absorb water from their neighbours and eventually from the xylem vessels in the leaf. Withdrawal of water by osmosis from the xylem vessels produces a tension, i.e. the water is submitted to 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 1s dependent on the rate of evaporation from the leaves.

It is easy to envisage a wire or a string being subjected to tension along its length without breaking but one would expect a column of water under tension to break up, leaving gaps filled with water vapour. The cohesion theory outlined above, however, supposes that the cohesive forces between wata molecules in very thin columns of water are not so easilv overcome. This theory, then, offers an explanation of the movement of water up the stems of plants, Including trees nearly 100 metres high.

A tree on a hot day may evaporate hundreds of litres of water from its leaves. Of all the water passing through the plant, only a tiny fraction is retained for photosynthesis and to maintain the turgor of the cells.

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(b) Transport of salts. The transpiration stream undoubtedly carries mineral salts from the roots to the leaves but the rate of uptake of salts from the soil is not directly dependent on the rate of transpiration.

(c) Cooling. The rapid evaporation of water from the leaf Surface and the consequent absorption of latent heat from the leaf tissues is almost certainly of value in keeping the temperature of a leaf below harmful levels in the direct rays of the sun.

Conditions affecting transpiration rate (a) Light intensity. When 1ight intensity increases, the stomata open and allow more rapid evaporation.

(b) Humidity. When the atmosphere is saturated with water vapour, little more can be absorbed from the plants and transpiration will be reduced. In a dry atmosphere, transpiration will be rapid.

(c) Temperature. A high temperature increases the capacity of the air for water vapour; hence transpiration increases. When the leaf itself becomes warm, evaporation from it occurs more rapidly. Direct sunlight even without a warm atmosphere will have this effect, since the leaf absorbs radiant energy and its temperature rises.

(d) Air movements. In still air, the region round a transpiring leaf will become saturated with water vapour so that no more can be absorbed from the leaf; in consequence transpiration 1s much reduced. In moving air, the water vapour will be swept away from the leaf as fast as it diffuses out, so that transpiration continues rapidly.

Control of transpiration

(a) Stomata. Since most of the water vapour is lost through the stomata, the closure of these will greatly reduce transpiration.

However, there is little or no evidence to suggest that a high rate of evaporation results in the stomata closing, although in extreme conditions where loss of water greatly exceeds uptake, the plant wilts, the cells of the leaf become flaccid (flabby) and the stomata close, preventing further evaporation. Usually the movements of the stomata depend on the light intensity, so that they are generally open during the day and closed at night. Less water vapour is lost during darkness, therefore, when photosynthesis is impossible and carbon dioxide is not needed.

(b) Leaf fall. 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 would tend to go on even though the supply of water would be limited by low temperatures or drought respectively.

(c) Lend shape and cuticle. Leaves with a 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 leaf 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.


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