It is the loss of water in the form of vapours from the living tissues of aerial parts of plant is termed as transpiration. This physiological phenomenon occurs in plants and living cells are involved in it. Various factors influence this process such as CO2, pH, hormones etc.

Types of Transpiration

  1. Stomatal Transpiration This type of transpiration involves stomata through which loss of water takes place. Stomata are tiny pores present in the epidermal surface of leaves, young stems and in certain fruits. Stomatal transpiration accounts for 80-90% of the total water loss from the plants.
  2. Cuticular Transpiration Cuticle is an impermeable covering on plant tissue. It can be thick as in case of xerophytes and xeromorphs and it can be thin and green also. Thickness of cuticle determines extent of water vapour loss from plant. As thickness is increased the extent of water vapour loss is significantly reduced.
  3. Lenticular Transpiration Very less amount of water is lost through lenticels of fruits and woody stems. The transpiration through lenticels is called as lenticular transpiration.

Transpiration Ratio

The transpiration ratio can be defined as the amount of water lost through transpiration per unit of dry matter produced during the growing season of plant. This ratio varies from plant to plant. It has been stated that desert plants have highest transpiration ratio as compared to other plants.

Movement of water Land plants have elaborate H2O conducting system, which is permitted large amount of H2O from roots to the evaporating surface of exposed shoots. Resistance to water movement by parenchyma cells is so high that it is impossible for large amount of water movement from roots and leaves.

Various factors affecting transpiration

Transpiration is escape of H2O from plants in the form of vapour. It is dominant factor in plant H2O relations because it produces the energy gradient that is required for movement of H2O into the plants through the plants.

The external factors affecting transpiration:

  1. Intensity of light
  2. Vapour temperature
  3. Wind and water supply

The internal factors affecting transpiration:

  1. Leaf area
  2. Leaf structure
  3. Behavior of stomata

All these factors interact with each other and affect the rate of transpiration.


Mechanism of loading and unloading of photoassimilates

Phloem Loading (Allocation and partition of carbon within a source leaf)

Apoplastic versus Symplastic Factors involved in moving photosynthate from mesophyll cells to the sieve elements of mature leaves are:

  1. Triose phosphate converted to sucrose in mesophyll cells. It is utilized for the synthesis of both starch and sucrose.
  2. Triose phosphate is translocated to cytosol from chloroplast.

Sucrose moves to various cells in the vicinity of sieve elements.

Sugars are transported into (and usually become more concentrated in) the sieve elements (phloem loading) and companion cells.

Once loaded into the sieve element, translocation over long distances can occur.


Phloem loading can occur through:

Apoplastic route In this route, sugar passes symplastically from mesophyll cells across plasmodesmata to bundle sheath cells. Sugars can move either apoplastically from bundle sheath cells across a plasma membrane to enter ordinary companion cells, which then connect symplastically to sieve elements or symplastically from bundle sheath cells to phloem parenchyma cells and then apoplastically across a plasma membrane to the ordinary companion cells (which connect symplastically to sieve elements). Sugars are translocated in the phloem by mass transfer along a hydrostatic pressure.

Symplastic route The initial short-distance route is always symplastic. The entire route however may be symplastic. In this route, sugar passes entirely symplastically from mesophyll cells across plasmodesmata to bundle sheath cells and intermediately companion cells and on in to the sieve element. Apoplastic phloem loading requires energy and ATP, which is used to concentrate sucrose in the sieve element and its companion cell. It involves a sucrose-H+ symporter (a type of secondary active transport using proton pumping ATP-ases). In some plants, apoplastic movement occurs directly into the sieve element. Symplastic phloem loading occurs in plants with intermediatory companion cells. The mechanism requires conversion (in the IC) of sucrose with galactose to form raffinose (and stachyose). These larger molecules can freely enter the sieve element through large connecting pores, but they are too large to return to the bundle sheath cells because the connecting pores are smaller.

Phloem Unloading and Transition from source to sink

This is performed at sinks, such as tubers, growing root tips, reproductive structures and young leaves. It requires short-distance transport to cells, then storage and/or metabolism. It can be apoplastic or symplastic.

Apoplastic route In this route, sugar passes apoplastically across the least one plasma membrane.

Symplastic route The entire pathway can be symplastic, from companion cells and sieve element complex to sink cell.

Transport into sink tissues requires metabolic energy and is thought to be done with a sucrose-H+antiporter.

Transition from Source to Sink Young leaves are sinks. As they mature, beginning first at the tips (using auto radiographs) they stop being sinks and become sources. With further maturation, the entire leaf becomes a source. Export as a source in a leaf is initiated when the unloading pathway is closed, minor veins responsible for loading have mature and the leaf is adequately photosynthesizing.

At a transitional stage, parts of the leaf are sources while parts are sinks and the vessels serving this opposite functions are not the same vessels.