FACTORS AFFECTING TRANSPIRATION

Biology

FACTORS AFFECTING TRANSPIRATION

 

The factors that affect transpiration are grouped into two.

  • Environmental and

Environmental factors

Temperature

  • High temperature increases the internal temperature of the leaf.
  • This in turn increases kinetic energy of water molecules which increases evaporation.
  • High temperatures dry the air around the leaf surface maintaining a high concentration gradient.
  • More water vapour is therefore lost from the leaf to the air.

Humidity

  • The higher the humidity of the air around the leaf, the lower the rate of transpiration.
  • The humidity difference between the inside of the leaf and the outside is called the saturation deficit.
  • In dry atmosphere, the saturation deficit is high.
  • At such times, transpiration rate is high.

Wind

  • Wind carries away water vapour as fast as it diffuses out of the leaves.
  • This prevents the air around the leaves from becoming saturated with vapour.
  • On a windy day, the rate of transpiration is high.

Light Intensity

  • When light intensity is high; more stomata open hence high rate of transpiration.

Atmospheric Pressure

  • The lower the atmospheric pressure the higher the kinetic energy of water molecules hence more evaporation.
  • Most of the plants at higher altitudes where atmospheric pressure is very low have adaptations to prevent excessive water-loss.

Availability of Water

  • The more water there is in the soil, the more is absorbed by the plant and hence a lot of water is lost by transpiration.

Structural Factors Cuticle

  • Plants growing in arid or semi-arid areas have leaves covered with a thick waxy cuticle.

Stomata

  • The more the stomata, the higher the rate of transpiration.
  • Xerophytes have few stomata which reduce water-loss.
  • Some have sunken stomata which reduces the rate of transpiration as the water vapour accumulates in the pits.
  • Others have stomata on the lower leaf surface hence reducing the rate of water-loss.
  • Some plants have reversed stomatal rhythm whereby stomata close during the day and open at night.
  • This helps to reduce water-loss.
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Leaf size and shape

  • Plants in wet areas have large surface area for transpiration.
  • Xerophytes have small narrow leaves to reduce water-loss.
  • The photometer can be used to determine transpiration in different environmental conditions.

Translocation of organic compounds

  • Translocation of soluble organic products of photosynthesis within a plant is called translocation.
  • It occurs in phloem in sieve tubes.
  • Substances translocated include glucose, amino acids, vitamins.
  • These are translocated to the growing regions like stem, root apex, storage organs e.g. corms, bulbs and secretory organs such as nectar glands.

Phloem

Phloem is made up of:

  • Sieve tubes
  • Companion cells
  • Parenchyma, a packing tissue
  • Schlerenchyma, a strengthening tissue Sieve Tubes
  • These are elongated cells arranged end to end along the vertical axis.
  • The cross walls are perforated by many pores to make a sieve plate.
  • Most organelles disappear and those that remain are pushed to the sides of the sieve tube.
  • Cytoplasmic strands pass through the pores in the plate into adjacent cells.
  • Food substances are translocated through cytoplasmic strands.

Companion Cells

  • Companion cells are small cells with large nuclei and many mitochondria.
  • They are found alongside each sieve element.
  • The companion cell is connected to the tube through plasmodesmata.
  • The mitochondria generate energy required for translocation. Phloem Parenchyma
  • These are parenchyma cells between sieve elements.
  • They act as packing tissue.

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