Transpiration

Any plant takes up a lot of water through its roots; for example, a tree can take up about 1000 litres (about 200 gallons) a day. Approximately 98 per cent of the water taken up moves through the plant and is lost by transpiration; only about 2 per cent is retained as part of the plant's structure, and a yet smaller amount is used up in photosynthesis.

The seemingly extravagant loss through leaves is due to the unavoidably large pores in the leaf surface (stomata) essential for carbon dioxide diffusion (see Figure 8.8). However, two other points should be considered here:

  • water vapour diffuses outward through the leaf stomata more quickly than carbon dioxide (to be used for photosynthesis) entering. However, the plant is able to partially close the stomata to reduce water loss without causing a carbon dioxide deficiency in the leaf;
  • the diffusion rate of water vapour through the stomata leads to a leaf cooling effect enabling the leaf to function whilst being exposed to high levels of radiation.

The plant is able to reduce its transpiration rate because the cuticle (a waxy waterproof layer) protects most of its surface and the stomata are able to close up as the cells in the leaf start to lose their turgor (see leaf structure p117). The stomatal pore is bordered by two sausage-shaped guard cells, which have thick cell walls near to the pore. When the guard cells are fully turgid, the pressure of water on the thinner walls causes the cells to buckle and the pore to open. If the plant begins to lose more water, the guard cells lose their turgidity and the stomata close to prevent any further water loss. Stomata also close if carbon dioxide concentration in the air rises above optimum levels.

A remarkable aspect of transpiration is that water can be pulled ('sucked') such a long way to the tops of tall trees. Engineers have long known that columns of water break when they are more than about 10 m long, and yet even tall trees such as the giant redwoods pull water up a hundred metres from ground level. This apparent ability to flout the laws of nature is probably due to the small size of the xylem vessels, which greatly reduce the possibility of the water columns collapsing.

A further impressive aspect of the plant structure is seen in the extreme ramifications of the xylem system in the veins of the leaf. This fine network ensures that water moves by transpiration pull right up to the spongy mesophyll spaces in the leaf (see p117), and avoids any water movement through living cells, which would slow the process down many thousand times.

If the air surrounding the leaf becomes very humid, then the diffusion of water vapour will be much reduced and the rate of transpiration will decrease. Application of water to greenhouse paths during the summer, damping down (see p15), increases relative humidity and reduces transpiration rate. While the air surrounding the leaf is moving, the humidity of air around the leaf is low, so that transpiration is maintained and greater water loss is experienced.

Windbreaks (see p38) reduce the risk of desiccation of crops. Ambient temperatures affect the rate at which liquid water in the leaf evaporates and thus determines the transpiration rate (see p124).

A close relationship exists between the daily fluctuation in the rate of transpiration and the variation in solar radiation. This is used to assess the amount of water being lost from cuttings in mist units (see misting p176); a light-sensitive cell automatically switches on the misting. In artificial conditions, e.g. in a florist shop, transpiration rate can be reduced by providing a cool, humid and shaded environment. Plasmolyzed leaf cells can occur if highly concentrated sprays cause water to leave the cells and result in scorching (see p123).

The evaporation of water from the cells of the leaf means that in order for the leaf to remain turgid, which is important for efficient photosynthesis, the water lost must be replaced by water in the xylem. Pressure is created in the xylem by the loss from an otherwise closed system and water moves up the petiole of the leaf and stem of the plant by suction (see transpiration pull). If the water in the xylem column is broken, for example when a stem of a flower is cut, air moves into the xylem and may restrict the further movement of water when the cut flower is placed in vase water. However, by cutting the stem under water

Pinus Cross Section Flower

Resin duct

Sunken stomata

Reduced surface area

Figure 9.2 Cross-section of pine leaf (Pinus) showing some adaptations to reduce water loss

Resin duct

Xylem Phloem Sclerenchyma Thick cuticle Endodermis

Sunken stomata

Reduced surface area

Figure 9.2 Cross-section of pine leaf (Pinus) showing some adaptations to reduce water loss the column is maintained and water enters at a faster rate than if the plant was intact with a root system.

Anti transpirants are plastic substances which, when sprayed onto the leaves, will create a temporary barrier to water loss over the whole leaf surface, including the stomata. These substances are useful to protect a plant during a critical period in its cultivation; for example, conifers can be treated while they are moved to another site.

Structural adaptations to the leaf occur in some species to enable them to withstand low water supplies with a reduced surface area, a very thick cuticle and sunken guard cells protected below the leaf surface (see Figures 9.2 and 9.3). Compare this cross-section with that of a more typical leaf shown in Figure 8.8. In extreme cases, e.g. cacti, the leaf is reduced to a spine, and the stem takes over the function of photosynthesis and is also capable of water storage, as in the stonecrop (Sedum). Other adaptations are described on p81.

Minerals

Essential minerals are those inorganic substances necessary for the plant to grow and develop normally. They can be conveniently divided into two groups. The major nutrients (macronutrients) are required in relatively large quantities whereas the micronutrients (trace elements) are needed in relatively small quantities, usually measured in parts per million, and within a narrow concentration range to avoid deficiency or toxicity. The list of essential nutrients is given in Table 9.1.

Non-essential minerals, such as sodium and chlorine, appear to have a role in the plants but not as a universal requirement for growth and development. Sodium is made use of in many plants, notably those of estuarine origins, but whilst it does not appear to be essential there is an advantage in using agricultural salt on some crops such as beet or carrots. Aluminium plays an important part in the colour of Hydrangea

Cross Section Marram Grass
Figure 9.3 Transverse section of Marram Grass leaf, showing adaptations to prevent water loss; outer thick cuticle, curling by means of hinge cells to protect inner epidermis, stomata sunken into surface to maintain high humidity

Table 9.1 Nutrient requirements

Macronutrients (major nutrients) Micronutrients (trace elements)

Table 9.1 Nutrient requirements

Macronutrients (major nutrients) Micronutrients (trace elements)

N

Nitrogen

Fe

Iron

P

Phosphate

Bo

Boron

K

Potassium

Mn

Manganese

Mg

Magnesium

Cu

Copper

Ca

Calcium

Zn

Zinc

S

Sulphur

Mo

Molydenum

flowers (see p84) and silicon occurs in many grasses to give them a cutting edge or sharp ridges on their leaves.

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Responses

  • christine
    What are transpiration rates used for in horicultural?
    6 years ago
  • CEDIVAR
    How the structure of marram grass helps to reduce water loss?
    6 years ago
  • Sara
    How do resin ducts precent water loss?
    6 years ago
  • austin
    How does a thick cuticle prevent water loss?
    5 years ago
  • fethawi
    What plant structure gets rid of water through the stomata of a plant?
    5 years ago
  • azzeza
    How does resin canal lower transpiration rate?
    2 years ago

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