Reducing water loss - xerophytes

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  • Reducing water loss - xerophytes
    • Unavoidable losses
      • The loss of water by transpiration is unavoidable. This is because plants exchange gases with the atmosphere via their stomata.
      • During the day, plants take up a lot of carbon dioxide (used in photosynthesis). Must be able to remove oxygen (by-product of photosynthesis).
        • So the stomata must be open during the day. While the stomata are open, there is an easy route for water to be lost.
      • Most plants can reduce those losses by structural and behavioral  adaptations:
        • a waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis.
        • the stomata are often found on the undersurface of leaves, not on the top surface - this reduces the evaporation due to direct heating from the sun.
        • most stomata are closed at night, when there is no light for photosynthesis.
        • deciduous plants lose their leaves in winter, when the ground may be frozen (making water less available) and when temperatures may be too low for photosynthesis.
    • Living in arid conditions
      • Some plants are particularly well adapted to lining in very dry conditions, known as xerophytes. Have a number of adaptations to reduce water loss from their leaves
      • Smaller leaves. This reduces the total surface area of the leaves. The total surface area is also reduced, so that less water is lost by transpiration
      • Densely packed spongy mesophyll. Reduces the cell surface area that is exposed to the air inside the leaves. Less water will evaporate into the leaf air spaces, reducing the rate of water loss.
      • A thicker waxy cuticle on the leaves reduces evaporation further.
      • Closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water
      • Hairs on the surface of the leaf trap a layer of air close to the surface. This air can become saturated with moisture and will reduce the diffusion of water vapour out through the stomata.
        • This is because the gradient of the water vapour potential between the inside of the leaf.
      • Pits containing stomata at their base also trap air that ca become saturated with water vapour. This will reduce the gradient in the water vapour potential between inside and outside the leaf, so reducing loss by diffusion.
      • Rolling the leaves so that the lower epidermis is not exposed to the atmosphere can trap air that becomes saturated. This is another way to reduce or even eliminate the water vapour potential graident
      • Some plants have a low water potential inside their leaf cells. This is achieved by maintaining a high salt concentration in the cells. The low water potential reduces the evaporation of water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced
    • Marram grass - a special case
      • specialises in living on sand dunes.
      • The conditions are particularly harsh because any water in the sand dunes away quickly, the sand may be salty and the leaves are often exposed to very windy conditions
      • Has:
        • Leaf rolled up to trap air inside
        • Thick waxy cuticle to reduce water evaporation from surface
        • Trapped air in centre with high vapour potential
        • Hairs on lower surface reduce movement of air
        • Stomata in pits to trap air with moisture close to the stomata

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