2 3 9 Describe Adaptations Of A Range Of Terrestrial Austral

Problems facing plants that minimize water loss

The main form of water loss in plants is due to transpiration – evaporation of water from the stomata of leaves. Transpiration has two main purposes, to draw water and dissolved ions up the xylem and as part of evaporative cooling. Xerophytes are plants that live in arid conditions and possess adaptations which allow them to achieve the balance between losing water for cooling purposes at the risk of dehydration.

There are 3 main problems facing plants with regard to water loss:
- If plants lose too much water through transpiration they run the risk of dehydrating, yet loss of water by this evaporative cooling mechanism is an essential part of temperature regulation to keep plant cells within the optimal range for cell functioning.
- If plants reduce the surface area of their leaves or lose their leave, the number of stomata exposed to the external environment will be reduced – reducing the photosynthetic surface area to sunlight.
- If plants retain their leaves, but develop ways of ensuring that stomata do not open, gaseous exchange between the leaf and the surrounding air becomes limited, and as a result enough carbon dioxide may not be available for photosynthesis.

Adaptations in Australian plants to minimize water loss

Reducing the internal temperature: some plants have developed structural features of physiological mechanisms to reduce their internal temperature, allowing the plants to use less water for evaporative cooling, but still keeping their temperature within the correct range for metabolism. For example the Eucalyptus trees have shiny waxy cuticle.

Reducing the exposure of transpiring plant structures to sunlight : Plant organs that have the most abundant stomata have the highest rates of transpiration hence plants can, change their orientation of leaves so that the stomata is not exposed, reduce the surface area of organs that have the highest abundance of stomata or completely losing transpiring organ. For example eucalypts have their leaves hanging in a vertical position; this reduces the exposure of the high abundance stomata’s.

Reducing the difference in water concentration between the plant and the outside air: the difference in water potential determines how much water is lost in transpiration, on a hot dry day the water concentration is much lower in the air, and hence much more water is lost through transpiration. Some plants cannot change the external environment and hence some have “hairy leaves” which trap humid air around the plant creating a “microclimate” – thus reducing water loss through transpiration. For example the sunken stomata in the Hakea traps air around the leaf, creating a higher humidity microclimate.

Water Storage: some plants called succulents have adaptations such as fleshy steams or leaves which are able to swell up and retain moisture – these plants then survive by using this moisture during dry periods. For example the Australian Calandrinia is a succulent plant.