In order to gain a better understanding of xerophyte and halophyte adaptations, a basic understanding of how plant structure enables its function is required. Three key components which require salt and water to be kept within tolerance limits include roots, stems and leaves. Water uptake occurs through the root hairs, while water loss occurs via the stomatal pores in leaves.

Process

• Root hairs found in roots have a high SA:VOL ratio, achieving high rates of diffusion and osmosis and performing some active transport; water and dissolved substances are transported up the stem

• Vascular plants contain xylem and phloem in their stem, and they act as transport tissues; xylem carries water and phloem carries photosynthetic by-products such as glucose

• Stems are attached to leaves

• Leaves are made up of layers of specialised tissues (which you may recall from year 11), and are the site where the most water loss occurs

• Transpiration pull occurs when water is pulled from the roots through the xylem by the forces of cohesion and adhesion

• Cohesion is the attractive force which occurs between water molecules themselves, adhesion is the attractice force which occurs between water molecules and the xylem walls

• Capillary action is created by the combined forces of cohesion and adhesion, and is the movement of water within the spaces of a porous material

• Concentration gradients are formed as water continues to move up the column and is drawn from the root hairs, between the inside and outside of root hairs; water movies in via osmosis

• Active transport of salt ions can allow for osmosis to occur, seeing water move into the root hairs, balancing internal and external salt concentration; this movement of water causes root pressure which pushes water upwards

• Transpiration stream occurs when the forces of cohesion, adhesion and root pressure produce a continuous flow of water from roots to leaves via the xylem

• Stomata allow for the loss of water from the plant via the process of transpiration

• Stomata have guard cells which open or close the stomata and provide some control over water loss, which are affected by different factors

Why is transpiration an important process?

1. Transpiration supplies photosynthesis with the required water needed for the reaction

2. Evaporation of water draws heat energy out, cooling the leaves

3. Transpiration stream enables the distribution of mineral salts throughout the plant

What are the factors affecting transpiration?

1. Light: Increased light levels allow for an increase in the rate of transpiration, as the heat from light stimulates the opening of stomata, which allows for transpiration to begin; conversely, lower light levels reduce the rate of transpiration

2. Humidity: Decreased levels of humidity allows for a higher water vapour concentration gradient between the air at the surface of the leaf and the air outside the lead, increasing diffusion of water vapour out of the leaf and evaporation from the leaf surface; leading to increased water loss (i.e. transpiration), whereas increased humidity leads to reduces water loss

3. Wind: Increased levels of wind leads to increased rates of transpiration as evaporation is increased; humid air near the stomata is carried away, increasing water vapour concentration gradient between the internal and external air

4. Temperature: Increased temperature increases evaporation from the surface of the lead; this leads to an increased rate of water loss, whereas decreased temperature decreases evaporation