The evaporation of water from a plant's surface, particularly through the stomata when they open to allow the entry of carbon dioxide for photosynthesis
During daylight, stomata open to allow carbon dioxide to enter for photosynthesis. This increases the rate of transpiration, as water evaporates from the mesophyll cells and diffuses out of the leaf.
An increase in temperature increases the rate of transpiration by providing the water molecules with more kinetic energy allowing them to evaporate more readily
Refers to the water vapour content of the air surrounding the plant. An increase in humidity of the air increases the water potential of the air. This will lead to a decrease in the rate of transpiration, as the water potential gradient for the diffusion of water decreases.
Air movement removes water vapour from the leaf surface increasing the water potential gradient and the rate of transpiration. In still air, water vapour builds up around the leaf decreasing the water potential gradient and the rate of transpiration.
Transports photosynthetic products such as sucrose, amino acids and fatty acids from the source (leaves) to the sinks (growing regions, roots, storage areas)
Some evidence supports the mass flow hypothesis, such as phloem sap being released when the stem is cut indicating hydrostatic pressure, and reduced translocation rate with temperature decrease or respiratory inhibitors
Some evidence against the mass flow hypothesis, such as the specific structure of sieve tubes and sieve plates not being required for mass flow, and substances observed moving in opposite directions in the same sieve tube
The evaporation of water from a plant's surface, particularly through the stomata when they open to allow the entry of carbon dioxide for photosynthesis
During daylight, stomata open to allow carbon dioxide to enter for photosynthesis. This increases the rate of transpiration, as water evaporates from the mesophyll cells and diffuses out of the leaf.
An increase in temperature increases the rate of transpiration by providing the water molecules with more kinetic energy allowing them to evaporate more readily
Refers to the water vapour content of the air surrounding the plant. An increase in humidity of the air increases the water potential of the air. This will lead to a decrease in the rate of transpiration, as the water potential gradient for the diffusion of water decreases.
Air movement removes water vapour from the leaf surface increasing the water potential gradient and the rate of transpiration. In still air, water vapour builds up around the leaf decreasing the water potential gradient and the rate of transpiration.
A thick cuticle provides a long diffusion pathway, reducing the rate of evaporation
Hairs on the leaf surface trap a layer of still air, which becomes saturated with water vapour reducing the water potential gradient for water loss
Rolling up of leaves traps a layer of still air which becomes saturated with water vapour. This reduces the water potential gradient for water loss, reducing the rate of transpiration e.g. marram grass
Reduced surface area to volume ratio of leaves e.g. pine needles reduce the surface area for water loss
Stomata positioned in 'epidermal pits or grooves' beneath the leaf surface, reducing exposure to air currents. The trapped air become saturated with water vapour and reduces the water potential gradient for evaporation
1. Solar heat energy causes the evaporation or transpiration of water from leaves
2. Water moves from cell to cell across the leaf by osmosis down a water potential gradient
3. Water is drawn from the xylem creating a tension ('negative pressure') in the xylem vessels effectively "pulling up" the water and dissolved ions
4. The water column is maintained in the xylem by cohesive forces and adhesive forces
5. Adhesion refers to the attraction of the water molecules to the xylem walls
6. The upward movement of the water from the xylem in the roots maintains the water potential gradient across the root cells, providing the mechanism for water uptake from the soil via osmosis
For the potometer to work correctly, there must be no air bubbles. If the column of water is broken and the cohesion lost, the water will not be drawn up from the potometer. The rubber bung must provide an air-tight seal.