Transpiration

When stomata open to allow an exchange of oxygen and carbon dioxide between the external air and air inside the leaf, water vapour also moves out by diffusion and is lost- this is transpiration, an inevitable consequence of gaseous exchange. 

Stomata open and close due to the amount of water lost by a plant, but some stomata (at least) need to be open all the time for photosynthesis and aerobic respiration. 

- this moves water up from the roots of a plent to its highest leaves. 

• Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into surrounding air by diffusion down a concentration gradient
• loss of water from mesophyll cells lowers water potential, water moves into the cell from an adjacent cell along the symplast and apoplast pathway 
• repeated across the leaf to the xylem, water moves out of xylem by osmosis into leaf cells 
• water molecules cohere and adhere and as a result capillary action occurs (the process by which water can rise up a narrow tube against the force of gravity)
• Water is drawn up the xylem in a continuous stream to replace water lost by evaporation- this is the transpiration pull
• transpiration pull results in tension in the xylem, this helps water move across the roots to the soil
• Model of water moving from the soil in a continuous stream up the xylem and across the leaf is known as the cohesion-tension theory

• Changes in the diameter of trees- when transpiration is at its highest the tension in xylem vessels is at its highest too, as a result the tree shrinks in diameter (and vice versa at night). This can be shown by measuring the circumfrance of a tree at different times of the day.
• When a xylem vessel is broken air is often drawn in instead of xylem leaking out (e.g. cutting flower stems to put them in water) 
• If a xylem vessel is broken and air is pulled in as previously described, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken 

Water uptake gives us a good indication of the rate of transpiration, so that is measured to avoid the practical difficulties of directly measuring transpiration. 

Rate of water uptake can be measure in a variety of ways, most commonly with a potometer. Any water loss measured is a result of transpiration from the stem and leaves.

The rate of water uptake = distance moved by air bubble/ time taken for air bubble to move that distance (units = cm/s)

• Fresh shoot- stem is cut under water and transferred to apparatus to avoid air bubbles getting to the stem
• Do not allow water to get onto the leaves
• Resevior from which water can be let into the capillary tube, pushing the air bubble back to the start of the scale 

The main way rate of transpiration is controlled is by opening and closing stomatal pores- this is a turgor-driven process.

 When the turgor is low the asymmetric configuration of the guard cell walls closes the pore.Cellulose hoops prevent the cells from swelling in width, so they extend lengthways. Because the inner wall of the guard cell is less flexible than the outer wall, the cells become bean-shaped and open the pore. 

Hormonal signals from the roots can trigger turgor loss from the guard cells, which close the stomatal pore and conserve water. 

• Light- required for photosynthesis, in the light the stomata open for gas exchange needed. In the dark, most the stomata close. Increasing light intensity = increasing number of open stomata- increases rate of transpiration
• Relative humidity- a measure of the amount of water vapour in the air compared to the total concentration of water the air can hold- a v high relative humidity will lower rate of transpiration because of the reduced water vapour potential gradient 
• Temperature- affects rate in two ways. Firstly, a higher temperature means water molecules have more kinetic energy and therefore more evaporation takes place from the mesophyll cells. Secondly, an increase in temperature increases the concentration of water vapour the air can hold before it becomes saturated
• Air movement- each leaf has a layer of still air around it trapped by the shape of the leaf and its features- this decreases air movement close to the leaf. When water vapour diffuses out it accumulates here and water vapour potential around the stomata increases, in turn reducing the diffusion gradient (anything that increases the diffusion gradient increases rate of transpiration)
• Soil-water availability - if soil is very dry the plant will be under stress and the rate of transpiration will be reduced.