9.3- Transpiration Flashcards
What is transpiration?
The loss of water vapour from the leaves and stems of plants.
- is an inevitable consequence of gaseous exchange.
What is the casparian strip and it’s importance?
A band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer.
- prevents harmful substances entering xylem, as they would now need to travel via the symplast route.
- prevents leakage of water from xylem vessel and aids the development of root pressure.
Explain the features of leaves that affect transpiration.
- Large SA covered with waxy cuticle which makes them waterproof. Prevents leaf losing water by evaporation from the surface.
- Stomatal frequency, size and position. They can open and close to control the amount of water lost by a plant. Some stomata needs to always be open as there is always exchange happening.
- Thickness of epidermis and cuticle.
What is the transpiration stream?
The movement of water through a plant from the roots until it is lost by evaporation from the leaves. It is a passive process (xylem vessels =dead)
- water moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem to leaves.
- water molecules evaporate from surface of mesophyll cells into air spaces in leaf before moving out the stomata by diffusion down a conc gradient.
- this loss of water lowers water potential of the mesophyll cell=water moves in from an adjacent cell along the symplast+apoplast pathway.
- this is repeated across the leaf to the xylem.
Explain the cohesion-tension theory.
The movement of water from the soil in a continuous stream up the xylem and across the leaf.
- water molecules form hydrogen bonds with carbohydrates in walls of narrow xylem vessels = adhesion.
- water molecules form hydrogen bonds with each other so stick together = cohesion.
- combined effect of adhesion+cohesion results in water exhibiting capillary action. Meaning it can rise up narrow tube against force of gravity. Called transpiration pull.
- transpiration pull results in tension in xylem, helping move water across roots from soil.
What is the evidence for cohesion-tension theory?
- Changes in tree diameter:
- during day= transpiration at highest= tension is high= so diameter shrinks.
- during night=transpiration at lowest=tension is low= increased diameter. - When xylem vessel is broken, air is drawn into xylem rather than water out. Continuous stream of water molecules held together by cohesive forces= broken so water can’t move up.
How can transpiration be measured and key points when setting it up?
Using a potometer.
- all joints sealed with waterproof jelly to ensure all water loss is result of transpiration.
- fresh shoot is cut under water to avoid air bubbles in stem. No water should get onto leaves.
(Learn procedure)
How do stomata control the rate of transpiration?
- opening/closing of stomata = tugor-driven process.
- when turgor is low= the asymmetric configuration of guard cells closes the pore.
- when conditions are favourable = guard cells pump in solutes by active transport to increase turgor.
- cellulose hoops prevent cells swelling in width so they extend lengthways. Inner wall less flexible than outer so become bean shaped.
- scarce water. Hormonal signals from root trigger turgor loss so pores close.
How do light, humidity and temp affect the rate of transpiration?
- Light- needed for photosynthesis. Stomata open in light and most close in dark. Increasing light intensity= increase no of open stomata= increase transpiration.
- Humidity. High relative humidity= reduces water vapour potential gradient= decrease transpiration. Very dry air has opposite effect.
- Temperature. Increase in temp= increase kinetic energy of water molecules= increase evaporation.
Increase in temp= increases conc of water vapour the external air can hold = decreases relative humidity and water potential= increases transpiration.
How does air movement and soil-water availability affect transpiration?
- Air movement. Each leaf has a layer of still air trapped around it. Has features like hairs on surface to reduce air movement.
Water vapour that diffuses out accumulates here= water potential around stomata increases= diffusion gradient reduced. Therefore increased movement= increased diffusion gradient = increase in transpiration. - Water-soil availability. Dry soil=water stress = reduced transpiration rate.
what factors affect transpiration?
- light intensity.
- humidity.
- temperature.
- air movement.
- soil water availability.
