Transpiration Rates

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Transpiration rates
Environmental factors affecting transpiration rate:
TEMPERATURE
Heat energy can provide the latent heat of vaporisation of water, causing an increase in rate
of evaporation of water from water from the mesophyll cells.
Heat energy will also increase the random thermal movement of molecules in the water
vapour inside air spaces in the mesophyll.
High temperatures increase water potential gradient between leaf air spaces and
atmosphere. Water vapour will diffuse quicker.
AIR CONDITIONS
Humidity of the air and any movement of air around the leaf affect transpiration rates. If the
air is humid, the water potential gradient of the surroundings of the leaf are shallower and
diffusion is slower.
BOUNDARY LAYER
This is a thin layer of still air, this air remains humid and diffusion rate decreases. If the wind
blows stronger/faster, the boundary layer is thinner.
LIGHT AND CARBONDIOXIDE
Transpiration occurs more quickly in bright light and in an atmosphere with a lower carbon
dioxide concentration. This is because guard cells open the stomata pores when they are
turgid (water filled via osmosis).

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Measuring transpiration rates
Place a potometer in a bowl of water so it is covered (prevent air bubbles appearing)
Then using a syringe; pull water into the capillary tube (cohesion tension): irradiating
air bubbles.
Take a sample of the plant and place in water (cutting the end of the stem off when
it is submerged)
Finally, (underwater) place the stem end of the plant into the rubber tubing (of the
photometer), seal with Vaseline if required.…read more

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