F4 Biology teaching notes Ch 10: 2nd Term NE 16

F

Factors affecting the rate of transpiration

Ø   A (30) potometer (蒸騰計) can be used to measure the

rate of transpiration of a plant.

n  Bubble potometer (氣泡蒸騰計) measures the rate of (31) water absorption / uptake of a leaf shoot

n  Assumption for valid measurement:

Rate of transpiration       = Rate of (32) water absorption

= Volume of water absorbed / time (ml min^-1)

n  (33) Weight potometer can measures volume of water absorbed by and the volume of water loss from the plant

Component	Used to measure
Burette	(34) rate of water uptake
Balance	(35) rate of water loss
l  Assumption for valid result: ²  the loss of water over a short period of time is only due to transpiration ²  gain or loss of weight due to photosynthesis and/or respiration is insignificant不明顯

l  Rate of water absorption   = volume change in burette / time

l  Rate of water loss = weight change / time

l  Rate of transpiration = Rate of (36) water loss

= Weight loss / time (mg min^-1)

l  Volume of water absorbed should be (37) larger than / smaller than / equal to the volume of water loss

because some water is consumed by the plants for maintaining the process of life

such as (38) photosynthesis, growth and cellular metabolism.

Ø   Effect of different environmental factors on the rate of transpiration:

Factor	Effect on rate of transpiration	Reason
Light intensity	The rate (39) increases / decreases with an increase in light intensity.	n   As light intensity increases, the stomata open (40) wider. The cross-sectional area for the diffusion of water vapour (41) increases. Water vapour diffuses out more rapidly. n   In darkness, the stomata are closed. Only a small amount of water vapour diffuses out.
Temperature	The rate (42) increases with an increase in temperature.	n   As temperature increases, the rate of (43) evaporation of water from the surfaces of mesophyll cells increases. Water vapour diffuses out more rapidly.
Air movement	The rate (44) increases in windy conditions.	n   Without air movement, water vapour diffused out accumulates around the stomata. n   Wind blows away water vapour around the stomata. This helps maintain a (45) steeper concentration gradient of water vapour between the air space and the surrounding air, thus (46) increasing the rate of diffusion.
Relative humidity	The rate (47) decreases with an increase in the relative humidity of the surrounding air.	n   A higher relative humidity of the surrounding air decreases the (48) concentration gradient of water vapour between the air space and the surrounding air. (49) Less water vapour diffuses out.