the water and the
wax paper.
However, in this
case the water-water
cohesive force is
stronger than the
adhesive force
between the
molecules of the
wax paper and the
water.Figure 6.16: Temperature vs transpiration rate.Light intensity
At high light intensity, the rate of photosynthesis increases. As photosynthesis increases, the
amount of stored glucose in the guard cells increases. This lowers the water potential of the
leaf (i.e. the contents of the leaf are less dilute). As the water potential decreases, more water
enters the guard cells making them more turgid. The turgor pressure of the guard cells leads
to an opening up of stomata resulting in transpiration.
Figure 6.17: Transpiration vs light intensity.Relative humidity
The amount of water vapour in the air is referred to as the humidity. Water always moves
downa concentration gradient. Therefore when the humidity is high (lots of water vapour
in the air) the water potential gradient between the inside of the leaf stomata and the atmo-
sphere is shallow and the rate of transpiration will be low. However, if the atmosphere is dry,
there will be a steep water concentration gradient between the humid inside of the stomata
and the outside air and the rate of transpiration will therefore be fast.
Chapter 6. Support and transport systems in plants 171