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I'm trying to get my head around factors which affect transpiration in leaves.

For example, how would applying petroleum jelly to the surface of plant leaves affect their rate of transpiration?

I get that it's basically going to decrease the trasnpiration rate because the stomata will be covered, but I'm not sure about these parts:

  • How would it affect the rate of transpiration if only the top surface was covered?
  • How would it affect the rate of transpiration if only the bottom surface was covered?

Also, I think I'm right in saying that increasing wind / applying a fan to plant leaves will increase the transpiration rate, but why?

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up vote 2 down vote accepted

The rate of transpiration is very closely linked with the rate of evaporation from the leaf surface. Increased air movement across the surface of the leaf (e.g. from a fan) increases the rate of evaporation of water from the leaf surface. This happens because water saturated air is moved away; making the osmotic gradient more pronounced.

As for the relative changes from adding an impermeable substance like petroleum jelly to the top and bottom surfaces of leaves, it's a question of plant-anatomy. In the majority of plants, stomata are only located on the bottom surface of the leaf. This is to prevent them getting 'clogged' during rainfall and to keep them out of the sun to minimise water loss (again more sunlight on the stomata would increase the rate of evaporation). Therefore a plant with the underside covered would have a much slower rate of transpiration than one with the top surface of the leaf covered.

Other factors that would affect the rate of transpiration:

  • Temperature (Positive Correlation)
  • Humidity (Negative Correlation)
  • Light Availability Excluding CAM photosynthesis pathway plants (Positive Correlation - the stomata are in the main only open when photosynthesis is occurring, so transpiration is less at night or on a cloudy day in addition to light heating the leaf surface)
  • Water content of soil (Positive Correlation)
    • amount of salt in soil (negative correlation)
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very thorough answer - thank you –  Alex Coplan Feb 14 '12 at 18:56
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