Question

How would applying petroleum jelly to the leaves affect the rate of transpiration? (This would create a thick film over the leaf, clogging the stomata)

Answer 1

Plants eliminate water vapour through the stomata found in their leafs, this is the proccess of transpiration. When the plant has excess of water, the stomata are open and water vapour is eliminated, when the plant is dehidratated, the stomate close to avoid the loss of water, this way, the plant preserves it.

If you apply petroleum jelly to the leafs, you will obstruct the stomata. This will render the plant unable to eliminate water vapour, i.e. the plant wont be able to transpire. As a result, the rate of transpiration will decrease.