Transpiration Rates and Leaf Temperature

So the experiment goes like this:

take a leaf, measure the temperature of the leaf surface.
then coat the lower leaf surface in vaseline and remeasure the temperatures

What would you expect the temperature difference to be, and why (in relation to transpiration rates?)

I think the temperature should be raised and that the degree of temperature difference shows the rate of transpiration. Why should increased transpiration increase the temperature? I have a feeling its more to do with the fact that transpiration has a cooling effect, and it's not so much direct heating as less cooling? But transpiration rates would also be linked to Carbon fixation - does this affect temperature?

Reply 1

Fluent in Lies

OP

**** off and die.

When you coat the bottom of the leaf you are blocking the stoma. (remember the stoma are the pores / holes on the surface of a leaf which allow gases in and out. When the plant wants to retain or at least reduce water loss, special cells known as guard cells can close reducing water lost to transpiration. But this also reduces the amount of CO2 which can enter, hence reducing photosynthesis is the process.)

So with the vaseline on the leaf, traspiration will be greatly reduced.
Less water will leave, but also less photosynthesising will occur in that area.

The main effect on temperarture is that there should be an increase in temperature when the vaseline is added.
Transpiration is reduced and therefore the leaf loses the colling affect of transpiration.

Think of it like sweat in humans. We sweat to lose heat. The sweat evaporates and this has a cooling affect (heat energy is lost).

The same can be applied to the leaf. Transpiration is reduced. Water loss reduces. Cooling effect lowers. Less heat energy is lost.

Reply 3

Fluent in Lies

OP

2

Thanks muchly. So is the transpiration rate indicative of the rate of photosynthesis? Greater transpiration rate = greater rate of photosynthesis as more available CO2?

Also, if would we expect to observe a transpiration gradient down the length of, say, a branch of a plant? And would we expect transpiration rates to be higher at the tip end or the stem end?

Thanks for your time!

Reply 4

Teacher_of_Bio

3

Fluent in Lies

Thanks muchly. So is the transpiration rate indicative of the rate of photosynthesis? Greater transpiration rate = greater rate of photosynthesis as more available CO2?

Also, if would we expect to observe a transpiration gradient down the length of, say, a branch of a plant? And would we expect transpiration rates to be higher at the tip end or the stem end?

Thanks for your time!

photosynthesis is just one of the factors affecting transpiration. Higher levels of photosynthesis do not always mean higher transpiration.
The outside temperature, humidity and the amount of external wind affect transpiration just as much as photosynthesis.
But when photosyntheis is higher the stoma will be wide open to allow more CO2 to enter the plant, therefore more water will be lost (via transpiration, out of the pores / holes / stoma).

yes. more transpiration occurs down the length of a branch. More occurs nearer the tip. Less humid in this area and more exposed to the environment.
Also down the tip end of a branch there will be proportionally more leaves tha at the stem end

Transpiration Rates and Leaf Temperature

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