Why do plants transpire?

[newtron]

Here's a good question that kind of relates to the previous question about plants. Why do plants let the water leave their system into the atmosphere? Is there any advantage to this? I have a answer, which was suggested in a vadose zone hydrology class I took a few years ago. I wll state this answer for for futher discussion after I get some answers. Thanks.

[jake-the-peg]

I would assume that it would serve to cool the plant rather like sweating in mammals.

If this is the case I would expect less moisture loss in colder climates than warmer ones providing there is sufficient soil moisture - obviously all bets are off if there's not enough ground water to make this practical.

So I'd expect to find broad leaf trees in warmer lattitudes than pine trees - which of course is what we do see.

But I'm not a biologist so that could all be wrong

What's your reason?

[newtron]

Hi jake-the-peg. That's an interesting idea. In class, it was suggested that it was just an accidental consequence of having openings in the leaves through which gas exchange takes place. I don't think anyone really knows for sure. I think that since nutrients are transported in the water, advective transport of the nutrients due to water moving through the plant is a more efficient process to provide nutrients to the cells. If the water in the plant was effectively stagnant, nutrient transport would be controlled by molecular diffusion, which is a much slower process.

I didn't think about temperature control. Anyone else have any ideas?

[Space]

I'm not a biologist, but I agree with you newtron, I don't think plants transpire on purpose.
I don't think they need to cool themselves.

[gen2]

I would say you are both right. Just think of the many uses of water in an animal's body. As the main constituent of blood, it provides transport for all manner of substances, it takes part in metabolic reactions and provides cooling.

Plants need water as a transport mechanism. They need water for metabolism - in particular, photosynthesis uses up water. They also need cooling.

Very small animals can rely on diffusion to meet their needs, but large animals have such a high demand for all that that they require a pump - a heart - and use energy to keep it flowing.

Plants have much more meagre demands and instead of using energy, can utilise physical processes - osmosis and transpiration - to do most of the job for them.

The availability (or lack) of water in a plant's environment has led to many species adapting structures to regulate their use of water much more economically.

[Sequin]

Transpiration also allows the water to rise to the top of a tall plant. Without transpiration or some form of active pumping mechanism, the highest water could reach by capillarity would be about 1m - transpiration, where the water evaporates at the top of the tree, draws water up - up to 367 feet in the case of the tallest tree in the world

[shammydodger]

I think all the main points have been considered and mentioned here already. I agree with Sequin regarding transpiration as a pumping mechanism for getting water up to the leaves and, through this, supplying the plant with nutrients that are carried in the water (as detailed by Newtron.) Evaporation through the stomata is the main cooling mechanism for the majority of plants. In the process of getting rid of some water, carbon dioxide for photosynthesis can be taken on board. I think it's a bit simplistic and misleading to talk about whether or not plants transpire "on purpose", as it suggests a thinking quality on the part of plants, although I appreciate what is meant. It's probably better to think in terms of how plants are adapted for the functions of, for examples, nutrient transport and photosynthesis. The mechanism of transpiration is the main adaptation.