A pressure bomb is
essentially a bicycle pump with a chamber for a leaf. At all times a leaf is sucking
in water to replace the water it loses through the stomata (see week 2's post). However, if you increase the pressure in
the chamber around the leaf, the leaf will have a harder time sucking in water.
Eventually, if you increase the pressure enough, the leaf will spew out water.
If you measure how much pressure it takes to make a leaf spew out water, you
are able to tell how hard the leaf is sucking in water. If the leaf is sucking
in water hard, it means it is water stressed. Therefore, pressure bombing
provides a robust quantification of how water-stressed a plant is. This is by
far the most common plant-based measurement of water-stress because it provides
almost instantaneous feedback and is easy to take. There are some shortcoming: it
is destructive (you have to take off a leaf from the plant); it is labor
intensive; it varies within a tree quite a bit.
I mentioned that the pressure bomb is destructive, but
it is nothing compared to this next instrument: the sap flow sensor. Installing
this involves drilling a finger-sized hole inside the trunk of a tree. This
sensor measures how many ions are moving through a cross-section of a tree at a
given instance. Since
ions of a tree move through the xylem (where water also moves), the number of moving ions is a great
proxy for the amount of water moving through a cross section of a plant at a
given time. When water is moving through a plant slowly, it means that the plant
is water-stressed. Therefore, this is another method of measuring plant water-stress.
Sap flow is an accurate measure of plant water-stress. However, it has the
obvious drawbacks of being a pain to install and maintain.
So far, we have only talked about plant-based measurements for
water stress (by this, I mean taking a measurement from the plant and not the
environment). However, there are several non-plant-based. The most common is
soil moisture; some of you might have soil moisture sensors in your backyard.
As to what are the strength and weaknesses of these, you’ll have to come back
to WTW to find out.
Why is the pressure bomb called a bomb? Does it do anything to the leaf within it, or simply measure the pressure? Thanks for the explanations of the different ways to measure water stress. If the sap flow sensor is destructive, why would you use it? Are there some cases in which it is preferable?
ReplyDeleteZiba, it was nice of you to comment over your break. The bomb I think refers to the fact that it has a fixed volume and variable pressure (but I could be making it up). As to why someone would use a sap flow sensor if it is destructive, each method of measuring plant water-stress provides part of the picture. The more different types of measurements, you have the clearer the picture. Sap flow is accurate (but a pain to install), so it is an important part of painting that picture.I should also clarify when I say destructive, I mean that it is invasive inside the tree. Drilling inside the tree should not affect the measurements themselves.
DeleteFrom reading your post, I learned a lot more about water stress and pressure bombs than I ever thought I would! Also, why does it have bomb in its name?
ReplyDeleteHi, Richard. It's like a BOMB calorimber. I think the use of bomb refers to a fixed volume and a changing pressure (but that is just a guess).
DeleteI remember that back in August you were looking for a bomb. Are there any passive ways(i.e. not affecting the plant) to measure water stress of an individual plant?
ReplyDeleteI was looking to buy a pressure bomb, but I ended making my own. Right now, there are some non-destructive measurements. Probably the best is measuring the leaf temperature, which is a good proxy for how much the leaf is photosynthesizing.
DeleteThe pressure bomb seems really cool! Sounds like you're having fun!!
ReplyDeleteThis was a great post! When you were talking about the pressure bomb being destructive you said that you have to take off a leaf. This may sound ignorant, but why is it destructive to take off just one leaf? Don't leaves fall off plants all the time? Does it get in the way with the plant's growth?
ReplyDeleteHi, Tara. It's not that big of a deal for the plant. If you start taking hundred of leaves from a plant, then that might be an issue (this I have done before). The biggest issue is you can not repeat the measurement with the same leaf.
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