Hi, I’m Dr. Colin Campbell. I’m a senior research scientist here at METER Group, and in this course we’re going to be talking about water potential. Now first, I want to tell you, this is just a primer on water potential.

We’re going to talk about a few different things about it. But if you really want to know more, go over to our website, metergroup.com and look in our knowledge base area, you’ll be able to find a lot of great documents, some virtual seminars and other things to help you learn about water potential in all its detail. So let’s go ahead and get started and talk about water potential.

One of the biggest mistakes people make when trying to understand if water is available to the plant is talking about water content. Water content tells you how much water is there, but it doesn’t tell you whether or not the plant can actually access it. So what tells you that? Of course, it’s water potential.

Now, plants take up water along a water potential gradient, so it would be highest, if you will, and it’s a negative value, so maybe –30 kPa in the roots, and it flows through the plants, out in the atmosphere, and maybe in the atmosphere, which is super dry, it could be –100,000 kilopascals. And all along that gradient, we see that water potential get lower and lower, kind of like you sucking on a straw just moves that water up through the plant and out into the atmosphere. Now there are a few things we got to talk about to really understand water potential. Water potential is just the energy state of water, and in its pure form.

We call that zero, zero water potential. But there are several things that lower the energy state of water. We can talk about a few of them. For example, matrix potential.

When water is associated on the surface of soil, the water binds to the soil. We call that the adhesive property of water, and it also binds to itself and keeps it there using the cohesive property of water. Now these soil surfaces, here’s our little all our soil particles. As that water comes closer and closer to those soil the surface of the soil particles, it becomes bound more and more tightly, and it’s more and more difficult to get off.

And so as that water content gets less in the soil, the water potential gets less and can get more and more negative, to the point that plants simply can’t get it out. We call that permanent wilting point. Now that’s not the only thing that lowers the energy state of water. Osmotic or solutes lower the energy state of water by the ions in that solution, the solutes attracting those water molecules with their positively charged state, that lowers the energy state of water and makes it less available for use like plant water uptake.

Now one of the things that increases the water potential is the pressure inside plant cells. We know that plants actively take up ions into their cells. As these ions are taken up, the water potential becomes more and more negative in the cells, and to try to get into equilibrium, water will passively move across the plasma membrane into the cell. As it does, the cells will press more and more firmly on the cell walls, and this creates a situation that you’re probably familiar with called turgor.

That’s what keeps the leaves upright. Now, as I said, water moves along the water potential gradient, and we talked a little bit about numbers. Now it’s interesting that we actually can put humidity together with water potential because they’re related through an equation. So the atmosphere I know it’s not always at 50% relative humidity, but let’s just pick a number that sometimes is there.

That’s that –100,000 kPa that I talked about. It is extremely dry compared to just inside the leaf, which is often around –1000 kPa. It just depends on whether the plant is stressed. Different plants take care of their water potential differently.

Some control their water potential for stress. Some control their stomates. But in the end, they try to keep their leaves in a hydrated state so they can take part in photosynthesis.

Now, as we said, down in the soil, we might have a very high water potential, less negative, and here the water is going to move from the soil into the roots along this water potential gradient can move up into the xylem and up into the leaves, and that’s how water potential flows through systems. It’s important to know that, because now we are going to talk about why we would measure that.