Water potential in soil will vary depending on the conditions. Generally speaking, dry soil will have a lower water potential than the water potential in plant roots. Conversely, damp soils will have a higher water potential than the water potential found in plant roots. This is because the water in damp soil usually has few dissolved solutes. Compared to plant roots, which, of course, are made of cells, and as we know, cells are filled with solutes. Now, soil near the ocean will have a much lower water potential than roots, and that's because of all the salt in the water, which, of course, will end up in the soil around coastal areas. And if the water potential is low enough, water can actually flow from the plant into the soil, which would be devastating to the plant. I mean, the root's job is to absorb water from the soil; you don't want it going the other way. So plants have actually evolved adaptations that allow their roots to store high concentrations of solutes, and therefore ensure that water is going to go from the soil into the roots. Now, just as soil can have water potential, air can have a water potential. Warm and dry air have low water potentials. So air that is both warm and dry will actually have a very low water potential. It's going to be the perfect conditions for evaporation. Now, you might recall that plants will evaporate water through their leaves in a process known as transpiration. What you might not have realized is transpiration actually helps pull water up from the roots. We're going to talk about this in greater detail momentarily. But for now, I just want to focus on transpiration.
So, how does that happen? Well, plants have these pores on their leaves that are called stomata, or the singular is just stoma. These stomata control gas exchange, which is another one of their purposes; they control gas exchange by opening and closing, but that opening and closing also has an effect on the amount of water that will evaporate from the plant. And if the air outside is dry, which literally just means less than 100% humidity, you bet water's going to evaporate. Now, how do these stomata open and close? Well, one mechanism is based on these proton pumps. These proton pumps, when the plant wants to open its stoma, will actually pump protons outside of the cell. By concentrating, so these proton pumps will pump protons outside the cell, this leads to a high concentration of protons outside the cell. And because these are charged particles, or charged ions, they will cause depolarization. Basically, they're going to affect the charge balance between the outside and inside of the cell, and this allows potassium ions to enter the cell. And water is going to follow those potassium ions. I know that's a little confusing, so let's just go through it once more. So, to open the stomata, or to open a stoma, proton pumps are going to concentrate protons outside the cell. This is going to mess with the electrical balance between the inside and outside of the cell, which results in potassium ions entering the cell. Potassium is going to enter the cell as a result of this, and water will follow the potassium in. Water follows potassium in plants, unlike in humans where we usually see water following sodium, one of those differences between plant and human cells. So, to close the stomata, the plant is going to get all of those potassium ions out of the cells, and the water is going to follow them. And that means that the cells are going to shrink and sag; they're going to lose their turgor pressure, and that's going to allow the stoma to close. So closing over here, opening over here, and it is due to the movement of water in and out of the stomata, or the guard cells of the stomata, I should say.
Now, stomata open in response to a variety of factors. One of those is circadian rhythms, which are just natural rhythms that organisms experience. So in general, plants will open and close their stomata according to the day-night cycle. Additionally, they will also respond to hormonal signals, like that of abscisic acid, which is often just abbreviated ABA. This stuff, ABA, is actually produced in roots and it's produced in response to low soil water potential. And what ABA does is cause the stomata to close. It induces the stomata to close, and that reduces transpiration. Why is this important? Well, if soil water potential is low, that means that the plant is not going to be absorbing water as effectively, which means it's going to want to reduce its transpiration so it doesn't lose a bunch of water. Right? It wants to keep its water levels balanced. And so we often talk about this idea that we call the photosynthesis-transpiration compromise. It's the compromise between conserving water and maximizing photosynthesis. The plant wants those stomata open for photosynthesis. That's how it's going to get the gases it needs to carry out photosynthesis. And, you know, it's essential that the plant gets that carbon dioxide in order to carry out the Calvin cycle. However, you know, it can lose water in the process by doing that. So plants need to find the perfect balance to maximize photosynthesis and maximize water conservation. And again, that idea is known as the photosynthesis-transpiration compromise. But plants have also come up with a bunch of other adaptations for water loss. We've talked about some of these, including the cuticle, occasionally, you'll see something like stomata in deep pits surrounded by trichomes. Remember, we said trichomes can be involved in preventing water loss. And also, in the section where we talked about photosynthesis, we talked about adaptations in what are called CAM plants and C4 plants, or plants that carry out CAM and C4 photosynthesis. So, if you want to review those particular concepts, go back and check out the video on photorespiration at the end of the photosynthesis section.
With that, let's flip the page.