Water Potential in Soil and Air

by Jason Amores Sumpter
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water potential in soil will vary depending upon the conditions. Generally speaking, dry soil will have a lower water potential. Then the water potential in plant roots and conversely, damp soils will have higher water potential than the water potential found in plant roots. This is because the water in damp soil usually has few dissolved salutes compared to plant roots, which of course are made of cells. And, as we know, cells are filled with salutes. Now soil near the ocean will have 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 water potential is low enough, water can actually flow from the plant into the soil, which would be devastating to the plant, right? I mean, the roots 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 salutes and therefore ensure that water is going to go from the soil into the roots. Now, just a soil can have water potential here can have water potential and warm air 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 gonna be perfect conditions for evaporation. Now, you might recall that plants will evaporate water through their leaves in a process known as transporation. What you might not have realized is transpiration actually helps pull water up from the roots. We're gonna 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 stone mata, or the singular is just stow MMA. So these stone mata control gas exchange, which is there 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 is gonna evaporate. Now, how do these stone open and close? Well, one mechanism is based on these proton pumps, these proton pumps. When the plant wants to open its toma 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 are charged ions, uh, they will cause a deep polarization. Basically, they're gonna 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. So I know that's a little confusing, so let's just go through it once more. So, uh, Thio open the stone mata or to open us toma proton pumps. They're gonna concentrate protons outside the cell. This is going thio mess with the electrical balance between the inside and outside of the cell, which results in potassium ions entering the cell so potassium is going to enter. The cell is 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 human cells. So to close the stone A, the plant is going Thio. 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 thio shrink and slides. They're gonna lose their tiger pressure, and that's going to allow the stomach to close. So closing over here, opening over here. And it is due to the movement of water in and out of that stone A or the guard cells of the stomach, I should say now, stone mama 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 stone mama according to the day night cycle, and additionally, they will also respond to hormonal signals like that of a BSI ZIC acid, which is often just abbreviated A B A. This stuff Eva is actually produced in roots, and it's produced in response to low soil water potential. And what a BA does is cause the stone mata too close. It induces the stone model to close, and that reduces transporation. Why is this important? Well, if soil water potential is low, that means that the plant is not going to be as absorbing water is effectively, which means it's gonna 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. And it's the compromise between conserving water and maximizing photosynthesis. We want the plant wants those D'Amato open for photosynthesis. That's how it's going to get the gas is it needs to carry out photosynthesis and, uh, you know it, Czar. 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, um, you'll see something like Samata in deep pit surrounded by try combs. Remember, we said, try. Combs could be involved in preventing water loss and also, uh, in the section where we talked about photosynthesis. We talked about adaptations in what are called cam plants and C four plants, uh, where plants that carry out camp and C four photosynthesis. Um, so if you want Thio, review those particular concepts, go back and check out the video on photo respiration at the end of the photosynthesis section. With that, let's flip the page.