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Plants are great at using and moving water. In this lesson, we’ll explore the concept of water potential as it applies to plants and check out how solutes and pressure can both impact the total water potential within an organic system.

Water Potential in Plants

You need water. I need water.

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We all need water. Water is important. We build reservoirs to collect it, disperse it through complex irrigation networks across the country, and even use it to generate energy. But, even with all of this, we’re not the most effective water users on the planet. That title goes to the plants, the trees and flowers and grasses that cover this world.Plants know how to use water, and there are some incredibly complex processes constantly at work within the cells of plants. How do giant redwoods get water all the way to the top branches? How do plants survive in the desert? How do thin flowers stay upright? One way we can look at this question is through the factor of water potential, the potential amount of energy contained within water.

Basically, differences in water potential throughout the water contained within a plant helps control the movement of water through the plant. It’s a complex system, one that requires the most sophisticated hydroengineering. What can I say? Plants know their water.

Water Potential Equation

So, how exactly does water potential work? Well water, like most things, naturally moves along a gradient, or from areas of high to low concentrations of energy. That means that water will always move from areas of high water potential to areas of low water potential, from lots of energy to low energy, trying to balance out the total energy within a water system. This is how plants transport from water from soil into their roots and throughout their cells. The transport of water across a membrane, like the membranes surrounding cells, is called osmosis.

If the total water potential in a cell is lower than the potential of water in the ground, that water will naturally move into the plant through osmosis, seeping through the membranes of root cells. Once the water is inside the plant, controlling water potential throughout various leaves and branches determines the distribution of that water.The total water potential in a system is a combination of a series of factors, which we can calculate through the water potential equation. It looks like this:water potential = capillary potential + osmotic potential + pressure + gravitational potentialGreat, now what does this mean?Let’s start with capillary potential. This indicates the amount of water that adheres to the matrix of the solid cells within the plant. It is always a negative number- zero, so it always removes potential from a system. Next is osmotic potential, the impact of solutes on the ability of water to move throughout a system.

Solutes are things dissolved into the water. After that we’ve got pressure, or the amount of force generated by water. Water pressure creates an outward force, which is how plants keep their rigidity and shape.

Finally, we have gravitational potential, or the force of gravity upon the water in the plant. This, too, negatively impacts potential, since plants must overcome this gravity to transport water up the stem.

Impact of Solutes

Now, when calculating water potential, two factors have the most dramatic impact. First is solutes, the dissolved components of water. Pure water has no solutes, so the osmotic potential is zero, with no negative effect on total water potential.

However, the more solutes you add, the more this changes. Adding solutes to water decreases the total potential by decreasing osmotic potential. It doesn’t matter what kind of solute it is, this is always true. But, this isn’t always bad. Cells are filled with a liquid mixture of substances called the cytoplasm.

There are lots of solutes in this mixture, from various molecules and enzymes to nutrients, so osmotic potential is low, and overall water potential is low. Since the water potential in the cell is lower than the potential of fresh water absorbed by the plant, this fresh water continually flows into the cell, providing the hydration needed for the cell to survive.

Impact of Pressure

The other major factor on water potential is pressure, the outward force on the cell wall.

If there’s too much pressure, the cell bursts, but if there’s too little, it collapses. So, it’s important to keep this number stable. The constant pressure needed to maintain cell shape and strength is called turgor pressure. It’s basically this pressure that lets plants stay upright and rigid. This is also another way that plants use the natural effects of water potential to stay hydrated.

You see, when water is lost from leaves through evaporation, which always happens since leaves must keep pores open to take in oxygen, the pressure in those cells decreases, and the overall water potential decreases. Since water potential is lower in the leaves than the roots, water moves along the gradient in an attempt to equalize the entire system, replenishing the area with low potential. So, the plant keeps water flowing wherever it needs to go.

Lesson Summary

Plants are masters at using the natural properties of water. One place we see this is with water potential, the potential amount of energy contained within water. Since water always moves along a gradient from high potential to low potential, plants can create areas of low water potential to absorb and distribute fresh water. We can calculate the total water potential in a system with the water potential equation, which is:water potential = capillary potential + osmotic potential + pressure + gravitational potentialThe two biggest impacts on this are solutes (dissolved components within water) and pressure (outward force of water).

When the amount of solutes increases, osmotic potential decreases, and total water potential decreases. When the pressure increases, water potential increases. Both of these can be used to decrease water potential in specific areas, forcing the movement of high potential water into various cells of the plants. Plants are great at moving water.

I guess they just understand their potential.

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