Key concepts for plant health
This course is a primer on water potential. If you would like a more in-depth exploration of water potential, visit the knowledge base, where you can find articles, case studies, webinars, podcasts, and extensive education guides.
One of the biggest mistakes made when trying to understand plant-water availability is to focus on water content. Water content describes how much water is present within the soil, but it doesn’t tell you whether or not the plant can actually access the water. Water potential is the measurement that describes the water’s availability to be accessed by plants.
Plants take up water along a water potential gradient, which is represented by a negative value. For example, the water potential at a plant’s roots may be –30 kilopascals (kPa), while the atmosphere on a dry day can reach as low as –100,000 kPa. Like sucking on a straw, this gradient pulls water from greater to lesser potential, from soil to roots, through the plant, and out into the atmosphere. As moisture moves through this gradient from soil to roots, through the plant, and out into the atmosphere, water potential continues to get lower.
What is water potential?
A few things must be discussed to understand water potential. Water potential is just the energy state of water, and when water is in its purest form, we call that zero water potential. There are several factors that lower the energy state of water from zero. In this course we will discuss a few of them, including matric potential.
- Adhesion: The adhesive property of water causes it to bind to soil and other media, as seen when water is clinging on the surface of soil.
- Cohesion: The cohesive property of water causes it to bind to itself, as seen when water pools on or collects in a medium without being absorbed.
As water molecules draw closer to the surface of the soil particles, the water and soil particles bind together tighter, making separation from each other more difficult. As the water content of the soil lessens, the water potential also decreases and can get more negative, to the point where plants cannot recover overnight after attempting to pull water from the soil all day. We call that permanent wilting point.</div
Matric potential is not the only thing that lowers the energy state of water. Solutes in water affect its osmotic potential, lowering the total energy state. This happens when charged solutes, such as salts, attract oppositely charged water molecules, which lowers the energy state of the water. Since plants draw in water through semi-permeable membranes in the roots, solutes that lower osmotic potential will make it even more difficult for plants to avoid wilting because water is less available for plant-water uptake or other processes.
The pressures of turgid
Leaves develop high pressure in cells to remain turgid or firm. The loss of turgor is easy to spot – a condition called wilting. Cells use osmotic potential to develop these high pressures. The process of turgid works as follows:
- Plants actively take up solutes across their semi-permeable cell membranes – water can move in passively, but solutes cannot.
- As these ions are taken up, the potential inside the cell becomes more negative.
- A higher potential outside the cell helps water to move passively across the plasma membrane into the cell until equilibrium is reached both inside and outside of the cell.
- With the membrane confining the cellular solution, intense pressures build up, sometimes amounting to pressures multiple times higher than a car tire.
We observe this as the normal phenomenon of leaf turgor where plasma membranes press firmly on cell walls, keeping the leaf rigid and well-positioned to absorb the sun’s rays. Leaves wilt as the water potential outside the cell drops to near the osmotic potential inside the cell and water is no longer moving along the gradient across the membrane to apply pressure on the cell walls.
Water potential and relative humidity
Humidity and water are related through the Kelvin equation. In Fig. 5, the Kelvin equation is used to show this relationship. While the relative humidity (hr) of the atmosphere isn’t always at 50%, for the purposes of the example in Fig. 5, this value has been chosen as an average value. With that assumption, we can determine the correlating water potential of the atmosphere to be –100,000 kPa. Compared to just inside the leaf, which is often around –1000 kPa, the atmosphere is extremely dry. Fig. 5 also reminds us that the entire range of water potentials of even a wilting plant covers only about 3% relative humidity – 97 to 100% hr. Maybe that’s why it takes specialized devices to measure water potential well. Plants of varying kinds have different mechanisms in place to manage their own water potential—some control their water potential for stress, some control their stomates, etc. Whatever their methods, all plants try to keep their leaves in a hydrated state so they can take part in photosynthesis. As stated, the soil can have a wide range of water potentials. Close to zero, the water moves easily from the soil into the roots along this water potential gradient, into the xylem, into the leaves, and out into the atmosphere. As it dries, it becomes more and more difficult to remove that water because the gradient is smaller.
