How exactly do we measure the water status of the trees?

You may be wondering how we measure the water status of the trees in our study. We use an instrument called a pressure chamber (Figure 1) that is widely used in plant science as well as in agriculture. A pressure chamber allows you to measure the water potential of the plant.

 

Figure 1. Professor Dawson making a water potential measurement

So what is water potential? Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. You may remember from biology or physics class that water moves from areas of high potential to areas of low potential, or concentration. By definition, a pool of pure water has a water potential of zero, and any substance that has a lower concentration of water will have a negative water potential, or tension (the opposite of pressure), with more negative values indicating lower concentrations.

In order to understand how water is moving from areas of high to low water potential in plants, you need to know a bit about how water actually moves through them, especially the tallest trees on the planet (coast redwoods). There is often a misconception that water is "pumped" from the roots to the top of the tree, but this is incorrect. Water moves through plants because there is a water potential continuum from the soil to the plant to the air, called the soil-plant-atmosphere continuum. The soil is moister than the plant, which is moister than the air. Water is essentially sucked from the soil, through the plant and out into the air following the water potential gradients between the three (Figure 2).

 

Figure 2. Soil-plant-atmosphere continuum

Now you have an idea of how the water moves through the trees. But how do we actually measure the water potential with a pressure chamber? When leaves are cut from a tree, the water in their stems retracts back into the stem because the tension it was under is broken. Think of it like a rubber band. If you stretch a rubber band tight until it breaks, the rubber band will then relax because it is no longer under tension. We take the leaf sample and seal it in the pressure chamber with the cut end of the stem sticking out. The chamber is then slowly pressurized with nitrogen gas (Figure 3). As more gas fills the chamber, pressure on the leaf slowly increases until water emerges on the surface of the cut stem (Figure 4). The amount of pressure required to force the water out of the cut stem is the equal to the tension, or water potential, that the leaf was experiencing at the time that it was sampled.

 

Figure 3. Pressurizing the chamber

 

Figure 4. Water on the surface of cut stem

The water potential of a given leaf will vary depending on its' specific crown position (e.g., tree top versus tree base), what time of day it is (pre-dawn versus mid-day), and how warm and dry the soil and atmosphere are. We'll take a closer look at this variation and how our sampling accounts for it in our next Lab Notes!