Simply put, the pressure chamber is just a device for applying air pressure to a leaf (or small shoot), where most of the leaf is inside an air-tight chamber but a small part of the leaf stem (the petiole) is exposed to the outside of the chamber through a seal. The amount of pressure that it takes to cause water to appear at the cut surface of the petiole on the outside of the chamber tells you how much tension the leaf is experiencing on its water: a high value of pressure means a high value of tension and a high degree of water stress. The units of pressure most commonly used are the Bar (1 Bar = 14.5 pounds per square inch). What is measured with the pressure chamber? In simplest terms, the pressure chamber can be thought of as measuring the "blood pressure" of a plant, except for plants it is water rather than blood, and the water is not pumped by a heart using pressure, but rather pulled with a suction force as water evaporates from the leaves. Water within the plant mainly moves through very small interconnected cells, collectively called xylem, which are essentially a network of pipes carrying water from the roots to the leaves. The current model of how this works is that the water in the xylem is under tension, and as the soil dries, or for some other reason the roots become unable to keep pace with evaporation from the leaves, then the tension increases. Under these conditions you could say that the plant begins to experience "high blood pressure”. Figure 2 provides a conceptual illustration of the soil-plant-atmospheric continuum and what the pressure chamber is measuring. Because tension is measured, values are reported as negative numbers. An easy way to remember this is to think of water stress as a "deficit:" the more the stress, the more the plant is experiencing deficit water. The scientific name given to this deficit is the "water potential" of the plant. The actual physics of how the water moves from the leaf within the pressure chamber to the cut surface just outside the chamber is more complex than just "squeezing" water out of a leaf, or just bringing water back to where it was when the leaf was cut. In practice, however, the only important factor is for the operator to recognize when water just begins to appear at the cut end of the petiole. Refer to Figure 3 below for an illustration of how midday stem water potential is measured. Midday stem water potential levels vary depending on the orchard crop and the level of water stress in the orchard. In almonds, prune, and olive midday stem water potential may range from –6 to -8 bars tension, when trees are under little or no water stress. In contrast, almond, prune, and olives trees under severe water stress may show midday stem water potential levels of –30 to –35 bars tension. In walnut, midday stem water potential may range from –2 to –4 bars tension in trees under little or no water stress. Walnut trees that consistently show midday stem water potential levels of –2 to –4 bars may be at risk of too much water and poor aeration in the root zone. Highly water stressed walnuts will show midday stem water potential levels between –12 and –16 bars.
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