The Pressure Chamber (The Bomb)
There are different types of pressure chambers, some using a tank of high pressure gas, some using a hand-pump, but all work on the same basic principle and in most cases the operation and use of the different types of pressure chambers is identical. This introduction gives examples using a hand-pump device that was developed here at UCD. Differing operational details will be noted in brackets [ ] for the pressurized tank style. All devices involve a potentially hazardous level of pressure however, and safety precautions related to the handling of the pressure chamber must be observed by the operator.
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 the 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 petiole 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) and the Mega Pascal (1 MPa = 10 bars).
Because tension is measured, negative values are typically reported. 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 a deficit of 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.
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The most important leaf factor is the rate of water loss from the leaf at the moment of sampling. During the daytime, fully exposed, outer canopy leaves will lose water at a faster rate than shaded inner canopy leaves. A faster rate of water loss causes a more negative water potential. This factor can be eliminated, however, by covering the leaf for a minimum of about ten minutes prior to sampling, which is the recommended procedure. Covering stops the process of water loss. The water potential then equals the water potential in the stem where the leaf is attached. Water potential measured this way is called stem water potential.
The major advantage of stem water potential in trees is measurement uniformity: the type of leaf (spur leaf, shoot leaf), size or shape of leaf, and physiological condition of the leaf (nutritional status) has no influence on stem water potential. Leaf position within the canopy has a small effect, showing a slightly more negative stem water potential with increasing distance from the root system. For this reason the recommended leaf position in trees is from the lower canopy interior, close to the main trunk or scaffold branches.
The most important plant factors are: weather conditions at the time of sampling, soil dryness,and root health.
For a fully irrigated tree with a healthy root system, we have found that weather conditions can be taken into account using a table of expected, or baseline values corresponding to weather conditions of air temperature and relative humidity. Baseline tables have been established and confirmed for prunes and almonds (which have the same baseline values), and tables based on preliminary data are available for walnuts and pears. In all cases, hotter and dryer conditions cause a more negative stem water potential. For midsummer conditions in California the values of stem water potential measured on a fully irrigated prune or almond tree, for instance, will typically be between -6.0 bars and -10.0 bars.
The relationship of soil dryness to stem water potential is straightforward: as the soil becomes dryer, stem water potential will become more negative. The pressure chamber measures effective soil dryness throughout the root system as a whole. This is very different from soil-based monitoring methods, which only measure the soil in part of the root zone. Just after a full coverage irrigation (sprinkler or flood) stem water potential should correspond to the baseline value, and as the soil dries, stem water potential will become more negative than the baseline value. For drip or micro-sprinkler systems that do not wet the entire soil, stem water potential may always be somewhat more negative than the baseline values, even after a full irrigation. This is probably due to the fact that some roots are in non-irrigated, dry areas of soil.
Root health will cause stem water potential to be more negative than the baseline, even under wet soil conditions. The process of root water uptake is not well understood, but any factor that influences root health, such as physical damage, damage by pests, infections by disease organisms, or poor soil aeration, will probably reduce the ability of roots to absorb water, and will cause stem water potential to be more negative than the baseline.
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