A pressure chamber offers a relatively inexpensive way to determine the amount of water stress a vine is experiencing, whether you’re using deficit irrigation or simply want to improve your irrigation efficiency.
The technique involves placing a leaf blade into the sealed chamber, leaving the petiole exposed. The chamber is then pressurized, using a hand pump or a CO2 or nitrogen tank (depending on the model), just until sap begins to exude from the cut end of the petiole. The pressure at which this occurs, shown on a gauge, is the stem water potential, measured in negative centibars. The more negative the pressure reading, the greater the water stress on the vine.
In the summer issue of Vit Tips, the San Joaquin Valley viticulture newsletter, Allison Ferry-Abee, University of California Cooperative Extension farm advisor for Tulare and Kings counties, describes how the tool can be used to determine a vine’s water needs by measuring the water potential of either the leaf or the stem.
Both are done during the day, when the vine’s water demand peaks at solar noon. But because of daylight saving time, that’s actually 1:00 p.m. in the summer and early fall. In practice, though, this period is extended from one hour earlier to one allow later to provide more time for measuring the water potential.
VINE’S WATER NEEDS
Both are excellent methods to better understand the vines’ water needs, Ferry-Abee says. “Leaf water potential is measured as quickly as possible after removal from the vine to prevent excess evapotranspiration.
“Stem water potential tends to be slightly more accurate, but is also more time-consuming, Leaves are covered with a foil-covered bag and left on the vine for an hour or more before measurement. This extra step allows transpiration to stop and water levels in the leaf to equalize with the rest of the vine.”
She lists the basic steps for each method:
Mid-day leaf water potential: Measure from 12:00 noon to 2:00 p.m.; use leaves fully exposed to the sun. Wrap leaves tightly in a clear bag and cut off the leaf with a sharp razor blade. Quickly insert the leaf into the pressure chamber, and slowly pressurize the chamber. Carefully watch the petiole with a magnifying loupe. As soon as you see sap beginning to exude from the petiole, stop increasing the pressure. The pressure in negative centibars is the leaf water potential.
Mid-day stem water potential: Measure from 12:00 noon to 2:00 p.m. Select leaves in the shade.
Wrap leaves tightly in a bag covered in aluminum foil to block out the sun and prevent the bag from becoming a mini-greenhouse.Leave the leaf and bag on the stem for 1½ to 2 hours.
Cut off the leaf with a sharp razor blade and measure as above.
Regardless of the method used, Ferry-Abee recommends taking at least three measurements per vineyard block or soil type, then averaging the results. Select leaves that are fully expanded and healthy, with no holes or pest damage. Also, try to keep the timing between cutting the petioles and beginning pressurization consistent, and raise the pressure in the chamber at the same rate for all samples.
Accurate results require consistency, she notes. The measurements should be taken by someone — preferably the same person — who is well-trained in the procedures.
“Pick one of these methods and stick with it,” Ferry-Abee says. “Don’t switch between leaf and stem water potential throughout the season.”