By Arturo Calderón, Universidad de Chile researcher and University of California Davis PhD

The Scholander pressure chamber is the most utilized water stress measure for wine grapevines. Although its use has been restricted in vineyards that use water stress to improve berry quality, recent scientific advances have allowed it to be incorporated into systems for other fruit crops.

In California, for example, many commercial almond orchards use pressure chambers to determine the best time for irrigation. Farmers compare actual values from the chamber with a reference value that represents a well-watered, stress-free plant.

This method has allowed a considerable decrease in the amount of water used for almond production without compromising output levels or fruit quality. University of California Davis researchers recommend starting irrigation when the chamber measurement falls between one to two bars below the reference value. This avoids plant stress and over-watering.

Scholander pressure chambers provide numerous benefits compared to other water stress measurement techniques, such as porometers or infrared thermometers. Among them are:

• less variability from environmental conditions, especially from stem measurements.
• measurements that relate directly to soil moisture, and with plant growth and development. In other words, it is possible to estimate how affected soil and crops will be when faced with water stress.
• sensitivity to irrigation application differences, which allows precise identification of drought conditions in the orchard.

Among the disadvantages are:

• cost and maintenance. Depending on the model, some pumps in Chile for example can cost over US$3,000 when imported directly from the U.S. In general, pressure chambers use compressed gas cylinders, which makes maintenance complex. For this reason, it is a requirement  that operators are trained in handling equipment that uses high pressure gases to avoid accidents.
• highly intensive manual labor that requires various, daily measurements to find average stress levels. When soil and plant condition are more variable, more measurements are necessary.

As with any measurement, it is essential to use the correct methodology to avoid errors in determination of water stress.

Pressure chambers can be used successfully to measure water stress in a variety of vegetables.

There is abundant scientific literature about the effects of different levels of water stress on various crops. For wine grapevines, for example, stress values between 10 and 12 bars before the cutoff have improved color compound concentration, astringency and berry aroma.

In other crops that do not use such stress levels to improve quality, reference values also exist using weather information to avoid crop strain. High costs, mainly from labor, can limit the use of this technique for crops that have sufficient output.

This Spanish-language video is available that teaches the fundamentals of vine water stress measurements using a pressure chamber.

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