Measuring Dormant Stem Water Potential in Almond (Prunus dulcis).

As California farmers face growing water insecurity, the need for winter (dormant) irrigation of deciduous tree crops has arisen as an important issue.  Measurement of midday stem water potential (SWP) with a pressure chamber has become a practical and widely adopted tool for irrigation management in many annual and perennial crops during the growing season, but this technique has not been applied to perennial crops during dormancy.  Indeed, the reliability of pressure chamber readings on the twigs of dormant trees has been questioned due to concerns that a low percent of living tissue and/or a high level of embolism may cause inaccurate results.  Psychrometric measurements of water potential are made in the vapor phase and do not depend on the degree of embolism or the percent of living tissue, and hence should be useful in evaluating the accuracy of pressure chamber measurements.  Measurements of water potential using both pressure chamber and thermocouple stem psychrometer methods were compared on dormant branches exposed to different levels of hydration in the laboratory.  A very highly significant (P<0.0001) linear regression was found between the two methods over a wide range of SWP values (about -2 to 0 MPa) in almond, cherry, and walnut, with r-square values of 0.98, 0.91 and 0.90, respectively, and in the case of almond and cherry the slope of the regression was close to 1 (XX-YY).  Field measurements on dormant almonds in two winter seasons with contrasting rainfall showed systematically lower SWP during a dry winter compared to a wet winter, and during a third season, SWP was found to increase in response to a winter irrigation.  This evidence strongly supports the validity of SWP as a measure of dormant tree water status, and its use as a tool to evaluate the need for winter irrigation in dormant tree crops.