A New Embedded Sensor for Continuous Monitoring of Stem Water Potential in Grapevines

This project is to extend the application of a recently developed microtensiometer (MT) device, which measures water tension, to the measurement of stem water potential (SWP) in grapevines under field conditions. The objectives of this project were to test the accuracy of this device using standard laboratory methods, test the performance of sensors compared to pressure chamber measured SWP under field conditions, and help optimize sensor packaging and installation for accuracy and robustness in the field. Laboratory and greenhouse testing indicated that the sensors responded correctly to water potential, although in some cases, sensor response to changes in water potential was relatively slow and temperature sensitivity high, both of which might limit sensor performance in the field. Improvements in the sensor and in handling/installation have been made and are ongoing, but near-commercial prototypes were installed in field vines starting in August, 2018. A total of 14 sensors were installed on 6 mature Cabernet Sauvignon vines in the RMI vineyards at Davis, CA. These vines were minimally drip-irrigated, had not been irrigated for about 3 weeks, and did not receive any irrigation once the sensors were installed. Essentially all sensors exhibited similar daily patterns in SWP that were comparable to patterns shown by the pressure chamber, but there was large sensor-to-sensor variation in the range of water potential values reported. For these vines, the relative values of the sensors indicated that the highest SWP did not occur at predawn (prior to 6:00am in August), as is commonly assumed, but rather about an hour after dawn. The midday minimum water potential (maximum stress) occurred around 3:00 pm, about 2h after peak reference evapotranspiration (ETo). These findings are consistent with previous in-situ measurements in other woody perennial species. Each MT was calibrated against the corresponding pressure chamber measured SWP on an individual vine basis, giving consistent MT measured SWP values for the 5 (of 6) vines in the study that had enough calibration points. The average values from these vines showed a very clear pattern of overnight recovery to about -3 bars from August to November, when rains caused an increase to above -2 bars. Midday SWP values were around -7 bars, also recovering with normal seasonal leaf loss as well as rains. Midday MT measured SWP also fluctuated in parallel with the weather-related baseline (fully irrigated) SWP for about 3 months, indicating the possibility for continuous monitoring over relatively long periods. These data illustrate that automated and sensitive monitoring of SWP in grapevine is possible with these sensors, and should provide useful information for irrigation management as well as for physiological studies.