Electrical Signals in Walnut Trees in Relation to Soil Water Content and Vapor Pressure Deficit

Several studies have shown that electrical signals in plants are a mechanism for communicating the detection of environmental stimuli, such as soil water content, air temperature or relative humidity, from the site of detection to more distant plant organs. Most of those studies were conducted with herbaceous plants and relatively little is known about electrical signaling in woody plants. Moreover, little is known about the relationship between the electrical signal strength and the intensity of the environmental variable. We measured electrical signals in young walnut (Juglans regia) trees in relation to soil water content and vapor pressure deficit (VPD) in a glasshouse. Trees were subjected to two irrigation treatments over a three day period: T1) one irrigation event per day, or T2) two irrigation events per day. The same quantity of water was applied during each irrigation event so that trees in T2 received twice as much water as trees in T1. The VPD in the glasshouse fluctuated naturally throughout the day. Soil content was continuously monitored with frequency domain reflectometry probes and VPD was recorded using a Hobo sensor/datalogger. The electrical potential (EP) was measured with three electrodes inserted into the trunk at 8.5, 32.5, and 37.5 cm above the soil surface. Electrical potential differences (ΔEP) between each set of electrodes were also recorded. Data were analyzed by Principal Component Analysis (PCA) and Partial Least Squares Regression (PLS), with the objective of quantifying the relationship between irrigation regime and VPD with respect to EP and ΔEP. PLS analysis to find correlations among EP and ΔEP, soil water content and VPD, showed that 79.3% of the variation in EP and ΔEP could be explained by changes in soil water content and VPD. Thus, multivariate analyses were useful for quantifying electrical signals in walnut trees produced in response to changing environmental conditions. The results also suggest that measurement of electrical signals has potential as a sensor system for quantifying physiological responses of walnut trees to environmental changes.