The 2012 ASHS Annual Conference

Recent studies have shown that electrodes inserted into the stem of some fruit tree species can be used to measure internal electrical signals in response to changes in environmental variables such as soil water content.  However, the relative effects of internal or external factors, such as placement of electrodes, light intensity or air temperature, on electrical responses to environmental stimuli have not been quantified. We measured the effects of short periods of darkness and light, placement of electrodes along the stem, and air temperature on electrical potential (EP) in young walnut (Juglans regia) trees. Six trees were subjected to ten minutes of absolute darkness [photosynthetically active radiation (PAR): 0 µmol·m^-2·s^-1] followed by 20 minutes of artificial light (PAR: 21 µmol·m^-2·s^-1), followed again by 10 minutes of darkness (PAR: 0 µmol·m^-2·s^-1).  Electrical potential (EP) was measured with three electrodes inserted into the trunk of each tree at 8.5, 32.5, and 37.5 cm above the soil surface. Electrical potential differences (ΔEP) between each set of electrodes and air temperature were also continuously monitored during the experimental period. Data were analyzed by Principal Component Analysis to determine the main factors associated with changes in EP and ΔEP. The factor that explained the most variation in EP and ΔEP was light intensity (explaining 57.6% of the variance). Position of the electrode along the trunk accounted for the second greatest amount of variability (15.1% of the variance) in EP and ΔEP, whereas temperature had no significant effect on EP and ΔEP. The results suggest a clear relationship between electrical signals in walnut trees with respect to light/dark cycles, and that the electrode placement along the stem should be taken into account when relating electrical signaling to environmental variables such as light/dark cycles.