Electrical Potentials in 'W. Murcott' Tangor Trees in Response to Salinity Stress

In response to abiotic stress, plants have been shown to generate electrical signals which may serve as rapid communication pathways between roots and shoots. The objectives of this study were to test the effects of salinity stress on electrical potentials (EP) in tangor citrus trees and to determine if EP responses to salinity are related to physiological variables such as stem water potential (SWP) or stomatal conductance (gs). Two experiments were conducted with 2-year-old ‘W. Murcott’ tangor trees growing hydroponically in dilute Hoagland solution in a greenhouse. In the first experiment, trees were subjected for 9 days to either no NaCl (T0) or 50 mM of NaCl (T1) added to the nutrient solution. In a second experiment, trees were subjected for 7 days to either no NaCl (T0) or 62.5 mM of NaCl (T1) added to the nutrient solution. In each experiment, EP was continuously monitored with electrodes placed at 3 different locations along the stem of each tree: EP₁) base of the trunk, EP₂) middle of the trunk, and EP₃) within the canopy. Electrical potential differences (ΔEP) between each pair of electrodes [base of the trunk and center of the trunk (Δ₁), ~ center of the trunk and within the canopy (Δ₂), and base of the trunk and within the canopy (Δ₃)] were also determined. Additional variables measured were: vapor pressure deficit in the greenhouse, electrical conductivity of the nutrient solution (EC), SWP, and gs. In experiment 1, the initial EC of the nutrition solution was 1.3 and 8.3 mmhos/cm for T0 and T1, respectively and 1.1 and 10.6 mmhos/cm for T0 and T1, respectively in experiment 2. In both experiments, VPD influenced EP regardless of electrode location. However, VPD had less of an influence on EP in trees subjected to salinity stress (T1) than in the control (T0) trees. Trees grown in nutrient solution with higher salinity (T1) had significantly lower SWP after 3 days than trees in the control (T0) treatment. Also, EP was positively correlated with SWP in both experiments. The electrical response variable most highly correlated with SWP was EP₃ (R² = 0.76) in experiment 1 and Δ₁ (R² = 0.95) in experiment 2. No differences in gs were observed between treatments in either experiment. This study showed that there are detectable EP changes in response to salinity stress that are correlated with SWP in tangor citrus trees.