ELECTRICAL SIGNAL MEASUREMENTS IN AVOCADO TREES: A POTENTIAL TOOL FOR MONITORING PHYSIOLOGICAL RESPONSES TO SOIL WATER CONTENT?

Monitoring soil water content coupled with phytomonitoring techniques have been shown to be good tools for irrigation management in avocado orchards. There are many well-tested devices for monitoring soil moisture in orchards but options for measuring plant water status are limited. The objective of this study was to determine whether measuring voltage differences between roots and leaves can be used as a technique to measure plant water status related to soil water content. Root and shoot voltage differences were monitored in young ‘Hass’ avocado trees grafted onto clonal rootstocks grown in containers, and in young seedling ‘Mexicola’ avocado trees grown hydroponically, both experiments carried out under laboratory conditions. In container-grown ‘Hass’ avocado trees, root and shoot voltage differences were initially measured for about 2 h to determine steady state conditions. Plants were then exposed to cycles of soil drying and re-watering. In seedling ‘Mexicola’ trees, electrical signals were measured for at least 75 min in control plants and plants exposed to simulated drought stress. The extracellular electrical potential difference between the base of the trunk and the leaf petiole was continuously monitored after exposing plants to treatments. Results indicated that in ‘Hass’ avocado trees, a change in soil water content induced by root drying and re-watering was accompanied by a slow change in the electrical signal at the leaf petiole which was greatest after 52 and 32 min for root drying and re-watering, respectively. This was related to a decrease in stomatal conductance (gs) of plants exposed to drought. Also, in hydroponic conditions, significant voltage differences occurred in plants exposed to drought simulated by the addition of polyethylene glycol (PEG 6000) to the hydroponic medium resulting also in a decrease in gs. These results suggest that it may be possible to use voltage differences measurements as a technique for monitoring physiological responses to soil water content.