Avocado trees are among the most water demanding crops in California, requiring about 142 gallons of water to grow one pound. Avocado trees have shallow roots that are strongly affected by drought stress and overwatering, both of which lead to early fruit drop. In California, the current drought conditions and diminishing levels of water supply due to climate change are threatening the amount of irrigation water for agricultural and horticultural production. The aim of this study was to find the best measure of plant water status in avocado trees to guide irrigation scheduling and minimize water use. This experiment was conducted in a Fuerte avocado orchard at the Fullerton Arboretum in Fullerton, California. Trees were subjected to temporary drought conditions for two-week intervals, during which several plant water status parameters were monitored. The state of water stress was determined directly using leaf water potential measurements. In addition, responses to water stress were measured in situ continuously using daily trunk diameter variations via dendrometry, trunk water potential via stem psychrometry, wood water content using frequency domain reflectometry, and sap flux using heat-ratio-method sap flow sensors. Soil water status was also monitored continuously using soil moisture content and soil water potential sensors.
Diurnal comparisons between plant-based sensors at the start and end of a dry down period showed a slight decrease of wood water content in response to stress. Trunk diameter variations increased, peaking at midday and afternoon, both before and during the dry-down periods. Diurnal trends of trunk diameter variations and trunk water content at the end of a dry-down period suggest a lag between transpiration and bark/sapwood water depletion. Diurnal patterns of sap flux density were consistently lower in the inner compared to the outer sapwood suggesting that only the most recent growth rings are highly active in water transport in avocado. Finally, diurnal comparisons of leaf and trunk water potential both similarly increased as the soil moisture content decreased. These findings suggest that stem psychrometers may be the best continuous tool for detecting plant drought stress reliably in avocado and other water demanding horticultural crops. The findings are expected to be more broadly applicable to measurements of water status of trees in natural environments and forest plantations.