Using micro-tensiometers to manage water stress to maximize fruit size of apple orchards

The number of apples per tree can be controlled by precision crop load management but final fruit size is also affected by plant water status. The first objective of this experiment was to study apple fruit growth dynamics in two irrigation treatments. The second objective was to understand apple fruit growth dynamics and relate it to water stress for maximizing fruit size and crop value in 'Gala' apple. During the 2021 growing season we continuously measured stem water potential using micro-tensiometers (MT; FloraPulse sensors) embedded in the trunks of trees. We also measured fruit diameter each week and related fruit growth rate and stem water potential. Two different water regimes were compared (fully irrigated and rain exclusion to induce water stress). All treatments had similar fruit numbers per tree. Our results suggest stem water potential was effectively measured in apple trees with the MT. Trees subjected to rain exclusion showed lower stem water potential values (greater stress) compared to fully irrigated trees. The fully irrigated trees had the highest fruit growth rate per day (mm diameter increase day-1) which declined over the course of the season. The rain exclusion trees had lower fruit growth rate day-1 throughout the season and lower final fruit weight and less fruit color when compared to fully irrigated tree. This study was our first attempt to relate apple fruit growth dynamics and water stress to manage irrigation for maximize fruit size, color and crop value in apple orchards. Our future work will utilize fruit growth dendrometers to relate daily fruit growth increment with stem water potential to fully automate irrigation management.