Partial root-zone drying (PRD) is a relatively recent strategy aiming at substantial reduction of irrigation volume. It consists of the alternate irrigation of one portion of the root-zone with volumes reduced to about 40-50% of conventional water. PRD is based on the ability of roots in drying soil to produce hormonal signals, like abscissic acid (ABA), responsible for partial stomatal closure. In turn, partial stomatal closure would improve water use efficiency and increase fruit yield per unit of irrigation water. This technique has been tested on a number of fruit crops, including apple. In addition to climatic conditions, which mainly affect atmospheric water demand, soil properties and water dynamics in the soil seem to play a major role in the outcomes of PRD studies. Irrigation systems and scheduling are also critical for a PRD effect (irrigation placement effect). Apple cultivars with different vigor, period of fruit growth and crop load perform differently in response to PRD and generalizations on the potential use of PRD as a valuable irrigation strategy for apple orchards result in difficulties. Fruit size and number per tree seem to be sensitive to some extent to the reduction of irrigation volumes operated with PRD. On the contrary, vegetative growth is mostly unaffected by PRD and fruit quality may be somewhat improved. These general responses are associated with a relatively good water status of trees under PRD. The hypothesized reductions of tree water loss through stomatal control, however, do not seem to fully explain the lack of water deficit. Higher soil moisture content, in the root-zone of PRD trees compared to trees with similar irrigation volumes but applied to the entire root-zone, has been documented in some studies. This difference, in good part due to diminished soil evaporation, poses some doubts on the real ability of apple trees under PRD to control water loss by root ABA signaling. In this context, longer irrigation events with low delivery rates (i.e., drip systems) should maximize the advantages of PRD in terms of soil moisture. Also, the use of PRD may be most appropriate for cultivation of late-maturing apple varieties with high crop load, which would suffer from relatively severe dehydration under reduced irrigation volumes.
LO BIANCO, R. (2013). Responses of apple to partial root-zone drying. A review.. In M. Hossain Ali (a cura di), Irrigation Management, Technologies and Environmental Impact (pp. 71-86). New York : Hossain Ali, M.