The competition for scarce water resources has increased in many world regions due to the population growth, economic development and environmental concerns. It is anticipated that irrigation water demand will continue to increase in the foreseeable future. Because it is not easy to obtain new water resources, strategies to optimize the agriculture water use efficiency (WUE) are needed. While, in some countries, such as Italy and Spain, much effort has been done at the technological level to modernize the irrigation systems improving the off-farm WUE, at the farm level there are still opportunities to improve WUE. Precision irrigation by closely matching irrigation application to the actual plant and whole field water needs is one possible solution, but in woody perennial crops there are also opportunities to improve WUE by deficit irrigation. In this respect, regulated deficit irrigation (RDI) is an on-farm irrigation strategy where water restrictions are imposed during certain periods of the crop phenological cycle, while full water needs are replaced during the rest of the season. Unlike deficit irrigation, RDI has a strong physiological basis because it is fully related with the tree and fruit crop growth annual cycles. This short review summarizes useful aspects to consider when using RDI highlighting the importance of determining plant and soil water status and the appropriate crop phenological cycle when to reduce irrigation applications. The results of several RDI experiments carried out in woody perennial crops and the techniques useful for detecting plant and soil water status are summarized. The main focus is to review results obtained mainly in citrus and deciduous fruit trees with water restrictions applied in different crop growth cycles. Most of the discussed results are taken from experiments were water quality was not a constraint, but recent research on RDI use in case of low quality water is also briefly discussed. About two decades of research have shown that by means of RDI it is often possible to reduce water applications by about 10- 30% without significantly affecting crop performance and, in some cases, improving fruit quality, or obtaining other agronomic benefits with respect to fully watered trees. However, often end-users are not purposely applying RDI because of the lack of knowledge on the long-term responses and the difficulties to accurately manage and determine plant water stress in response to the irrigation supply. This brief review ends with some possible solutions to boost the final adoption of RDI by end-users, not only to save water, but also to obtain agronomical advantages. Modelling the soil-plant-water orchard continuum and incorporating the results in user-friendly decision support systems are considered as strategies to facilitate users with tools for their day-to-day RDI regime decisions taking.
Keywords: fruit trees, citrus, deficit irrigation, fruit quality