Continuous determination of fruit tree water-status by plant-based sensors

Recently, climate change has caused shortages of water worldwide, especially in semi-arid and arid regions. Several irrigation strategies have been studied with the aim of saving water overuse in agriculture. In the past most of the attention was directed towards soil water content, but recently the focus has moved to plant responses to water deficit. In recent years, crop evapotranspiration (ETc) obtained from reference evapotranspiration (ET0) and crop coefficients (Kc), has become common for irrigation scheduling in several crops, but it does not provide precise insights on the tree water status. Today an increasing focus is being given to plant-based sensors for the continuous monitoring of plant water status to provide support to irrigation management strategies with a precision approach. In this work several plant-based (leaf, stem and fruit) devices used for plant water status sensing and for irrigation scheduling are reviewed. Scientists have managed to create and test a variety of small leaf-adapted sensors with the aim of 7collecting valuable information on water dynamics. Nondestructive continuous water status detection in leaves is difficult due to the intrinsic fragility of these organs. Yet, the data collected can provide insights on the actual status of one leaf, within a multitude of other leaves which might have a slightly different behavior because of factors such as age, sun exposure, canopy position and others. Leaf thickness sensors, leaf pressure and leaf thermal probes are discussed in this review. Stems and shoots establish the connection between climatic conditions and water availability in the soil. Continuous measurements of stem water status by non-destructive sensors provide information not only on the variations of soil water availability but also on the reserves of plant tissues. The use of stem dendrometers, sap flow probes, stem hygrometers and stem microtensiometers for continuous determination of plant water status and irrigation management is discussed. Moreover, it has been demonstrated that fruit water relations have key implication on horticultural production and quality. Measurements of fruit water status and fruit growth dynamics under different irrigation strategies might be crucial in order to reduce water use, maintain yield and/or improve fruit quality. Advantages and disadvantages of different sensors ranging from linear variable displacement transducers (LVDTs), strain-gauges, potentiometers and/or optoelectric sensors, are discussed. However, a unique methodology for continuous plant water status determination in fruit trees has yet to be found. An integrated approach, which considers contemporary use of sensors on different plant organs is proposed as effective strategy to collect exhaustive information on tree water status.