Testing sap-flow sensors to predict irrigation of soilless tomato fertigated with saline water

In Mediterranean vegetable greenhouses, the optimization of fertigation in presence of saline water is a crucial goal. The use of decision support systems, for calculating irrigation water requirements, based on crop evapotranspiration (ETc) models and on soil, plant, climate data and weather forecast, is helpful for greenhouse farmers although not much applied. In soilless systems, evaporation from the substrate is negligible and irrigation can be inferred considering plant transpiration only. Therefore, sap flow sensors are promising tools for estimating plant transpiration in horticulture, so far mainly used in forestry and fruit crops. This work aimed to validate the use of these sensors in tomato (Solanum lycopersicum L.) by comparison with ETc simulation models. Two trials were conducted on cherry tomatoes in the summer-autumn season, in two unheated greenhouses located in the Centre and South Italy, under a closed soilless system. In the first location (Italy), plants were grown on rockwool slabs and drip-irrigated with saline water up to 75 mM NaCl. In the second location (south Italy), plants were grown on coconut fiber slabs and subirrigated with saline water up to 70 mM NaCl. Sap-flow sensors, using the stem heat balance method, were mounted at 30 cm from the collar of the plants. The actual daily water uptake was measured through flow meters in a closed-loop systems. Greenhouse climate was continuously recorded throughout the crop by a meteorological station. Sap-flow sensor estimates showed significant correlations with flow meter data and climate parameters, at the different salinity levels, thus showing the high capability to replace, or integrate, other tools to optimize irrigation in Mediterranean greenhouses.