Modelling uptake of Na⁺ and Cl⁻ by tomato in closed-cycle cultivation systems as influenced by irrigation water salinity

The aim of the present investigation was to simulate the uptake concentrations (weights of ion per volume of water absorbed) of Na⁺ and Cl⁻ in hydroponic tomato crops as a function of the NaCl concentration in the root zone. An empirical model was calibrated and validated, which can be incorporated into on-line operating decision support systems aimed at optimizing the nutrient supply and minimizing the discharge of drainage solution in tomato crops grown in closed-cycle hydroponic systems. Three experiments were conducted, of which one was carried out to calibrate the model using irrigation water with NaCl concentration ranging from 0 to 14.7molm⁻³ while the other two experiments were commissioned to validate the model within either a low (0.5-2molm⁻³) or a high (1.2-12molm⁻³) concentration range. The model could successfully predict the uptake concentration of Na⁺, but Cl⁻ could not be simulated by this model at external Cl⁻ concentrations lower than 10molm⁻³. The results indicate that Na⁺ is excluded actively and effectively by the tested tomato cultivar even at low external Na⁺ concentrations, while Cl⁻ is readily taken up at low concentrations, particularly during the initial growing stages. Due to the efficient exclusion of Na⁺ by tomato, the Na⁺ concentration in the root environment increased rapidly to extremely high levels even when the Na⁺ concentration in the irrigation water was relatively low. These results indicate that tomato genotypes characterized by high salt-exclusion efficiency, require irrigation water with a very low NaCl concentration, if they are grown in closed hydroponic systems and the drainage water is not flushed periodically. To maintain Na⁺ at levels lower than 19molm⁻³ in the root zone of the tomato hybrid ‘Formula' in closed hydroponics, a maximum acceptable Na⁺ concentration of 0.53molm⁻³ was estimated for the irrigation water.