Modelling salt accumulation by a bean crop grown in a closed hydroponic system in relation to water uptake

Four different NaCl concentrations in the irrigation water, 0.8, 3, 6 and 9 mol m−3, were applied as experimental treatments to beans (Phaseolus vulgaris L.) grown in completely closed hydroponic systems in a greenhouse. Initially, the Na and Cl concentrations increased rapidly in the root zone, as indicated by the values measured in the drainage water, and this resulted in corresponding increases in the Na/water and Cl/water uptake ratios. However, as these ratios approached equilibrium with the NaCl/water ratios in the irrigation water, the Na and Cl concentrations in the root zone converged to maximal levels, which depended on the treatment. The highest Na and Cl concentrations in the root zone and the corresponding NaCl concentrations in each treatment were used to establish relationships between the external NaCl concentration and the Na/water or Cl/water uptake ratios, which proved to be exponential for Na but linear for Cl. These relationships were then used in a previously established model [Savvas, D., Kotsiras, A., Meletiou, G., Margariti, S., Tsirogiannis, I., 2005a. Modeling the relationship between water uptake by cucumber and NaCl accumulation in a closed hydroponic system. HortScience 40, 802–807] to enable the prediction of the Na and Cl concentrations in the root zone in relation to the cumulative water uptake. The curves predicted by the model followed a convex pattern, with an initially rapid increase in Na and Cl concentrations in the root zone followed by a gradual levelling out as the cumulative water consumption rose. The measured Na and Cl concentrations in the drainage water were more accurately predicted at the higher NaCl concentrations in the irrigation water, although those predicted at 0.8 mol m−3 of NaCl were considered acceptable for use in commercial practice. Bean showed a high efficiency of Na exclusion from the upper leaves, while Cl was readily translocated to the young leaves as the external Cl concentration rose. Plant growth decreased with increasing salinity in a way similar to that reported for beans constantly exposed to comparable salinity levels.