Plant uptake of NaCl in relation to enzyme kinetics and toxic effects

Water and salt uptake of growing plants were measured simultaneously with the willow tree test. Toxic effects were determined. Concentrations of sodium chloride in solution ranged from 0 to 20 g/L and were determined by electric conductivity. Concentrations above 0.8 g/L strongly inhibited transpiration. Above 1 g/L growth was inhibited as well. The amount of salt taken up into the plants was calculated from the mass balance. Below 1 g/L, salt was taken up more slowly than water, leading to an increase of the NaCl concentration in solution. Above the breakthrough point at a concentration between 1.5 and 2.3 g/L (depending on exposure time), salt was taken up as fast as water. These concentrations were severely toxic to the plants.The uptake of salt into plants could be calculated with a non-linear mathematical model. The model considers inflow of salt with the transpiration stream and enzymatic removal. It predicts that below a breakthrough point, plant enzymes pump back most NaCl, while above, salt is taken up at the same rate as water. The kinetic parameters of the Michaelis–Menten equation were determined by inverse modeling. The half-saturation constant KM was approx. 0.1 g/L; the maximal enzymatic removal rate was 19–20 g NaCl/kg plant root and day. The model for simultaneous salt and water uptake by plants can be coupled to groundwater models in order to predict the salt budget of ecosystems and to manage agriculture in arid zones.