Simulation of shoot chloride accumulation: separation of physical and biochemical processes governing plant salt tolerance

Dalton et al. (1996) showed that increasing root temperature from 18 degrees C to 25 degrees C increases the root zone salinity threshold value (i.e. a critical value above which yield is reduced) to tomato by 96%. In contrast, the threshold value of a new dynamic salinity stress index, SSI, was found to be invariant to root temperature. This paper shows through model simulation and greenhouse experiment that (1) the apparent increase in salt tolerance is a physical manifestation of the dynamics of salt loading to the shoot and does not reflect fundamental changes in biochemical processes affecting salt tolerance and (2) that the root temperature invariance of the threshold value of the SSI represents an intrinsic property of the plant related to the biochemical mechanisms of salt tolerance, thereby separating the physical and biochemical processes governing plant salt tolerance. These hypotheses are tested in part by simulating shoot chloride accumulation in terms of a temperature dependent physical-mathematical model describing the simultaneous transport of water and salt into the shoot of a transpiring plant and comparing the model output with experimentally determined chloride accumulation in tomato plants (Lycopersicon esculentum Mill.) grown in a greenhouse and in nutrient solution at root temperatures of 18 degrees C and 25 degrees C and respective root zone chloride concentrations of 33.2 and 63.4 mmol. Root surface area development rates and cumulative water use were measured as input parameters to the model. Experiment and model showed good agreement at 18 and 25 degrees C. The shoot chloride simulations for two environments with different growth potentials demonstrate that the salt accumulation process is controlled by the transpiration rate, the development rate and biophysical transport properties of the root. Chloride concentration in the root zone is just one of many physical parameters affecting the critical salt accumulation rate, relative to growth, that is necessary to reduce yield.