Introduction
Accurate prediction of root water uptake under salinity stress contributes to efficient water management in arid and semi-arid regions. Salinity caused by irrigation is a major limiting factor for crop production in these dry environments. Despite water scarcity, the amount of irrigation water must exceed evapotranspiration to leach excess salts and prevent further soil salinization. To minimize water consumption while avoiding yield reduction, precise prediction of root water uptake under salinity stress is essential. Macroscopic root water uptake models, which employ stress response functions describing the dependence of reduction coefficients on matric or osmotic potential at each soil depth, are widely used in soil water and solute transport simulation models such as HYDRUS and SWAP.
Materials and Methods
In this study, an optimization approach was used to determine the root water uptake parameters within a macroscopic model, and the corresponding root water uptake was quantified. The experimental work was conducted in a greenhouse using potato plants. Six pots were prepared for cultivation, three of which were subjected to salinity stress, while the remaining three were used to measure potential transpiration. Each pot was equipped with two moisture sensors installed at different depths to monitor soil moisture and electrical conductivity. Two potato seed tubers were planted in each pot on September 1, 2018, and ten days later (after germination), the number of plants was reduced to one per pot. To prevent soil evaporation, the pot surfaces were covered during the daytime. The pots were weighed manually on a daily basis to determine the actual transpiration rate. The drought stress period began on December 13, 2018, after the volumetric water content had been reduced to 0.35. Salinity stress was applied to the relevant treatments immediately after the onset of drought stress in two stages. In the first stage, irrigation was performed using a 3000‑ppm NaCl solution starting on December 13, 2018. Since no reduction in plant transpiration was observed by December 23, 2018, a 5000‑ppm NaCl solution was applied on December 23, marking the second stage of salinity stress. The experiments were continued until the relative transpiration (the ratio of actual to potential transpiration) dropped below 0.5. After completing the experiments and following full plant development, all pots were emptied. At the end of the experiment and after the salinity stress period, the root density distribution was determined by harvesting the plants. The root uptake parameters were estimated inversely by minimizing the sum of squared differences between the observed and simulated daily transpiration rates. Finally, root water uptake at each depth and time was calculated by substituting the linearly interpolated osmotic potential into the stress response function.
Results and Discussion
The results showed that the optimized daily transpiration agreed well with the observed values. In addition, the deviations in the three optimized stress response functions were small under low to moderate stress levels, indicating the reliability of the method. In the non-stress treatments, most of the root volume was located in the first and fourth soil quartiles. The high root volume in the fourth quartile (the lower part of the soil profile) was due to the limitation of root penetration by the impermeable bottom of the pots. In the salinity-stress treatments, no specific trend was observed, which indicates that the plant was showed no consistent pattern in root development and water uptake due to the presence of osmotic stress. A comparison between the calculated and measured transpiration values around the 1:1 line shows that, in both the non-stress and salinity-stress treatments, most points were located below the 1:1 line and tended toward the calculated transpiration values. This indicates that the transpiration equation is overestimated and can be corrected by a coefficient for potato. In the salinity-stress treatments, transpiration values were much lower than those in the non-stress treatments. The average value of parameter P₂ (the exponent of the water uptake reduction coefficient equation) was obtained as 4.98. The average value of parameter ho₅₀ (the osmotic potential at which root water uptake reaches 50% of its potential uptake) was obtained as 4244 cm of water. A comparison of the ho₅₀​ values shows that potato is less tolerant to salinity than canola and more tolerant than bean. In the non-stress treatments, since irrigation was not carried out with saline water, no significant reduction in the uptake reduction coefficient was observed; therefore, the uptake reduction coefficient (α) can be considered equal to unity in this case. In all three salinity-stress treatments, as the salt concentration in the soil solution increased (due to irrigation with saline water and the reduction in soil moisture caused by root water uptake), the uptake reduction coefficient decreased. In the salinity-stress treatments, as the osmotic potential increased from 100 to 10,000 cm, the root uptake coefficient decreased from 1 to zero.
Conclusion
As time progressed, and particularly during the mid-growth stage of the potato plants, the transpiration values in the stressed treatments became closer to the mean transpiration of the non-stressed treatments. In the late growth stage, the application of salinity stress in the salinity-stressed treatments led to a decrease in both transpiration and relative transpiration values. This indicates a reduction in the root water uptake capacity in the pots subjected to salinity stress, resulting from an increase in osmotic potential. In the non-stressed treatments, since irrigation was not carried out with saline water, no considerable reduction in the uptake reduction coefficient was observed; therefore, the uptake reduction coefficient (α) may be considered equal to unity in this case. In all three salinity-stressed treatments, as the salt concentration in the soil solution increased due to irrigation with saline water and the reduction in soil moisture caused by root water uptake, the uptake reduction coefficient decreased. In the salinity-stressed treatments, as the osmotic potential increased from 100 to 10,000 cm, the root uptake coefficient decreased from 1 to zero.