Plant water deficit index-based irrigation under conditions of salinity

In arid and semi-arid regions, water scarcity and soil salinization are major factors impacting sustainable agricultural production. In this study, a macroscopic root-water-uptake model was used to adapt a plant water deficit index (PWDI) for irrigation scheduling under conditions of coexisting soil water and salinity stress-causing factors. The traditional approach, estimating PWDI with average root zone soil water and salt amounts, was improved by weighting the effects of soil water and salinity according to the normalized root length density profile. An experiment growing wheat (Triticum aestivum L.) in soil columns and an experiment growing cotton (Gossypium hirsutum L.) in a salinized field were implemented to explore and quantify the effects of soil water and salinity conditions on plant water status, and thus to validate the improvement and evaluate its application, by monitoring soil water and salinity dynamics and plant growth indexes (e.g., leaf area, dry weight, leaf water potential, transpiration and yield). The results indicate that, even under conditions with equal root zone averages of soil matric and osmotic potentials, plant water status might be significantly different. In general, plants were less stressed when more water and less salinity were allocated in the upper root zone with more roots while less water and more salinity occurred in the lower root zone with less roots. By referring to some information in the soil column experiment, a numerical experiment was conducted to further demonstrate the improvement. The root-weighted approach resulted in improved PWDI estimation and thus was more reliable for irrigation scheduling, leading to higher irrigation frequency and quantity, leaf area index, biomass, yield, and transpiration, without significant decrease in water productivity. However, further improvement could be possible by considering the effects of historical soil water and salinity stresses as well as meteorological conditions on plant water status.