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As a critical index for irrigation scheduling, plant water deficit index (PWDI) is defined as the ratio of water deficit to water demand to reflect the extent of abiotic stresses such as water and salinity. Recently, smart irrigation scheduling, according to PWDI thresholds to maintain desirable or acceptable stress levels, has been suggested to maximize yields while minimizing negative environmental effects under non-saline conditions. To investigate and quantify the potential for PWDI-driven irrigation on agricultural production under conditions of salinity, a two-year experiment with six specific thresholds was conducted in Shawan of Xinjiang for drip-irrigated cotton under film mulch. Results indicated that, with increasing PWDI threshold, irrigation depth per event increased, while irrigation frequency and total volume decreased. Consequently, the soil water and salt environment deteriorated, resulting in less nutrient uptake, slower growth, and lower yield and net profit. With particularly high PWDI thresholds leading to serious stress conditions, fiber quality was also negatively affected. Within a designed range of PWDI thresholds between 0.39 and 0.62, an elliptic function characterized the processes of water application, yield and net profit (R2 ≥ 0.95), and water productivity could be described by a parabolic function (R2 = 0.77). These quantitative results were used to provide guidelines for smart irrigation scheduling under local conditions considering water management measures and market prices of cotton. For a reference market price of 7.5 CNY kg-1, a PWDI threshold of 0.49 was found to optimize economic benefits while maximizing water productivity. When prices of cotton are prohibitively low, a lower threshold should be considered to obtain an acceptable net profit. Otherwise, a higher threshold would be preferable to use water more efficiently. Further verification and improvement are necessary to deal with more complex scenarios, such as, considering crop sensitivity to water and salinity stresses at different growth stages and optimizing irrigation depth per event. • Plant water deficit index (PWDI) is used for smart irrigation under saline conditions. • PWDI threshold affects soil water and salt dynamics, crop growth and production. • Relations between PWDI threshold and water productivity/net profit are quantified. • A PWDI threshold of 0.49 was used to balance local ecological and economic benefits. • Irrigation regime is optimized with market factors and water management measures. [ABSTRACT FROM AUTHOR]

Delineating root-water-uptake (RWU) under conditions with augmented CO 2 concentrations is very important for scheduling irrigation to contend with climate change. Responses of plant growth to elevated CO 2 concentration (e[CO 2 ]) have been widely reported, while the effects of e[CO 2 ] on RWU has hardly been studied. A hydroponic experiment of wheat (Triticum aestivum L.) with five NO 3 − -N concentrations (Exp. 1) was conducted to investigate and quantify the effects of e[CO 2 ] on RWU activity. Another experiment growing wheat in soil columns with four combinations of water and N supply levels (Exp. 2) was conducted to validate the results obtained in Exp. 1, establishing a macroscopic RWU model to simulate soil water dynamics under e[CO 2 ]. Although CO 2 acclimation was observed in both experiments, plant canopy and root growth were generally stimulated under e[CO 2 ], while transpiration consumption was not synchronously enhanced due to decreased stomatal conductance, indicating an increase in water use efficiency while a decrease in RWU activity. Potential transpiration was found more linearly related to root nitrogen mass (RNM) than root length under various CO 2 concentrations, regardless of wheat growth stage, water and N supply level. Consequently, RNM density was used to drive the RWU model. The results from Exp. 1 indicated that the effects of e[CO 2 ] on water uptake coefficient per RNM could be quantified by a recently proposed nonlinear stomatal conductance response model (R 2 = 0.84, RMSE = 0.55 cm3 mg−1 d−1). The RWU model reliably simulated the dynamics of soil water transport and wheat transpiration under e[CO 2 ] in Exp. 2 with the RMSE and relative errors mostly less than 0.03 cm3 cm−3 and 10 %, respectively. Practical application of the established RWU model for any other specific conditions is expected to benefit from optimization of parameters following choice of most appropriate stomatal conductance response model. • Elevated CO 2 concentration (e[CO 2 ]) limited plant transpiration in a short term. • Under e[CO 2 ] root growth was promoted while water uptake activity was limited. • Plant potential transpiration was linearly proportional to root N mass under e[CO 2 ]. • Effects of e[CO 2 ] on root water uptake activity were described by a nonlinear model. • A root water uptake model was established to simulate soil water flow under e[CO 2 ]. [ABSTRACT FROM AUTHOR]