Drought stress (DS) is one of the abiotic stresses that plants encounter commonly in nature, which affects their life, reduces agricultural output, and prevents crops from growing in certain areas. To enhance plant tolerance against DS, abundant exogenous substances (ESs) have been attempted and proven to be effective in helping plants relieve DS. Understanding the effect of each ES on alleviation of plant DS and mechanisms involved in the DS relieving process has become a research focus and hotspot that has drawn much attention in the field of botany, agronomy, and ecology. With an extensive and comprehensive review and summary of hundred publications, this paper groups various ESs based on their individual effects on alleviating plant/crop DS with details of the underlying mechanisms involved in the DS-relieving process of: (1) synthesizing more osmotic adjustment substances; (2) improving antioxidant pathways; (3) promoting photosynthesis; (4) improving plant nutritional status; and (5) regulating phytohormones. Moreover, a detailed discussion and perspective are given in terms of how to meet the challenges imposed by erratic and severe droughts in the agrosystem through using promising and effective ESs in the right way and at the right time.

The study of plant photosynthesis under different degrees of drought stress can provide a deeper understanding of the mechanism of plant drought resistance. In the Taklimakan Desert, saline groundwater is the only local water source with regard to shelterbelt construction and determines plant growth and photosynthetic changes. In this study, daily dynamics of gas exchange parameters and their responses to photosynthetic photon flux density at three irrigation levels (W1 = 17.5, W2 = 25, W3 = 35 mm) were measured for two main species, i.e., Calligonum mongolicum (C) and Haloxylon ammodendron (H). H was better adapted to drought stress than C. Net photosynthetic rate (P_N) was mainly related to soil water status in the main root system activity layer. In July, the daily variations of P_N and transpiration (T_r) for C were higher than H. C increased water use efficiency (WUE) with increases in P_N, while H decreased T_r to obtain a higher WUE. Either C or H, drought reduced the low light and metabolic capacity, and thus decreased the light adaptability and photosynthesis potential. We suggest a prerequisite understanding of physiological mechanisms and possible plant morphological adjustments required to adapt plant species to desert drought conditions.

As a good carbon source and soil conditioner, biochar is widely used in acidic soils but seldom in alkaline soils due to its high pH. In this study, cotton straw biochar was modified with five different acidic materials to obtain wood-vinegar- (WBC), monosodium-glutamate (MSG)-wastewater- (MBC), citric-acid- (CBC), phosphoric-acid- (PBC), and nitric-acid-modified biochars (NBC), and three dosages were used for each modifier. The pristine and modified biochars were characterized with scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy. The biochar properties such as pH, specific surface area (SSA), and elemental contents were measured. In addition, the technique for order preference by similarity to ideal solution (TOPSIS) model based on entropy weight was used to evaluate the application potential of the biochars in alkaline soils. The FTIR spectra showed that modification with the five acidic materials, MSG wastewater in particular, resulted in more oxygen-containing functional groups such as O-H, C=O, and C-O on the biochar surface. In addition, acid modification greatly decreased the pH: phosphoric acid modification significantly decreased the pH of cotton straw biochar by 5.71–7.88 units. For the same modifier, a higher dosage (i.e., a smaller biochar:modifier ratio) led to a larger decrease in the pH of cotton straw biochar. The magnitudes of increase in total soluble salt content followed the general order of CBCs > PBCs > WBCs > NBCs > MBCs. The SSA, average pore diameter, and total pore volume of biochar were changed as well. Modification using wood vinegar and MSG wastewater significantly decreased the SSA of cotton straw biochar by 15.58–16.24 m² g⁻¹ (82.7–86.2%) and 15.87–16.80 m² g⁻¹ (84.2–89.2%), respectively, whereas modification using citric acid and nitric acid significantly increased the SSA of cotton straw biochar by 4.51–4.66 m² g⁻¹ (23.9–24.7%) and 0.55–54.21 m² g⁻¹ (2.9–287.7%). The evaluation based on entropy weight TOPSIS model suggested that the MBCs have the highest potential for application in alkaline soils. This study presents a theoretical basis for evaluation of biochar application potential, demonstrates a way of improving biochar application potential, and provides a support for beneficial utilization of agricultural and industrial wastes such as cotton straw, wood vinegar, and MSG wastewater.

Freshwater resources are in a shortage in arid regions worldwide, especially in extremely arid desert areas. To solve this problem, highly saline groundwater is used for drip irrigation of desert plants. Since more irrigation infiltrating into the deep soil cannot be absorbed and utilized by desert plants, it is crucial to determine optimal water-saving irrigation regimes. In this study, we examined the effects of irrigation regimes on the soil water dynamics of two typical woody halophyte species (Haloxylon and Calligonum), and quantified the irrigation intervals and periods based on a field test of precision irrigation control in the Taklimakan Desert Highway shelterbelt. Results showed that the change in soil moisture of two species in the shallow 0–60 cm layer could be divided into a rapid decline period (1–9 d), a slow decline period (9–19 d), and a relatively stable period (19–39 d) after irrigation. The decrease rate of soil moisture at the 0–60 cm depth was significantly higher than that at the 60–200 cm depth. The irrigation regime combining 35 mm irrigation with 10 days was beneficial to soil water storage and plant use with respect to Calligonum, while the irrigation regime combining 35 mm irrigation with 40 days was best for Haloxylon. Increasing the single irrigation amount and prolonging the irrigation period can further enable the more effective use of irrigation water. This study highlights that saline groundwater irrigation provides potential advantages for desert plants’ survival under reasonable irrigation regimes.

Freezing saline water irrigation is an effective technique for leaching soil salts. However, the coupling effects between initial salinity, sodium adsorption ratio (SAR), and freezing temperature of saline water have not been systematically studied. Therefore, a three-factor and five-level quadratic general rotation combination design was adopted, employing indoor simulated freeze–thaw experiments to investigate the influence of these three factors on the freshwater production ratio. The results indicated the following: (1) Within the range of the set experimental factor levels, the freshwater production ratio after the freezing of saline water ranged from 23.34% to 81.11%. (2) The significant negative effect of the main factor on the freshwater production ratio was descending order: initial salinity > freezing temperature. Although the impact of saline water SAR on the freshwater production ratio is insignificant, the interaction between saline water SAR and the freezing temperature has a highly significant negative effect on the freshwater production ratio. (3) At least 83% of the salt content was leached out in the first 16 h when the ice was thawed at 6 °C. These findings are beneficial for selecting appropriate irrigation times based on irrigation water quality in the actual field application of saline water freezing irrigation, with the aim of more efficiently reducing soil salt content.

To investigate how different training modes of salt stress priming affect the dynamic variation of the salt tolerance threshold (STT) in summer maize, a micro-plot experiment with staged brackish water irrigation was conducted. Based on physiological and biochemical parameters, along with shoot and root traits, a dynamic salt tolerance coefficient (α_STT) was defined to quantify STT across growth stages. The results revealed a clear two-stage adaptive response to salt stress, consisting of an initial physiological adaptation phase followed by a phenotypic adaptation phase. Different training modes induced distinct salt stress memory effects by regulating the coordination between these two stages. Among treatments, the S_1-2-3 regime—corresponding to mild (2.0 g·L⁻¹), moderate (4.0 g·L⁻¹), and severe (6.0 g·L⁻¹) salinity applied sequentially at the six-leaf, ten-leaf, and tasseling stages—exhibited the most favorable adaptive outcome, with α_STT gradually recovering to 1.0 at later stages and a concomitantly higher STT. Furthermore, a unified predictive framework was established to estimate STT dynamics, within which the process-constrained PCR-STP pathway outperformed purely data-driven pathways. Overall, our study elucidates the dynamic nature of salt tolerance in summer maize and provides a scientific basis for optimizing brackish water irrigation regimes and refining salt stress modules in crop models.