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 use of shallow saline groundwater may alleviate the shortage of freshwater resources in the Low Plain around the Bohai Sea in China. The soil salt levels and the responses of cotton seed emergence, growth, and yield to continuous irrigation with saline water were investigated from 2012 to 2014. Four salinity levels were tested: 1.3 (S1, control), 5.4 (S2), 8.8 (S3), and $12.4\mathrm{dS}/\mathrm{m}$ (S4). Irrigation was initiated when the soil moisture content in the 0–60 cm layer was below 60% of field capacity. Soil salinity within the cotton root zone increased with an increase in irrigation water salinity, but the effect was not cumulative over the experimental period to because of the leaching effect associated with summer rainfall. The cotton emergence percentage decreased as the irrigation water salinity increased. Compared with S1, the average seedling establishment rates over the three seasons examined decreased by 2.1, 5.1, and 20.2% with S2, S3, and S4, respectively. Large variations in seedling establishment rates were observed between the seasons, and this variability increased with increase in irrigation water salinity. Maximum plant height, leaf area index, and number of fruiting shoots decreased by S3 and S4, and some of these differences were statistically significant ($p<0.05$ ). Cotton growth was not negatively affected until irrigation water salinity reached $8.8\mathrm{dS}/\mathrm{m}$ . Cotton yields were close to that produced by freshwater irrigation (S1) when the irrigation water salinity was below $8.8\mathrm{dS}/\mathrm{m}$ , but it significantly ($p<0.05$ ) decreased by 7.3% in 2012, 11.1% in 2013, and 8.6% in 2014 when the irrigation water salinity was $12.4\mathrm{dS}/\mathrm{m}$ , mainly due to the elevated soil osmotic potential. These results show that cotton can be irrigated successfully with water of $\le 8.8\mathrm{dS}/\mathrm{m}$ . Furthermore, the mitigation of salt accumulation by heavy rainfall indicates that long-term use of saline water irrigation is feasible in this region.