Effects of soil salinity on foxtail millet osmoregulation, grain yield, and soil water utilization under varying water conditions

Salinity and water deficit are major abiotic environmental stresses affecting crop quality in arid and semi-arid areas. Particularly, salinity stress induces several detrimental physiological effects on crops, including toxic ion accumulation, disturbed osmotic potential, and reduced grain yield. However, how salinity stress reduces growth loss and maintains water productivity (WP) by compensating for water conditions in foxtail millet remains unclear. Herein, a two-year experiment was conducted to investigate the effects of two soil water levels [W1, 45% ± 5% field capacity (FC); and W2, 75% ± 5% FC; FC, 0.34 cm3 cm−3] and ten soil salinity levels (S, 1.0–2.8 g kg−1 in 0.2 g kg−1 intervals) on foxtail millet flag-leaf osmoregulation, grain yield, and soil water utilization in the Bohai Lowland of the North China Plain. The results indicated that the plant survival rate was significantly lower at W2 than at W1 three days after sowing. Compared with W1, the Na+ (2.94–9.96% for S1.8 and S2.0) and proline contents (2.74–61.48%) at W2 were lower from the heading to the grain filling stage, whereas the K+ content (5.39–23.24% from S1.6 to S2.0, except for S1.6 at the flowering stage) was significantly higher. With increasing salt stress, the leaf water potential dropped continuously. Compared with S1.0, evapotranspiration at S1.6, S2.2, and S2.8 decreased significantly by 8.36–72.70% at W1 (except for S1.6) and by 4.69–52.44% at W2 in both study years. In 2021, aboveground biomass decreased at W1, but remained stable at W2 in S1.0–S1.4, at the mature stage. Compared with W1, W2 increased the grain yield (1.54–110.85%) but decreased water productivity at the grain yield (WPy, 1.54–33.72%) and biomass levels (WPbm, 2.36–47.49%). Overall, the grain yield at the soil salinity levels of 1.87 and 1.91 g kg−1 (at W1 and W2, respectively) was only half that at 1.0 g kg−1 soil salinity. Altogether, this study provides guidelines for foxtail millet cultivation in saline soils.