Saline irrigation improves survival of forage sorghum but limits growth and increases toxicity

Moderately saline water has been proposed as a potential irrigation resource for crops such as forage sorghum (Sorghum bicolor × Sorghum bicolor nothosubsp. drummondii) in drought-prone regions. However, it is not yet fully understood how salinity affects growth and potential toxicity of sorghum. Sorghum produces the cyanogenic glucoside dhurrin, which can cause hydrogen cyanide (prussic acid) poisoning in grazing animals. To address this, two glasshouse experiments were conducted. Experiment 1 assessed tolerance of sorghum to a range of salt treatments (0, 12.5, 25, 50, 100 and 150 mM NaCl). Experiment 2 assessed whether moderately saline irrigation would relieve drought stress by growing sorghum under three treatments: no watering (drought), watering with freshwater, or watering with 50 mM NaCl. All treatments lasted 7 weeks. In Experiment 1, salinities as low as 25 mM NaCl significantly reduced biomass, despite sorghum being able to exclude sodium from entering transpiring leaves at NaCl concentrations up to 50 mM. Foliar concentrations of dhurrin positively correlated with salinity and exceeded the safe threshold for cattle of ≥12.5 mM NaCl. In Experiment 2, moderately saline water effectively alleviated drought stress of sorghum, with significant reductions in growth and photosynthesis in the drought treatment compared to the 50 mM NaCl treatment. While sorghum's survival and growth may be boosted by moderately saline irrigation during droughts, its cyanogenic glucoside concentrations should be monitored to ensure safety for grazing animals.