Influence Of Plant Growth Regulators And Nitrogen Application On The Hydrocyanic Acid And Morpho-physiological Parameters In Drought-stressed Sorghum Species

Sorghum（Sorghum bicolor Moench）is widely distributed in arid and semi-arid areas,mainly being used for food,forage,bioenergy,and baijiu production.However,water shortage associated with the climate changes severely influenced sorghum production all over the world.The increase of cyanogenic glycosides（CGs）in forages would increase the risk of hydrocyanic acid（HCN）to animals,which would prohibit the activity of metalloenzymes in animals,especially cytochrome oxidase and the definitive enzyme in the respiratory electron transport chain,and thus lead to poisoning,mild to severe illness,and even death.Droughts not only reduce the biomass of sorghum but also increase the risk of HCN toxicity to animals,mainly due to increased HCN content in drought-stressed plants.Therefore,it is important to control the contents of CGs in fodder plants and to balance between defensive responses and food safety.Therefore,three experiments were conducted in this study,including one field experiment and two pot experiments,aiming to elucidate the potential of plant growth regulators and nitrogen applications in balancing obtaining suitable plant biomass and harvesting forages with low HCN accumulation.In field experiment,the variations of HCN contents in 12 sorghum genotypes（10 sweet sorghum cultivars,one sudangrass and one forage sorghum）were investigated at jointing,filling and ripening stages under rain-fed conditions.In pot experiments,three sorghum genotypes,one sweet sorghum,one sudangrass and one forage sorghum,which differed greatly in their leaf HCN contents in fields,were further selected.In the first pot experiment,about 100μg/L abscisic（ABA）and 100μg/L methyl jasmonate（MeJA）were sprayed separately or together twice on drought-stressed（50%field capacity（FC）and well-watered conditions）plant leaf.The drought lasted for 15 days.In the second pot experiment,nitrogen was applied(0,60,90 and 120kg N ha^-1)on plants under two water conditions（drought,55%FC;well-watered conditions）.The pot experiments replicated four times.The physiological responses of plant growth regulators and nitrogen applications were evaluated in mitigating the concentrations of HCN in drought-stressed plants.The main results were as follows:In field experiment,sorghum species differed greatly in their leaf HCN contents in fields,ranging from 31 mg kg^-11 to 482 mg kg^-1.The HCN accumulation showed a reducing trend from the jointing stage to filling stage and then showed an increasing trend in some sorghum genotypes,particularly in two sweet sorghums（America Giant and Sea lion）.In first pot experiment,hydrocyanic acid and soluble protein accumulation in three genotypes increased significantly under water-stressed conditions.The application of exogenous hormones overall reduced the HCN contents from both plants of control and those subjected to drought in sweet sorghum and sudangrass.The HCN contents in ABA treated plants were significantly higher than those in MeJA treated plants.Plants sprayed with ABA and/or MeJA significantly reduced the soluble protein content in forage sorghum.Drought stress overall reduced the plant height,stem diameter,leaf number per plant and dry weight of all genotypes.ABA treatment significantly increased the plant weight in all genotypes under well-watered conditions.The photosynthetic rate（P_N）,transpiration rate（Tr）,stomatal conductance（Gs）and total chlorophyll（TChl）reduced,whereas the intercellular CO₂ concentration（Ci）increased in all genotypes by drought.The application of exogenous ABA and/or MeJA increased P_N,Tr,and Gs in sweet sorghum.For forage sorghum,MeJA increased P_N and Tr.The total chlorophyll content was improved by ABA and/or MeJA treatments in non-stressed and stressed plants.Drought significantly increased the levels of hydrogen peroxide content（H₂O₂）,superoxide anion（O₂^-）,malondialdehyde（MDA）and antioxidant enzyme activities in all genotypes,except for SOD activity in sudangrass.The plants treated with MeJA had the lowest O₂^-whereas the ABA+MeJA had the highest O₂^-release rate in all three genotypes,and the plants treated with ABA had the highest H₂O₂release rate in all three genotypes.The application of exogenous ABA and/or MeJA reduced the activities of peroxidase（POD）,superoxide dismutase（SOD）,catalase（CAT）,and ascorbate peroxides（APX）in sweet sorghum except for insignificant change of POD activity by ABA+MeJA treatment.In second pot experiment,interactive effects of nitrogen and drought on hydrocyanic acid accumulation in sorghum plants were observed.HCN accumulation in all genotypes increased significantly under water-stressed conditions in comparison to well-watered conditions.The application of nitrogen under drought conditions usually increased the HCN concentration by 62.14%,31.74%and 62.88%for sweet sorghum,sudangrass,and hybrid sorghum,respectively.Drought stress reduced the plant height,diameter,number of leaves,total leaf area,fresh weight,dry weight and dry root weight while the application of nitrogen at 120 kg N ha^-11 increased plant height,diameter,number of leaves,total leaf area,fresh weight,dry weight and dry root weight in all genotypes.P_N,Tr,Gs and TChl reduced,whereas the intercellular CO₂ concentration（Ci）increased under drought-stressed conditions.The application of nitrogen at 90 kg N ha^-1increased P_N,Tr,and Gs,while decreased Ci at well-watered and drought-stressed conditions.Drought caused a significant increase in the levels of H₂O₂,O₂^-,MDA,soluble protein,SOD,POD,CAT and APX,in all genotypes.Among the nitrogen treatments,the plants treated with 90 kg N ha^-11 had the lowest O₂^-,H₂O₂ and MDA content in all three genotypes.The application of nitrogen at 120 kg N ha^-11 under drought increased soluble protein and proline contents in all genotypes.For hybrid sorghum,the application of nitrogen at 90 kg N ha^-11 gave the highest SOD,POD,CAT and APX under drought conditions.Drought stress and nitrogen application had a significant effects on nitrogen,phosphorus and potassium concentrations in plants of all genotypes.The application of nitrogen at 120 kg N ha^-11 increased N by 70.80%and53.25%and P by 28.76%and 36.92%,and while 90 kg nitrogen.AT 90 kg N ha^-1increased K by 31.94%and 43.80%,in well-watered and drought-stressed hybrid sorghum,respectively.In conclusion,drought conditions hampered the morphological growth and yield-related traits of all genotypes.Our studies indicated that plant growth regulators and an optimum rate of nitrogen applications could adjust the growth of sorghum under drought-stressed and well-watered conditions.Plant growth regulators（ABA/MeJA）mitigated negative effects of drought and reduced the concentration of HCN,whereas exogenous nitrogen at 90 kg N ha^-11 could strike a balance between plant defensive responses and the food safety under drought-stressed and well-watered conditions.