Drought-induced changes in nitrogen partitioning between cyanide and nitrate in leaves and stems in sorghum grown at elevated CO2 are age dependent

Authors: GLEADOW, R - Monash University; Kimball, Bruce; Wall, Gerard - Gary; Pinter Jr, Paul; LEAVITT, S - University Of Arizona; OTTMAN, M - University Of Arizona

Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2015
Publication Date: 1/1/2016
Publication URL: https://handle.nal.usda.gov/10113/61704
Citation: Gleadow, R.M., Kimball, B.A., Wall, G.W., Pinter Jr, P.J., Leavitt, S.W., Ottman, M.J. 2016. Drought-induced changes in nitrogen partitioning between cyanide and nitrate in leaves and stems in sorghum grown at elevated CO2 are age dependent. Field Crops Research. 185:97-102.

Interpretive Summary: Sorghum is considered drought tolerant and therefore may be a more favorable crop for farmers to raise with climate change in the future. However, it can store cyanide and nitrate in its leaves which are toxic to cattle. Therefore, cyanide and nitrate concentrations were measured of stored samples of shoots of sorghum grown in the FACE (Free Air CO2 Experiment) trials in 1998 and 1999. These trials exposed open-field plots of sorghum to elevated concentrations of CO2, such as expected in the atmosphere near mid-century, as well as to ample and limited levels of water supply. The results showed there were no significant differences in total nitrogen, nitrate or cyanide levels as a result of elevated levels of CO2 alone. However, concentrations of both cyanide and nitrate were higher in plants that had been water stressed. Therefore, in the future it is likely that drought will continue to be the single most important factor in determining sorghum toxicity. This research benefits both future growers and consumers of sorghum and other agricultural crops.

Technical Abstract: Sorghum [Sorghum bicolor (L.) Möench] is the world’s fifth most important crop, grown for forage, grain, and as a biofuel. Fast growing and drought tolerant, it is considered a climate-change-ready crop. Two free-air CO2 enrichment (FACE) experiments at Maricopa, Arizona, USA showed that, like other C4 grasses, its photosynthetic rates were relatively unaffected by elevated CO2, but stomatal conductance was reduced, thereby increasing water use efficiency. Sorghum is cyanogenic (i.e. it contains prussic acid) and can be toxic to cattle, especially during droughts. Sorghum also stores nitrogen as nitrate that can cause nitrite toxicity under certain conditions. We measured cyanide and nitrate concentrations of shoots of sorghum grown in the FACE (Free Air CO2 Experiment) trials in 1998 and 1999. Plants were grown at ambient and elevated CO2 (ca. 550 ppm) and either irrigated regularly (Wet), or only once after sowing (Dry). Ontogenetic effects were taken into account by comparing plants of the same stage of development as well as the same chronological age. There were no significant differences in total nitrogen, nitrate or cyanide levels on a per mass basis as a result of CO2 alone. Contrary to observations in C3 plants, there was no increase in allocation of nitrogen to cyanide or nitrate in plants grown at elevated CO2 either. Concentrations of both cyanide and nitrate were higher in Dry plants. The increase in water use efficiency of Dry plants grown at elevated CO2 did not cause any significant decrease in the concentration of cyanide in leaves or stems. We conclude that in the future drought will continue to be the single most important factor in determining sorghum toxicity and propose the development of low cyanogenic forage varieties, although care will need to be taken to monitor any impact on foliar nitrate.