Nitrogen supply and cyanide concentration influence the enrichment of nitrogen from cyanide in wheat ( Triticum aestivum L.) and sorghum ( Sorghum bicolor L.)

Cyanide assimilation by the β ‐cyanoalanine pathway produces asparagine, aspartate and ammonium, allowing cyanide to serve as alternate or supplemental source of nitrogen. Experiments with wheat and sorghum examined the enrichment of 15 N from cyanide as a function of external cyanide concentration in the presence or absence of nitrate and/or ammonium. Cyanogenic nitrogen became enriched in plant tissues following exposure to 15 N‐cyanide concentrations from 5 to 200  µ m , but when exposure occurred in the absence of nitrate and ammonium, 15 N enrichment increased significantly in sorghum shoots at solution cyanide concentrations of ≥50  µ m and in wheat roots at 200  µ m cyanide. In an experiment with sorghum using 13 C 15 N, there was also a significant difference in the tissue 13 C: 15 N ratio, suggestive of differential metabolism and transport of carbon and nitrogen under nitrogen‐free conditions. A reciprocal 15 N labelling study using KC 15 N and 15 NH 4 + and wheat demonstrated an interaction between cyanide and ammonium in roots in which increasing solution ammonium concentrations decreased the enrichment from 100  µ m cyanide. In contrast, with increasing solution cyanide concentrations there was an increase in the enrichment from ammonium. The results suggest increased transport and assimilation of cyanide in response to decreased nitrogen supply and perhaps to ammonium supply.