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 ¹⁵N 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 ¹⁵N‐cyanide concentrations from 5 to 200 µm , but when exposure occurred in the absence of nitrate and ammonium, ¹⁵N enrichment increased significantly in sorghum shoots at solution cyanide concentrations of ≥50 µm and in wheat roots at 200 µm cyanide. In an experiment with sorghum using ¹³C¹⁵N, there was also a significant difference in the tissue ¹³C:¹⁵N ratio, suggestive of differential metabolism and transport of carbon and nitrogen under nitrogen‐free conditions. A reciprocal ¹⁵N labelling study using KC¹⁵N and ¹⁵NH₄⁺ and wheat demonstrated an interaction between cyanide and ammonium in roots in which increasing solution ammonium concentrations decreased the enrichment from 100 µm 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.