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Effect of Stage of Growth, Temperature, and N and P Levels on the Hydrocyanic Acid Potential of Sorghums in the Field and Growth Room¹

Variation in hydrocyanic acid potential (HCN-p) was studied in three sudangrass [Sorghum sudanense (Piper) Stapf] strains and S. almum Parodi. Under field conditions, nitrogen application did not result in a significant change in HCN-p, but strain and stage of growth when sampled had highly significant effects. ‘Sweet’ sudan. grass and S. almum were high in HCN-p, while ‘Piper’ and Line 109 (LI09) sudangrasses were low in HCN-p. Forty-five days after planting HCN.p peaked, then dropped sharply during the following 15-day interval.

In the first experiment performed under controlled environmental conditions, HCN-p differences among the four strains were highly significant. When P concentration in Hoagland'fss solution was increased from 0.0 ppm to 31.0 ppm, HCN-p was reduced significantly. Lowering the temperature from 30 C day/20 C night to 20/10 C resulted in a highly significant increase in HCN-p. In the second growth room experiment, a low HCN-p strain, L109, was significantly lower in HCN-p than the cultivar, Piper, under three N levels. HCN-p was associated with N-level, being lowest at the low N rate. Under both 30/20 C and 20/10 C temperature regimes, HCN-p was higher when the plants were 30 days old, than when they were 45 days old. Also, HCN-p was lower at 30/20 C than at 20/10 C. When both temperature regimes were lowered to 10/10 C for 10 days, the HCN-p in 55-day old plants was greater than in the 45-day old plants. On the other hand, further reduction of temperature to –5/–5 C for one day at day 55, resulted in the lowest HCN-p 56 days after planting.

Key Words: Environmental factors • Sorghum sudanense (Piper) Stapf • Sorghum almum Parodi • Sudangrass • Prussic acid

² Formerly graduate student (now agronomist, Agricultural Res. Substation, Khashm el Girba, Sudan), professor, and professor emeritus, respectively, Dep. of Agronomy, Univ. of Wisconsin, Madison.

Received for publication April 2, 1979.