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Seasonal Variation in Leaf Hydrocyanic Acid Potential of Low- and High-Dhurrin Sorghums¹

The KS8 and N32 sorghum [Sorghum bicolor (L.) Moench] lines are low and high, respectively, in the hydrocyanic acid potential (HCN-p) of mature leaves. Thisd difference is conditioned primarily by a single pair of alleles. The main objective of his study was to determine, at various staes of plant growth and various times during the growing season, the HCN-p of upper leaves and tillers of field-grown plants of these two parental lines and of two low-HCN-p F₃ lines derived from crosses between KS8 and N32. The four entries were grown in a randomized complete block design with three replications in 1985. Samples of leaf tissue were dried, ground, and extracted, and cyanide in the extracts was assayed colorimetrically. Using a mean HCN-p level of 500 mg kg^–1 dry wt to separate safe from unsafe sorghum forage, all samples of KS8 mature leaves and tillers would be considered safe, and all N32 samples would be considred potentially dangerous. Values for most of the samples of the F₃ lines fell within the safe range, but some samples of young regrowth exceeded the 500 mg kg^–1 limit. Regressions of HCN-p on height for upper leaves of main stems and of tillers indicated a significant negative relationship for all entries except for leaves from the amin stems of KS8. However, the relationship was not close enough to support the use of plant or tiller height as a reliable indicator of HCN-p. Levels of HCN-p also were determined for mature leaves and young regrowth of hybrids involving KS8, N32, and ‘Redlan’ sorghums as seed parents and NP25, ‘Piper’, and ‘Greenleaf’ sudangrasses [S. sudanense (Piper) Stapf] as pollinators. Results indicated that for minimizing the risk of cyanide poisoning, KS8 would be the seed parent of choice, and NP25 and Piper would be the preferred pollinators.

Key Words: Cyanogenesis • Prussic acid • Sorghum bicolor (L.) Moench • Sorghum sudanense (Piper) Stapf • Sorghum x sudangrass hybrids

² George Holmes professor of agronomy; supervisory research geneticist, USDA-ARS; and research geneticist, USDA-ARS, respectively.

Received for publication July 31, 1986.

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