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Nitrate Reductase Activity in Sudangrass Cultivars¹

Seventeen sudangrass [Sorghum sudanense (Piper) Stapf] varieties and sudangrass-sorghum (Sorghum color L. Moench) crosses were evaluated for nitrate reductase activity by both in vitro and in vivo procedures. They were also evaluated for total cyanide concentration. Measurements were made on leaf tissue of 10-, 14-, and 18-day-old growth chamber-grown seedlings. Nitrate reductase activity ranged from 9.6 to 3.3 µmoles NO^–₂.g fresh wt^–1.hr^–1 for the in vitro assay and from 3.9 to 0.8 µmoles NO^–₂.g fresh wt^–l.hr^–1 for the in vivo assay. Significant differences in nitrate reductase activity among genotypes were detected by both assays. While the in vivo activity was only 30% that of the in vitro assay, the range and relative rankings of the genotypes were similar as shown by the overall correlation, r = 0.6 (significant at the 1% level). Total cyanide concentration in the leaf tissue ranged from 16.4 to 1.5 µmoles.g fresh wt^–1 and significant differences were noted among the 17 genotypes. Both nitrate reductase activity and cyanide concentration decreased with plant age.

Maximum recovery of nitrate reductase activity was obtained with an extraction medium containing 20 mM gluthathione and 4 mM NiCl₂ The optimum tissue: extraction medium ratio was 1:20. The NiCl₂ protected nitrate reductase from exogenous cyanide up to 7.2 µmoles/g, which is higher than is likely to be produced in the extracts by hydrolysis of the cyanogenic glucoside found in sudangrass.

Key Words: Dhurrin • Plant cyanide levels • Nitrate reductase assays

² Soil Scientist, USDA Southwestern Great Plains Research Center, Bushland, TX 79012, and Professor of Plant Physiology, University of Illinois, Urbana, IL 61801.

Received for publication August 27, 1973.