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Metabolism and Enzymology

Cyanide-Resistant Respiration in Light- and Dark-Grown Soybean Cotyledons ¹

Department of Agronomy, Iowa State University, Ames, Iowa 50011, Department of Botany, Iowa State University, Ames, Iowa 50011, Department of Plant Pathology, Iowa State University, Ames, Iowa 50011

Measurements of respiration were made on intact tissue and mitochondria isolated from soybean (Glycine max [L.] Merr. cv `Corsoy') cotyledons from seedlings of different ages grown in light and darkness. Effects of cyanide (KCN) and salicylhydroxamic acid (SHAM) on O₂ uptake rates were determined. O₂ uptake was faster in light-grown tissue and was inhibited by both KCN and SHAM in all except light-grown tissue older than 9 days. Both inhibitors stimulated O₂ uptake in tissues more than 9 days old. Mitochondria in which O₂ uptake was coupled to ATP synthesis were isolated from all tissues. O₂ uptake by mitochondrial preparations from light- and dark-grown cotyledons was equally sensitive to KCN. Similarly, age did not affect KCN sensitivity, but sensitivity to SHAM declined with age both in the presence and absence of KCN. Estimated capacities of the cytochrome and alternative pathways of the mitochondrial preparations indicated considerably larger cytochrome than alternative pathway capacities. The cytochrome pathway capacities paralleled the state 3 mitochondrial respiration rates, which increased from day 5 to day 7 then declined thereafter. The alternative pathway capacities were not affected by light. The uncoupler, p-trifluoromethoxycarbonylcyanide phenylhydrazone (FCCP), increased the flow of electrons through the cytochrome pathway at the expense of flow through the alternative pathway in isolated mitochondria. However, the combined capacities did not exceed the rate in the presence of FCCP. The results are interpreted to indicate that the stimulation of respiration by KCN and SHAM observed in the 12-day-old green cotyledons and previously observed in older soybean leaves is not explained by characteristics of the mitochondria.

¹ Journal Paper No. J-12882 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Projects No. 2496, 2724, and 2789. Supported in part by Pioneer Hi-Bred International, by U.S. Department of Agriculture CSRS Special grant 59-2191-1-2-010-0, and by U.S. Department of Agriculture Competitive Research grant 86-CRCR-1-2039.