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Environmental and Stress Physiology

Inhibition of Chloroplastic Respiration by Osmotic Dehydration ¹

Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02254

The respiratory capacity of isolated spinach (Spinacia oleracea L.) chloroplasts, measured as the rate of ¹⁴CO₂ evolved from the oxidative pentose phosphate cycle in darkened chloroplasts exogenously supplied with [¹⁴C]glucose, was progressively diminished by escalating osmotic dehydration with betaine or sorbitol. Comparing the inhibitions of CO₂ evolution generated by osmotic dehydration in chloroplasts given C-1 and C-6 labeled glucose, 54% and 84% respectively, indicates that osmotic dehydration effects to a greater extent the recycling of the oxidative pentose phosphate intermediates, fructose-6P and glyceraldehyde-3P. Respiratory inhibition in the darkened chloroplast could be alleviated by addition of NH₄Cl (a stromal alkylating agent), iodoacetamide) an inhibitor of glyceraldehyde-3P dehydrogenase), or glycolate-2P (an inhibitor of phosphofructokinase). It is concluded that the site which primarily mediates respiratory inhibition in the darkened chloroplast occurs at the fructose 1,6-bisphosphatase/phosphofructokinase junction.

¹ Supported by the U.S. Department of Energy DE-ACO2-76-ERO 3231 and by National Science Foundation PEM 83-04147.