Photosynthetic Electron Transport in Isolated Maize Bundle Sheath Cells

Abstract

Fragments of bundle sheath strands, free of mesophyll cells and showing a chlorophyll a/b ratio of 6.0-6.6 were prepared from Zea mays by a mechanical method. They were unable to photoreduce ferricyanide but photoreduced the membrane-permeant 2,5-dimethylquinone at a rate of 250-420 microequivalents/h per mg chlorophyll [Chl] (.mu.eq/h .cntdot. mg Chl) at 21.degree. C. In the presence of the catalase inhibitor KCN, methylviologen catalyzed a Mehler reaction at a rate of 120-180 .mu.eq/h .cntdot. mg Chl. This was increased to 200-350 .mu.eq/h .cntdot. mg Chl when the uncoupler methylamine was added. The rate of endogenous pseudocyclic electron flow, detected as a Mehler reaction, was also considerable (100-150 .mu.eq/h .cntdot. mg Chl with methylamine). Diaminodurene supported a high rate of photosystem I-mediated electron flow to methylviologen (400-750 .mu.eq/h .cntdot. mg Chl). When the tissue fragments were illuminated in a weakly buffered suspension, a reversible rise in the medium pH was observed which apparently originated from H+ translocation in the thylakoids. The kinetics of the pH changes was rather slow (t 1/2 [half-time] > 15 s for pH rise; > 30 for dark decay) but the extent of H+ uptake was substantial (0.1 to 0.3 .mu.eq/mg Chl). All of the electron transport reactions tested, including partial reactions which involve only photosystem I or photosystem II, invariably supported H+ uptake. Possibly 2 sites of energy conservation are associated with the photosynthetic chain in the bundle sheath chloroplasts (as in spinach chloroplasts) and both of these sites are functional in vivo. The pH changes observed in the absence of exogenous electron carriers were abolished by 3-(3,4-dichlorophenyl)-1,1-dimethylurea or by anaerobiosis, indicating that the underlying endogenous electron transport was strictly a pseudocyclic reaction. There was no evidence of endogenous cyclic electron flow which might contribute to the energy metabolism of the bundle sheath cells.