Metabolic Activities in Extracts of Mesophyll and Bundle Sheath Cells of Panicum miliaceum (L.) in Relation to the C 4 Dicarboxylic Acid Pathway of Photosynthesis

Abstract

The activities of certain enzymes related to the carbon assimilation pathway in whole leaves, mesophyll cell extracts, and bundle sheath extracts of the C4 plant Panicum miliaceumn have been measured and compared on a chlorophyll basis. Enzymes of the Co dicarboxylic acid pathway-phosphoenolpyruvate car-boxylase and NADP-malic dehydrogenase-were localized in mesophyll cells. Carbonic anhydrase was also localized in meso-phyll cell extracts. Ribose 5-phosphate isomerase, ribulose 5-phosphate kinase, and ribulose diphosphate carboxylase enzymes of the reductive pentose phosphate pathway-were predominantly localized in bundle sheath extracts. High activities of aspartate and alanine transaminases and glyceralde-hyde-3-P dehydrogenase were found about equally distributed between the photosynthetic cell types. P. miliaceum had low malic enzyme activity in both mesophyll and bundle sheath extracts. Isolated bundle sheath cells were capable of converting as-partate to oxalacetate at rates approaching the aspartate trans-aminase activity of bundle sheath extracts. The bundle sheath cells had a light induced C02 fixation of 23 ,tmoles of CO./mg chl hr in the absence of exogenous substrates. The photorespiratory enzymes, hydroxypyruvate reductase and glycolic oxidase, were about 3 fold higher in bundle sheath extracts than in mesophyll extracts when compared on a chlorophyll basis. The initial end products of photosynthesis in plants having the C4 pathway2 of photosynthesis are oxalacetate, malate, and aspartate, which are thought to be synthesized in mesophyll cells (21, 25).