Photosynthetic activities of isolated bundle sheath cells in relation to differing mechanisms of C 4 pathway photosynthesis

Abstract

Photosynthetic activities of bundle sheath cell strands isolated from several C 4 pathway species were examined. These included species that decarboxylate C 4 acids via either NADP-malic enzyme ( Zea mays, NADP-malic enzyme-type), NAD-malic enzyme ( Atriplex spongiosa and Panicum miliaceum, NAD-malic enzyme-type) or phosphoenolpyruvate carboxykinase ( Chloris gayana and Panicum maximum, phosphoenolpyruvate carboxykinase-type). Preparations from each of these species fixed 14CO 2 at rates ranging between 1.2 and 3.5 μmol min −1 mg −1 of chlorophyll, with more than 90% of the 14C being assimilated into Calvin cycle intermediates. With added HCO 3 − the rate of light-dependent O 2 evolution ranged between 2 and 4 μmol min −1 mg −1 of chlorophyll for cells from NAD-malic enzyme-type and phosphoenolpyruvate carboxykinase-type species but with Z. mays cells there was no O 2 evolution detectable. Most of the 14CO 2 fixed by Z. mays cells provided with H 14CO 3 − plus ribose 5-phosphate accumulated in the C-1 of 3-phosphoglycerate. However, 3-phosphoglycerate reduction was increased several fold when malate was also provided. Cells from all species rapidly decarboxylated C 4 acids under appropriate conditions, and the CO 2 released from the C-4 carboxyl was reassimilated via the Calvin cycle. Malate decarboxylation by Z. mays cells was dependent upon light and an endogenous or exogenous source of 3-phosphoglycerate. Bundle sheath cells of NAD-malic enzyme-type species rapidly decarboxylated [ 14C]malate when aspartate and 2-oxoglutarate were also provided, and [ 14C]aspartate was decarboxylated at similar rates when 2-oxoglutarate was added. Cells from phosphoenolpyruvate carboxykinase-type species decarboxylated [ 14C]aspartate when 2-oxoglutarate was added and they also catalyzed a slower decarboxylation of malate. Cells from NAD-malic enzyme-type and phosphoenolpyruvate carboxykinase-type species evolved O 2 in the light when C 4 acids were added. These results are discussed in relation to proposed mechanisms for photosynthetic metabolism in the bundle sheath cells of species utilizing C 4 pathway photosynthesis.