Photosynthesis in Flaveria brownii, a C4-Like Species: Leaf Anatomy, Characteristics of CO2 Exchange, Compartmentation of Photosynthetic Enzymes, and Metabolism of 14CO21

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Light microscopic examination of leaf cross-sections showed that Flaveria brownii A. M. Powell exhibits Kranz anatomy, in which distinct, chloroplast-containing bundle sheath cells are surrounded by two types of mesophyll cells. Smaller mesophyll cells containing many chloroplasts are arranged around the bundle sheath cells. Larger, spongy mesophyll cells, having fewer chloroplasts, are located between the smaller mesophyll cells and the epidermis. F. brownii has very low CO2 compensation points at different O2 levels, which is typical of C4 plants, yet it does show about 4% inhibition of net photosynthesis by 21% O2 at 30°C. Protoplasts of the three photosynthetic leaf cell types were isolated according to relative differences in their buoyant densities. On a chlorophyll basis, the activities of phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase (carboxylation phase of C4 pathway) were highest in the larger mesophyll protoplasts, intermediate in the smaller mesophyll protoplasts, and lowest, but still present, in the bundle sheath protoplasts. In contrast, activities of ribulose 1,5-bisphosphate carboxylase, other C3 cycle enzymes, and NADP-malic enzyme showed a reverse gradation, although there were significant activities of these enzymes in mesophyll cells. As indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the banding pattern of certain polypeptides of the total soluble proteins from the three cell types also supported the distribution pattern obtained by activity assays of these enzymes. Analysis of initial 14C products in whole leaves and extrapolation of pulse-labeling curves to zero time indicated that about 80% of the CO2 is fixed into C4 acids (malate and aspartate), whereas about 20% of the CO2 directly enters the C3 cycle. This is consistent with the high activity of enzymes for CO2 fixation by the C4 pathway and the substantial activity of enzymes of the C3 cycle in the mesophyll cells. Therefore, F. brownii appears to have some capacity for C3 photosynthesis in the mesophyll cells and should be considered a C4-like species.

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