Photosynthetic Enzyme Activities and Localization in Mollugo verticillata Populations Differing in the Levels of C3 and C4 Cycle Operation

Ecotypic differences in the photosynthetic carbon metabolism of Mollugo verticillata were studied. Variations in C(3) and C(4) cycle activity are apparently due to differences in the activities of enzymes associated with each pathway. Compared to C(4) plants, the activities of C(4) pathway enzymes were generally lower in M. verticillata, with the exception of the decarboxylase enzyme, NAD malic enzyme. The combined total carboxylase enzyme activity of M. verticillata was greater than that of C(3) plants, possibly accounting for the high photosynthetic rates of this species. Unlike either C(3) or C(4) plants, ribulose bisphosphate carboxylase was present in both mesophyll and bundle sheath cell chloroplasts in M. verticillata. The localization of this enzyme in both cells in this plant, in conjunction with an efficient C(4) acid decarboxylation mechanism most likely localized in bundle sheath cell mitochondria, may account for intermediate photorespiration levels previously observed in this species.