The intercellular distribution of glycine decarboxylase in leaves of cassava in relation to the photosynthetic mode and leaf anatomy

There have been conflicting reports on the photosynthetic mode of cassava (Manihot esculenta Crantz). It has been suggested that cassava is a C₃-C₄ intermediate plant and, alternatively, that it is a C₃ plant. Glycine decarboxylase (GDC), a photorespiratory enzyme, is known to exhibit different patterns of intercellular distribution between C₃-C₄ intermediate and C₃ leaves. In order to elucidate the photosynthetic mode of cassava, we investigated the localization of GDC in the leaves of three cultivars by immunogold electron microscopy and the anatomical structures of the leaves. The leaves of cassava are essentially hypostomatous, and the photosynthetic tissues consist of palisade mesophyll cells (PMCs), spongy mesophyll cells (SMCs) and bundle sheath cells (BSCs). Although the BSCs include centrifugally located chloroplasts, they show no increase in mitochondrial frequency. Labeling specific for GDC was found on all mitochondria of all three types of cell. However, the density of labeling in the PMGs was always higher than in the SMCs and BSCs. In leaves that developed under a water deficit, the difference in labeling density became even clearer. These data suggest that at least the cultivars of cassava examined here are not C₃-C₄ intermediates and should be regarded as C₃ plants. However, the intercellular differences in the level of accumulation of GDC seem to merit further investigation with respect to an internal gradient of photorespiratory capacity in the leaves.