Photorespiratory properties of protoplasts from C 3-C 4 intermediate species of moricandia

Abstract

Protoplasts were isolated from leaves of the C 3-C 4 intermediate species, Moricandia arvensis (L.) DC. and Moricandia spinosa Pomel. Analysis by light and transmission electron microscopy indicated that these purified preparations contained both mesophyll protoplasts (MP) and bundle-sheath protoplasts (BSP). Conventional density gradient centrifugation procedures failed to yield separations of pure protoplasts from each cell-type. With these heterogeneous suspensions of MP and BSP, values measured for (i) the percentage inhibition of photosynthetic CO 2 fixation by O 2, (ii) the apparent K m( CO) 2 of photosynthesis, and (iii) dark/light ratios of the rate of 14CO 2 evolution during decarboxylation of exogenous [1- 14C] glycine were not significantly different from those determined for protoplast preparations from related or representative C 3 plants, including M. foetida, Nicotiana tabacum, and Triticum aestivum. In contrast, previous comparisons with C 3 species, using intact leaf tissue from M. arvensis, have shown a reduced sensitivity of net photosynthesis to inhibition by O 2 [Holaday et al., Plant Sci. Lett., 27 (1982) 181] and an enhanced capacity for the photosynthetic refixation of CO 2 evolved during decarboxylation of exogenous photorespiratory substrates [Holbrook et al., Plant Physiol., 77 (1985) 578]. We conclude that these photosynthetic properties, associated with reduced photorespiration by M. arvensis and M. spinosa, are dependent upon the integrity of the anatomical and ultrastructural arrangement of bundle-sheath and mesophyll cells in these C 3-C 4 intermediate species.