Photophosphorylation by mesophyll and bundle sheath chloroplasts of c(4) plants.

Leaves of plants possessing the C4-dicarboxylic acid CO2fixation pathway (14) are characterized by outer mesophyll cells containing granal chloroplasts and inner bundle sheath cells which contain chloroplasts with varying degrees of grana development depending on the species. As judged by Hill activities and fluorescence yield, bundle sheath chloroplasts which contain few grana have less photosystem II activity relative to photosystem I when compared to mesophyll cell chloroplasts in the same leaf (5, 11, 13, 15, 16). The deficiency in PS-II' in these bundle sheath chloroplasts is further shown by the low chl/P700 ratio in bundle sheath chloroplasts of Sorghum bicolor and Digitaria sanguinalis (1, 13) and by the near absence of pigment-protein complex II (associated with PS-II) in bundle sheath chloroplasts of S. bicolor (12). These studies show that bundle sheath chloroplasts of C4 plants which contain few grana are highly deficient in PS-II and closely resemble the PS-I particles prepared from spinach chloroplasts by digitonin treatment (1). The major deficiency in PS-II activity reported for S. bicolor bundle sheath chloroplasts, for example, has important consequences for Calvin cycle metabolism which occurs in the bundle sheath cells of this species. All carbon fixed during photosynthesis eventually passes through the Calvin cycle in the bundle sheath cells and, in view of major deficiency in PS-II, it has been suggested that NADPH is provided via malic enzyme (10, 14) and that ATP is generated via cyclic photophosphorylation (6, 14). There have been relatively few studies of photophosphorylation in chloroplasts of C4 plants (6, 8), and in these no distinction was made between mesophyll and bundle sheath origin. This paper reports studies on photophosphorylation as a measure of coupled electron flow in chloroplasts from three C4 species, S. bicolor, Zea mays and Atriplex spongiosa. The data demonstrate a major deficiency of PS-II dependent photophosphorylation in fragments obtained from bundle sheath cell chloroplasts of S. bicolor and Z. mays.