Photosynthetic electron transport in the bundle sheath of maize

Initial investigations into the mechanism of C4 photosynthesis showed that in species such as maize, CO* fmed in the mesophyll cells is transported in the form of malate to the bundle sheath cells, where its decarboxylation is catalyzed in the chloroplasts by NADP’-dependent malic enzyme [ 11. The presence of agranal chloroplasts in the bundle sheath of these species [2] and the functional deficiency of photosystem II activity [ 1,3,4] led to the view that the NADPH formed during the decarboxylation of malate is essential for reduction of part of the glycerate-1,3-bisphosphate formed in the reductive pentose phosphate pathway. The marked stimulation of photosynthesis which malate causes in isolated bundle sheath tissue is consistent with this view [3,5]. Although the extent of the deficiency in photosystem II has since been questioned [6,7], recent work with bundle sheath strands having photosynthesis rates equal to those of the parent tissue implied that photosynthetic Oa evolution (and uptake) with COz as the acceptor is limited and that the high ATP demand for COa fucation is largely met by cyclic photophosphorylation mediated by photosystem I rather than via noncyclic or pseudocyclic photophosphorylation [5]. Cyclic photophosphorylation requires activation by electron flow from a suitable reductanr, which establishes appropriate redox poise in the electron carriers. In thisletter, flash spectrophotometric studies are described which indicate that photosystem II activity is inadequate to poise cyclic electron flow in bundle sheaths of maize, and that malate decarboxylation supplies the necessary electrons.