THREE LIGHT REACTIONS AND THE TWO PHOTOSYSTEMS OF PLANT PHOTOSYNTHESIS*

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Abstract
Abstract—Recent work in our laboratory yielded new evidence that noncyclic electron transport in chloroplasts from water to ferredoxin (Fd) and N ADP is carried out solely by System II which, unexpectedly, was found to include not one but two photoreactions (IIa and IIb). The evidence suggests that these operate in series, being joined together by a ‘dark’ chain of electron carriers that includes (but is not limited to) cytochromeb559and plastocyanin (PC):H2O → IIbhv→ C550 → Cytb559rarr;PC→IIahv→ Fd → NADP.Photoreaction IIb involves an electron transfer from water to C550, a new chloroplast component distinct from cytochromes, whose photoreduction is observed as a decrease in absorb‐ance with a maximum at 550 nm. The photoreduction of CSSO proceeds effectively only in short‐wavelength System II light, is insensitive to low temperature (at least down to — 189°C). does not require plastocyanin, and is the first known System II photoreaction which is resistant to inhibition by DCMU oro‐phenanthroline. Photoreaction IIa involves an electron transfer from cytochromeb559to ferredoxin‐NADP and also proceeds effectively only in System II light. The photooxidation of cytochromeb559requires plastocyanin. Cytochromeb559is reduced by C550 in a reaction that is readily inhibited by DCMU oro‐phenanthroline. Thus, the site of DCMU (ando‐phenanthroline) inhibition of System II appears to lie between C550 and cytochromeb559. System I, comprising a single long‐wavelength light reaction and a cyclic electron transport chain that includes cytochromesb6andf, is viewed as operating in parallel to System II. The photoreduction of NADP by artificial electron donors via System I involves a portion of the cyclic electron transport chain and appears to be independent of plastocyanin. Chloroplast fragments have been prepared which either (a) exhibit System II activity (water → NADP) and lack functional cytochromefand P700 or (b) exhibit System I activity and lack plastocyanin. The present concept is consistent with the following: (i) No enhancement effect was found for NADP reduction by water where only System II is thought to be involved, but a large enhancement effect was observed in chloroplasts engaged in complete photosynthesis where both cyclic (System I) and noncyclic photophosphorylation (System II) are needed for CO2assimilation. (ii) The transfer of one electron from water to ferredoxin via System II requires optimally two quanta, but the transfer of one electron from reduced dye to ferredoxin via System I requires optimally only one quantum of light.