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This chapter discusses our current understanding of the chloroplast proton circuit, i.e., those reactions that involve the storage and utilization of light energy in the transfer of protons, and its importance for regulating photosynthetic light-capture/electron-transfer reactions. The photosynthetic machinery of plants is finely tuned to balance the needs for efficient light capture with an avoidance of photodamage by regulating the capture of light energy, via thermal dissipation of excess excitation energy (assessed from non-photochemical quenching, NPQ, of chlorophyll fluorescence) by regulating light-driven electron transfer processes. In addition to driving ATP synthesis at the chloroplast ATP synthase, the thylakoid electrochemical gradient of protons or proton-motive force (pmf) plays a central role in regulating NPQ. The transthylakoid proton concentration gradient (ΔpH) component of pmf triggers the “energy-dependent”, or qE component of NPQ, which protects photosystem II from photodamage and regulates electron transfer through the cytochrome b 6 f complex, thereby preventing damage to photosystem I. The extent and mode of storage in ΔpH and ΔΨ of pmf are regulated by several processes that respond to the metabolic, or physiological, state of the organism. The extent of pmf is determined by proton influx (via linear and alternative electron flows) into the thylakoid lumen, and proton efflux through the chloroplast ATP synthase. Both processes are modulated by, or responsive to, environmental conditions and resulting metabolic fluctuations. Proton influx is controlled by linear electron flow and a series of alternative electron flow pathways, possibly including cyclic electron flow around photosystem I, the Mehler peroxidase reaction (or water-water cycle), and oxidation of plastoquinol by the plastid terminal oxidase. The fraction of pmf stored as ΔpH is also regulated by plastidic ionic strength or luminal buffering capacity, altering the sensitivity of pH-dependent processes to pmf. The integrated regulation of these processes is an open, active area of research.

Light absorbed by photosystems I and II is used to drive linear electron transport, and associated proton transport, in the thylakoid membranes of leaves. In healthy leaves operating under non-stressful conditions and in which photorespiration is inhibited, photosynthetic electron transport is used primarily to reduce NADP+ to NADPH, which is then used to drive the assimilation of CO2 into carbohydrates with ca. 88% of electrons being consumed in this process. However, such a high quantum efficiency of CO2 assimilation is frequently not observed in leaves. We examine the intrinsic physiological, metabolic and environmental factors that can modify photosynthetic electron transport in leaves. Electron transport is also required for the reduction and activation of key enzymes involved in photosynthetic metabolism and driving other metabolic processes, such as nitrogen and sulfur metabolism. Oxygen can act as an electron acceptor, being reduced by electrons from photosystem I via a Mehler reaction or by electrons from photosystem II via the plastid terminal oxidase. Although such photoreductions of oxygen do not appear to have a significant role in healthy, non-stressed leaves, there is evidence to support the contention that these processes can be important for photoprotection of photosystem II in leaves under light stress. Cyclic electron transport can occur around photosystem I; however, this process would also appear to only be of physiological importance when the ability of the leaf to assimilate CO2 is severely restricted. It is concluded that leaves exhibit a high degree of plasticity in their ability to modify the pathways of photosynthetic electron transport in order to deal with fluctuations in metabolic demands and environmental stresses.

The biosynthesis of photosynthetic complexes encompasses the coordination of diverse processes with the potential for many levels of regulation. Collectively, complex assembly involves the expression of chloroplast and nuclear genes, the targeting of subunits to their proper locations within the chloroplast, and the proper organization and assembly of the subunits and cofactors into functional complexes [5]. Degradation of proteins is another crucial process that is regulated by internal and extrinsic factors. The abundance of functional photosystem I (PSI) can be affected by altering any one of these processes. Different environmental conditions and various mutations have been characterized that affect the abundance of PSI complexes.

In NAD-malic enzyme type C4 plants like P. mliaceum the decarboxylation in the C4 cycle proceeds via NAD-malic enzyme located in the matrix of bundle sheath mitochondria (Kagawa, Hatch 1975). The bundle sheath mitochondria have very high activity of NAD-malic enzyme while the activity in mesophyll mitochondria is considerably lower and comparable with the activity found in C3 mitochondria (Gardestrom, Edwards 1983). The properties of malate oxidation were very different between mesophyll and bundle sheath mitochondria from P. miliaceum. In mesophyll mitochondria malate oxidation was largely sensitive to cyanide while malate oxidation in bundle sheath mitochondria was almost totally insensitive to cyanide (Gardestrom, Edwards 1983).

In cyclic and non cyclic photosynthetic electron transport of isolated chloroplasts three products are formed: NADPH2, oxygen and ATP. There seems to be general agreement that at least two light reactions participate in non cyclic flow from water to NADP, coupled stoichiometrically to ATP formation and in cyclic electron flow leading to only ATP. Though not unequivocally accepted, considerable evidence has accumulated for two light reactions in series in non cyclic flow in the grana lamelle of chloroplasts and possibly a third light reaction in cyclic electron flow located in the stroma lamellae (PARK & SANE 1971)-Usually photosystem II and photosystem I are assumed to operate in series in non cyclic flow, but recently KNAFF & ARNON (1969) have suggested that possibly two photosystem II in series are needed to bridge the redox potential gap between water and NADP. In spite of such uncertainties there is no question that the photosystem oxidizing water to oxygen requires an electron acceptor at the redox potential level of about -100 mV and that a second photosystem is able to reduce an endogenous redox carrier at the redox potential level of about -600 mV. For the acceptor i.e. the endogenous quencher (s) of the water photooxidizing system plastoquinone has been suggested and for the acceptor of the photosystem operating at a low redox potential FRS = ferredoxin reducing substance. The present report, if only for the sake of simplicity, operates with photosystem II and I in series and deals with the function of piastoquinone in photosynthetic electron transport of chloroplasts and with the state of identification of the primary acceptor of photosystem I.

Several lines of investigations have contributed to our knowledge of the identities and pool sizes of the intermediates in the chain between photoreactions 1 and 2 (for reviews see e.g. HIND & OLSON 1968, FORK & AMESZ 1970, CHENIAE 1970). Among these are studies of light-induced absorbancy changes, fluorescence and oxygen evolution. Nevertheless, there are still several uncertainties, e.g. about the participation of b-type cytochromes, and about the pool sizes and the oxidationreduction potentials of the intermediates.