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Yield Potential, Plant Assimilatory Capacity, and Metabolic Efficiencies

^a Agronomy & Range Science, Univ. of California, Davis, CA 95616, USA
^b Environmental Sciences Division, Oak Ridge National Lab., Oak Ridge, TN 37831-6422 USA

View larger version (18K): [in a new window]   Fig. 1 Light reactions of photosynthesis and photophosphorylation associated with thylakoid membranes inside chloroplasts leading to production of NADPH and ATP used in CO2 assimilation. Photons are absorbed by chlorophyll antennae in both photosystem II (PSII) and photosystem I (PSI). Water is oxidized by PSII, yielding O2 and protons in the lumen. Coordinated activity of PSII and the cytochrome (cyt) b6f complex pumps protons from stroma to lumen. This involves cycling of plastoquinone between oxidized (Q) and reduced (QH2) states. As drawn, a "Q-cycle" is associated with cyt b6f. Electrons are transferred from cyt b6f to PSI via plastocyanin (PC). In noncyclic electron transport, e- are then transferred from PSI via ferredoxin (Fd) to FNR (ferredoxin-NADP reductase), which leads to reduction of NADP. The dashed line (– – –) indicates cyclic transport in which e- flow from PSI back to the cyt complex via Fd. Protons move from lumen to stroma through CFo-CF1 ATP synthase, which catalyzes ADP phosphorylation (about 3 protons/ATP)

View larger version (25K): [in a new window]   Fig. 2 CO2 uptake (PS rate) by a leaf with increasing photon flux density (PFD). Departure from the initial slope (which defines the minimum quantum requirement) occurs as CO2 becomes limiting and photosynthesis approaches a maximum, light-saturated rate (Amax). The shaded area represents what Björkman and Demmig-Adams (1994) term "excess PFD"

View larger version (20K): [in a new window]   Fig. 3 Daily course of the xanthophyll photoprotective system in cotton under field conditions (adapted from Björkman and Demmig-Adams, 1994). (Top) Incident PFD, efficiency of PSII, and fraction of PSII centers that were closed. This amount of closure is much less than would occur without intervention of the xanthophyll cycle. (Bottom) Concomitant changes in concentrations of xanthophyll pigments during the day as violaxanthin was converted to zeaxanthin, which is capable of accepting excitation energy from chlorophyll

View larger version (25K): [in a new window]   Fig. 4 Respiratory-chain reactions associated with the inner mitochondrial membrane. Mitochondria are enclosed by two membranes; the outer membrane (not shown) is permeable to metabolites but the inner one is not. NAD(P)H in the mitochondrial matrix and in the cytosol can be oxidized by several mitochondrial dehydrogenases. Complex I oxidizes matrix NADH and in so doing pumps protons from the matrix to the cytosolic side of the inner membrane. Other inner-membrane-bound dehydrogenases (indicated by DH) do not pump protons. Inner-membrane dehydrogenases (including complex II, which functions as part of the TCA cycle) transfer electrons (and protons) to ubiquinone (Q), which is reduced to ubiquinol (QH2) in the process. A mobile Q/QH2 pool exists in the inner membrane. Complex III oxidizes QH2 and passes electrons to cytochrome (cyt) c, which in turn passes electrons to complex IV. Electron transport through complexes III and IV is coupled to proton translocation across the inner membrane; a "Q-cycle" associated with Complex III is included in the diagram. Free O2 is reduced to water by complex IV. The alternative oxidase (alt ox) can also oxidize QH2, and form water, but this bypasses two sites (complexes III and IV) of proton translocation in the mitochondrial electron transport chain. Protons may enter the matrix through membrane leaks, but Fo-F1 ATP synthase (similar to the CFo-CF1 ATP synthase in Fig. 1) is the main route of proton entry. Apparently, one ADP is phosphorylated when three protons pass through the F0-F1 ATP synthase. The ADP required for ATP formation enters the mitochondrial matrix only as ATP exits the matrix through an antiporter. A symporter couples the transport of Pi and H+ into the matrix