Measurement of Photosynthesis and Photorespiration in the Field
Lloyd ND, Canvin DT, Culver DA (1977) Photosynthesis and photorespiration in algae. Plant Physiol 70: 1637-1640
Ammonia acts as an uncoupler of photosynthesis by ..
Atkins CA, Canvin DT, Foch H (1971) Intermediary metabolism of photosynthesis in relation to carbon dioxide evolution in sunflower. In MD Hatch, CB Osmond, RO Slatyer, eds, Photosynthesis and Photorespiration. John Wiley, New York, pp 497-505
A low conductance for CO2 diffusion out of the bundle sheath cells (or its equivalent cellular compartment) is critical for the efficient operation of C4 photosynthesis. The total diffusive resistance to CO2 has multiple components with different levels of contribution. These components include bundle sheath walls, membranes, bundle sheath chloroplast position, the site of C4 acid decarboxylation, and the liquid-phase diffusion path. For kranz-type C4 plants, calculated total bundle sheath resistance on a leaf area basis can range from 50 to 150 m2 s-1 mol-1 (von Caemmerer and Furbank 2003). Evidently, single-cell C4 plants have sufficient resistance to CO2 back-diffusion which is essentially made of the cytoplasmic liquid phase and the special localisation of the (Rubisco-containing) chloroplasts surrounding the mitochondria (site of C4 acid decarboxylation).
Ammonia acts as an uncoupler of photosynthesis ..
Regulation of carbon flux through the TCA cycle probably occurs via phosphorylation/dephosphorylation of pyruvate dehydrogenase, which will depend in turn on mitochondrial energy status and feedback inhibition of various enzymes by NADH and acetyl-CoA. The rate of cycle turnover thus depends on the rate of electron flow through the respiratory chain (to reoxidise NADH) and the utilisation of ATP in the cell to provide ADP for substrate level and oxidative phoshorylation. TCA cycle turnover will also depend on the rate of substrate provision by reactions in chloroplasts and cytosol. A number of studies of TCA cycle mutants have demonstrated the wide impact these enzymes have not simply on TCA cycle function but as steps for the delivery of organic acids for other processes in plant cells such a photosynthetic performance, plant biomass, root growth, photorespiration, nitrogen assimilation, amino acid metabolism, and stomatal function.
During respiration, metabolites are oxidised and the electrons released are transferred through a series of electron carriers to O2. Water and CO2 are formed and energy is captured as ATP which is harnessed to drive a vast array of cellular reactions.
THE HILL REACTION OF PHOTOSYNTHESIS
A scheme for photorespiration involving the photosynthetic carbon reduction (PCR) cycle and the photosynthetic carbon oxidation (PCO) pathways represents the synthesis of almost 70 years of research. It integrates the metabolism of P-glycolate with the PCR cycle. This scheme is shown in Figure 2.13.
A more dynamic approach to carbon fixation was needed to resolve this impasse. In particular, the biochemical fate of early products would have to be traced, and using a development of the open gas exchange system at Queens, Atkins et al. (1971) supplied 14CO2 in pulse–chase experiments to sunflower leaf tissue under conditions in which photorespiration was operating at high rates (21% O2) or in which it was absent (1% O2). A series of kinetic experiments showed that synthesis of 14C-glycine and 14C-serine was inhibited in low O2 and that the 14C precursor for their synthesis was derived from sugar bisphosphates of the PCR cycle, especially RuBP. Indeed RuBP was the obvious source of glycine carbon atoms and the kinetics of glycine turnover closely matched those of RuBP. As these authors concluded, ‘we can no longer view this (glycolate) pathway as an adjunct to the Calvin cycle but must incorporate it completely into the carbon fixation scheme for photosynthesis’ (Atkins et al. 1971).
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EFFECT OF AMMONIA ON PHOTOSYNTHETIC RATE …
(ammonia) as our uncoupler ..
Why is ammonia referred to as an "uncoupler ..
Ammonia functions as an uncoupler or a compound that separates the process of phosphorylation from electron transport.
Inhibition by ammonium chloride of the oxygen yield …
amonia /amoniak/ ammonia. ..
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C4 photosynthesis suppresses photorespiration by operating a CO2 concentrating mechanism that comes at additional energetic cost. This cost is independent of ambient CO2 and temperatures. In contrast, photorespiration (and its associated energy cost) increases steeply with temperature in C3 plants and is highly dependent on CO2 concentrations. Under saturating irradiance and current ambient atmospheric CO2 concentration, the threshold temperature where the cost of photorespiration in C3 plants exceeds that of the CO2 concentrating mechanism in C4 plants is estimated around 25oC. This model provides a physiological basis for understanding today’s contrasting geographic distribution between C3 and C4 grasses.
The hill rxn in isolated chloroplasts post ..
Figure 2.10. Schematic representation of the ‘photorepiratory pump’ operating in C3-C4 photosynthesis. The intermediate photosynthetic pathway reduces photorespiration by refixing photorespired CO2 released locally in the bundle sheath cell. Mesophyll mitochondria lack glycine decarboxylase activity. Mesophyll and bundle sheath cells contain chloroplasts with functional Calvin cycle. Abbreviations: cp: chloroplast; GDC: glycine decarboxylase; PCO: photosynthetic oxidative cycle; PCR: photosynthetic reductive cycle; mt: mitochondrion.
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The C4 pathway supercharges photosynthesis and suppresses photorespiration by operating a CO2 concentrating mechanism which elevates CO2 around Rubisco. Although C4 photosynthesis incurs additional energy, the energy cost of photorespiration exceeds that of the CO2 concentrating mechanism above 25oC. Hence, higher radiation use efficiencies (i.e., efficiency of converting absorbed radiation into biomass) have been recorded for C4 than C3 crops. High bundle sheath CO2 concentration saturates C4 photosynthesis at relatively low intercellular CO2, allowing C4 plants to operate with lower stomatal conductance. Thus, leaf-level WUE is usually higher in C4 than C3 plants. Relative to C3 plants, Rubisco of C4 plants is faster (higher turnover rate) and operates under saturating CO2. Thus, C4 plants typically achieve higher photosynthetic rates with about 50% less Rubisco and less leaf nitrogen. Hence, photosynthetic NUE is higher in C4 than C3 plants. Accordingly, C4 plants are advantaged relative to C3 plants in hot and nitrogen-poor environments with short growing seasons, hence their great abundance in wet/dry tropics such as Northern Territory savannas.
Introduction Photosynthesis is the process by which green plants
The physiological advantages of the intermediate photosynthetic pathway in all its naturally occurring forms remain unclear. It may be hypothesised that lowered photorespiration may lead to reduced CO2 limitation of photosynthesis, and thus allow C3-C4 plants to operate with lower stomatal conductance, thus conferring higher water use efficiency relative to C3 counterparts. Moreover, increased nitrogen cost associated with ‘building’ another set of photosynthetic cells (bundle sheath) may reduce nitrogen use efficiency if the gains in CO2 uptake are not substantial.
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