A model of carbon dioxide assimilation in Chlamydomonas reinhardii

A simple model of photosynthetic CO2 assimilation in Chlamydomonas has been developed in order to evaluate whether a CO2-concentrating system could explain the photosynthetic characteristics of this alga (high apparent affinity for CO2, low photorespiration, little O2inhibition of photosynthesis, and low CO2 compensation concentration). Similarly, the model was developed to evaluate whether the proposed defects in the CO2-concentrating system of two Chlamydomonas mutants were consistent with their observed photosynthetic characteristics. The model treats a Chlamydomonas cell as a single compartment with two carbon inputs: passive diffusion of CO2, and active transport of HCO3-. Internal inorganic carbon was considered to have two potential fates: assimilation to fixed carbon via ribulose 1,5-bisphosphate carboxylase-oxygenase or exiting the cell by either passive CO2 diffusion or reversal of HCO3-transport. Published values for kinetic parameters were used where possible. The model accurately reproduced the CO2-response curves of photosynthesis for wild-type Chlamydomonas, the two mutants defective in the CO2-concentrating system, and a double mutant constructed by crossing these two mutants. The model also predicts steady-state internal inorganic-carbon concentrations in reasonable agreement with measured values in all four cases. Carbon dioxide compensation concentrations for wild-type Chlamydomonas were accurately predicted by the model and those predicted for the mutants were in qualitative agreement with measured values. The model also allowed calculation of approximate energy costs of the CO2-concentrating system. These calculations indicate that the system may be no more energy-costly than C4 photosynthesis.This article is published as Spalding, M. H., and A. R. Portis. "A model of carbon dioxide assimilation in Chlamydomonas reinhardii." Planta 164, no. 3 (1985): 308-320. Posted with permission.</p

Control of photosynthesis in barley leaves with reduced activities of glutamine synthetase or glutamate synthase.

Wild-type and mutant plants of barley (Hordeum vulgare L. cv. Maris Mink) lacking activities of chloroplastic glutamine synthetase (GS) and of ferredox-in-dependent glutamate synthase (Fd-GOGAT) were crossed to generate heterozygous plants. Crosses of the F2 generation containing GS activities between 47 and 97 of the wild-type and Fd-GOGAT activities down to 63 of the wild-type have been selected to study the control of both enzymes on photorespiratory carbon and nitrogen metabolism. There were no major pleiotropic effects. Decreased GS had a small impact on leaf protein and the total activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The activation state of Rubisco was unaffected in air, but a decrease in GS influenced the activation state of Rubisco in low CO2. In illuminated leaves, the amino-acid content decreased with decreasing GS, while the content of ammonium rose, showing that even small reductions in GS limit ammonium re-assimilation and may bring about a loss of nitrogen from the plants, and hence a reduction in protein and Rubisco. Leaf amino-acid contents were restored, and ammonium and nitrate contents decreased, by leaving plants in the dark for 24 h. The ratios of serine to glycine decreased with a decrease in GS when plants were kept at moderate photon flux densities in air, suggesting a possible feedback on glycine decarboxylation. This effect was absent in high light and low CO2. Under these conditions ammonium contents exhibited an optimum and amino-acid contents a minimum at a GS activity of 65 of the wild-type, suggesting an inhibition of ammonium release in mutants with less than 65 GS. The leaf contents of glutamate, glutamine, aspartate, asparagine, and alanine largely followed changes in the total amino-acid contents determined under different environmental conditions. Decreased Fd-GOGAT resulted in a decrease in leaf protein, chlorophyll, Rubisco and nitrate contents. Chlorophyll a/b ratios and specific leaf fresh weight were lower than in the wild-type. Leaf ammonium contents were similar to the wild-type and total leaf amino-acid contents were only affected in low CO2 at high photon flux densities, but mutants with decreased Fd-GOGAT accumulated glutamine and contained less glutamate