In the F2 generation, nonviability in air and the lack of chloroplast glutamine synthetase co-segregated, in both the lines tested. These two lines and four others proved to be allelic, we designate them gin 2a-f. The characteristics of these mutants conclusively demonstrate the major role of chloroplast glutamine synthetase in photorespiration and its associated nitrogen recycling.

Five mutant lines of barley (Hordeum vulgare L.), which are only able to grow at elevated levels of CO2, contain less than 5% of the wild-type activity of ferredoxin-dependent glutamate synthase (EC 1.4.7.1). Two of these lines (RPr 82/1 and RPr 82/9) have been studied in detail. Leaves and roots of both lines contain normal activities of NADH-dependent glutamate synthase (EC 1.4.1.14) and the other enzymes of ammonia assimilation. Under conditions that minimise photorespiration, both mutants fix CO2 at normal rates; on transfer to air, the rates drop rapidly to 15% of the wild-type. Incorporation of (14)CO2 into sugar phosphates and glycollate is increased under such conditions, whilst incorporation of radioactivity into serine, glycine, glycerate and sucrose is decreased; continuous exposure to air leads to an accumulation of (14)C in malate. The concentrations of malate, glutamine, asparagine and ammonia are all high in air, whilst aspartate, alanine, glutamate, glycine and serine are low, by comparison with the wild-type parent line (cv. Maris Mink), under the same conditions. The metabolism of [(14)C]glutamate and [(14)C]glutamine by leaves of the mutants indicates a very much reduced ability to convert glutamine to glutamate. Genetic analysis has shown that the mutation in RPr 82/9 segregates as a single recessive nuclear gene.

Manipulation of the CO2 concentration of the atmosphere allows the selection of photorespiratory mutants from populations of seeds treated with powerful mutagens such as sodium azide. So far, barley lines deficient in activity of phosphoglycolate phosphatase, catalase, the glycine to serine conversion, glutamine synthetase, glutamate synthase, 2-oxoglutarate uptake and serine: glyoxylate aminotransferase have been isolated. In addition one line of pea lacking glutamate synthase activity and one barley line containing reduced levels of Rubisco are available. The characteristics of these mutations are described and compared with similar mutants isolated from populations of Arabidopsis. As yet, no mutant lacking glutamine synthetase activity has been isolated from Arabidopsis and possible reasons for this difference between barley and Arabidopsis are discussed. The value of these mutant plants in the elucidation of the mechanism of photorespiration and its relationships with CO2 fixation and amino acid metabolism are highlighted.

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Carboxylase and oxygenase activities of ribulose bisphosphate carboxylase purified from wheat were measured over the temperature range 5 to 35°C either at constant 02 and CO2 concentrations or where the 02 and CO2 simulated the concentrations in water equilibrated at each temperature with the same gaseous phase. At (v0/v,) in vivo with increased temperature could explain the observed effects on photorespiration and photosynthesis.The velocity ratio v0/v, was reported to increase with temperature (2, 17) and the change was considered sufficient to account for increased photorespiration relative to photosynthesis.Studies of the enzyme isolated from C3 plants showed that the kinetic parameters of the oxygenase and carboxylase activities change differently as the temperature increases and this may explain changes in v0/v, and hence increased photorespiration (3,17 porated into mathematical models of photosynthesis and photorespiration, which also take into account limitations by the light reactions (9). It is important to determine whether the reaction kinetics based on concentrations of substrates in solution or gas solubility ratio is the most significant feature determining the effect of temperature on photosynthesis and photorespiration in order to make the models more accurate. This paper describes the direct measurement in the same reaction mixtures of carboxylase and oxygenase activities under various defined conditions oftemperature CO2 and 02 concentrations, using fully activated enzyme. This approach was used since measurement of the kinetic constants is complicated by the fact that the oxygenase reaction is not saturated at 100%02 at atmosphere pressure (1). The objective was to determine how v0/v, changes with temperature at constant concentrations of CO2 and 02, and hence where only changes in conventional parameters of enzyme kinetics are involved, as well as under conditions designed to simulate those expected in the chloroplast stroma where relative concentrations of gaseous substrates are also changed with temperature because of changes in solubility.MATERIALS AND METHODS RuBP Carboxylase from Wheat Leaves. The purified enzyme was available as a freeze-dried powder (5) and was activated by incubation for 1 h at 35°C in 100 mm Bicine-NaOH (pH 8.2) containing 10 mm NaHCO3 and 20 mm MgCl2. Subsequently, the enzyme solution was maintained at 25°C for the duration of the experiment. The specific activity of the preparations used was 0.5 to 0.9 ,umol/min -mg protein at 25°C and 5 mm bicarbonate.Preparation of Buffers. Bicine and MgCl2 were added to distilled H20 that had been boiled and then cooled under N2. N2 was bubbled through the solution for approximately 1 h before adjusting to the final pH with 'CO2-free' NaOH (BDH).