Five mutant lines of barley (Hordeum vulgareL.), 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 CO2at normal rates; on transfer to air, the rates drop rapidly to 15 of the wild-type. Incorporation of14CO2into 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 of14C 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 14Cglutamate and 14Cglutamine 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.
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