Photorespiratory Amino Donors, Sucrose Synthesis and the Induction of CO2Fixation in Barley Deficient in Glutamine Synthetase and/or Glutamate Synthase

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Abstract
Murray, A. J. S., Black well, R. D., Lea, P. J. and Joy, K. W. 1988. Photorespiratory amino donors, sucrose synthesis and the induction of CO2 fixation in barley deficient in glutamine synthetase and/or glutamate synthase.—J. exp. Bot. 39: 845–858. A number of mutants of barley have been produced which lack both chloroplastic glutamine synthetase and ferredoxin-dependent glutamate synthase activities. The plants accumulated ammonia to the same extent as mutants deficient in only glutamine synthetase but shared the gas-exchange characteristics of the glutamate synthase deficient parent. These mutants have been used to demonstrate directly the ability of alanine to ameliorate the dramatic drop in fixation rate normally exhibited by glutamate synthase deficient mutants on transfer to photorespiratory conditions. Immediately after transfer to air, the mutants deficient in glutamate synthase activity demonstrated a reduced ability to incorporate 14C derived from 14CO2 into sucrose. This effect was, however, dependent on the previous induction of CO2 fixation. Use of 14CO2 revealed that the induction phase of CO2 fixation was altered in all three mutants. Neither of the parents nor the double mutant accumulated sucrose in air under conditions which promote sucrose accumulation by the wild type. The implications of these results for photosynthesis and the control of sucrose synthesis are discussed.