Plant Physiol, October 2002, Vol. 130, pp. 989-998

Photorespiratory NH₄⁺ Production in Leaves of Wild-Type and Glutamine Synthetase 2 Antisense Oilseed Rape¹

Plant Nutrition Laboratory, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark (S.H., M.M., J.K.S.); and Institute of Agronomy and Plant Breeding, University of Göttingen, von Siebold Strasse 8, D-37075 Göttingen, Germany (C.M., M.W.)

Exposure of oilseed rape (Brassica napus) plants to increasing leaf temperatures between 15°C and 25°C increased photorespiratory NH₄⁺ production from 0.7 to 3.5 µmol m² s¹. Despite the 5-fold increase in the rate of NH₄⁺ production, the NH₄⁺ concentration in root and leaf tissue water and xylem sap dropped significantly, whereas that in the leaf apoplastic fluid remained constant. The in vitro activity of glutamine synthetase (GS) in both leaves and roots also increased with temperature and in all cases substantially exceeded the observed rates of photorespiratory NH₄⁺ production. The surplus of GS in oilseed rape plants was confirmed using GS2 antisense plants with 50% to 75% lower in vitro leaf GS activity than in the wild type. Despite the substantial reduction in GS activity, there was no tendency for antisense plants to have higher tissue NH₄⁺ concentrations than wild-type plants and no overall correlation between GS activity and tissue NH₄⁺ concentration was observed. Antisense plants exposed to leaf temperatures increasing from 14°C to 27°C or to a trifold increase in the O₂ to CO₂ ratio did not show any change in steady-state leaf tissue NH₄⁺ concentration or in NH₃ emission to the atmosphere. The antisense plants also had similar leaf tissue concentrations of glutamine, glycine, and serine as the wild type, whereas glutamate increased by 38%. It is concluded that photorespiration does not control tissue or apoplastic levels of NH₄⁺ in oilseed rape leaves and, as a consequence, that photorespiration does not exert a direct control on leaf atmosphere NH₃ fluxes.