INVESTIGATION OF THE APPARENT INDUCTION OF NITRATE UPTAKE IN BARLEY (HORDEUM-VULGARE L) USING NO3--SELECTIVE MICROELECTRODES - MODULATION COARSE REGULATION OF NO3- UPTAKE BY EXOGENOUS APPLICATION OF DOWNSTREAMMETABOLITES IN THE NO3- ASSIMILATORY PATHWAY

The influence of a 12-h pretreatment with either NO3-, NH4+, glutamine , or glutamate (300 mu M) on the apparent induction of NO3- uptake was investigated. Net fluxes of NO3- into roots of intact, 7-d-old barley (Hordeum vulgare L. cv Prate) seedlings in solution culture were esti mated from ion activity gradients measured with NO3--selective microel ectrodes in the unstirred layer of solution immediately external to th e root surface. Control plants, pretreated with nitrogen-free nutrient solution, exhibited a sigmoidal increase in net NO3- uptake, reaching a maximum rate between 8 and 9 h after first exposure to NO3-. Plants pretreated with NH4+ or Glu exhibited a delay of several hours in the initiation of the induction process after they had been exposed to NO 3-. In Gln-pretreated plants, however, responses ranged from no delay of the induction process to delays comparable to those observed follow ing NH4+ or Glu pretreatments. Only treatment with NO3- resulted in th e induction of NO3- uptake, whereas pretreatments with NH4+, Gln, or G lu tended to delay induction of NO3- uptake upon subsequent exposure t o NO3-.