The Effect of Methionine Sulphoximine on the Absorption of Ammonium by Maize and Barley Roots over Short Periods

Pre-treating intact, ammonium-grown plants of maize and barleywith the glutamine synthetase inhibitor, L-methionine sulphoximine(MSO), increased subsequent accumulation of ¹³N or ¹⁵N fromlabelled ammonium very markedly—up to 8-fold in extremecases. The phenonemon could not be satisfactorily explainedin terms of an effect of MSO on the cation-exchange propertiesof the root free space, because neither potassium (⁸⁶Rb⁺) norsodium absorption was enhanced by MSO pretreatment, and rapidabsorption of ammonium continued for too long in relation tothe likely free-space content of a monovalent cation. Instead,the response is seen as a consequence of the enlargement ofthe cytoplasmic and vacuolar ammonium pools that occurs whenammonium assimilation is inhibited by MSO. The results, whichincluded comparisons between short-term and longer-term ratesof ammonium absorption, provide tentative evidence of rapidfluxes of ammonium through the plasma membrane. Key words: Cytoplasm, vacuole, flux, transport