DIFFERENTIAL-EFFECTS OF AMMONIUM AND TUNGSTEN ON NITRATE AND NITRITE UPTAKE AND REDUCTION BY SUNFLOWER PLANTS

Net NO3- uptake by intact sunflower plants (Helianthus annuus L. cv Peredovic) was substantially decreased by the presence of NH4+ in the external solution. The simultaneous addition of the glutamine synthetase (GS)-inhibitor methionine sulphoximine (MSO) did not relieve the inhibitory effect of NH4+. The addition of MSO alone also resulted in a substantial inhibition of NO3- uptake. Hence, glutamine or another immediate product of NH4+ assimilation did not appear to be involved in the NH4+ effect. By contrast to NO3-, NO2- uptake was not affected by the presence of NH4+ or MSO. Addition of NH4+ and/or MSO also promoted a decrease of root nitrate reductase (NR) activity without affecting nitrite reductase (NiR) activity. Furthermore, NH4+- and MSO-treatments led to accumulation of both NH4+ and NO3- in root tissue. It is suggested that the long-term inhibitory effect of NH4+ on NO3- uptake is the result of a decreased rate of NO3- reduction. On the other hand, pretreatment of the plants with tungsten suppressed NO3- uptake but not NO2- uptake. Besides, tungsten fully inhibited root NR activity without affecting NiR activity. Inhibition of NO3- uptake by tungsten led to a decrease of internal NO3- levels. On the basis of these results and of recent observations by other authors, it is suggested that the NR complex from roots might also be involved in the uptake of NO3- by sunflower plants. © 1990.