Differential effects of ammonium and tungsten on nitrate and nitrite uptake and reduction by sunflower plants

Abstract

Net NO 3 − uptake by intact sunflower plants ( Helianthus annuus L. cv Peredovic) was substantially decreased by the presence of NH 4 + in the external solution. The simultaneous addition of the glutamine synthetase (GS)-inhibitor methionine sulphoximine (MSO) did not relieve the inhibitory effect of NH 4 +. The addition of MSO alone also resulted in a substantial inhibition of NO 3 − uptake. Hence, glutamine or another immediate product of NH 4 + assimilation did not appear to be involved in the NH 4 + effect. By contrast to NO 3 −, NO 2 − uptake was not affected by the presence of NH 4 + or MSO. Addition of NH 4 + and/or MSO also promoted a decrease of root nitrate reductase (NR) activity without affecting nitrite reductase (NiR) activity. Furthermore, NH 4 +- and MSO-treatments led to accumulation of both NH 4 + and NO 3 − in root tissue. It is suggested that the long-term inhibitory effect of NH 4 + on NO 3 − uptake is the result of a decreased rate of NO 3 − reduction. On the other hand, pretreatment of the plants with tungsten suppressed NO 3 − uptake but not NO 2 − uptake. Besides, tungsten fully inhibited root NR activity without affecting NiR activity. Inhibition of NO 3 − uptake by tungsten led to a decrease of internal NO 3 − 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 NO 3 − by sunflower plants.