Nitrate and Nitrite Utilization by Sunflower Plants

The utilization of oxidized nitrogen forms (NO3 − or NO2 −) by higher plants involves: (1) uptake into the root cells; (2) reduction to NH4 + either in the root or in the shoot; and (3) incorporation of NH4 + to carbon skeletons to produce amino acids. The reduction of NO3 + to NH4 + takes place by the sequential participation of the enzymes nitrate reductase (NR) and nitrite reductase (NiR) which catalyze, respectively, the reduction of NO3 − to NO2 − and NO2 − to NH4 + (Guerrero et al. 1981; Oaks and Hirel 1985). Assimilation of NK4 + seems to proceed mainly via the glutamine synthetase (GS)-glutamate synthase cycle (Lea and Miflin 1979). However, under conditions of ample NH4 + supply, glutamate dehydrogenase (GDH) may also operate as NH4 + -assimilating enzyme (Singh and Srivastava 1982; Cammaerts and Jacobs 1985). In most plant systems, NR and NiR are both inducible by NO3 − (Guerrero et al. 1981; Oaks and Hirel 1985). The presence of NO3″ also induces the development of the NO3 −-uptake system in nitrogen-starved plants (Deane-Drummond 1984; MacKown and McClure 1988). On the other hand, addition of NH4+ to plants growing on NO3 − usually has a negative effect on NO3 − utilization. The decline in NO3 − utilization produced by ambient NH4 + can be the result of a short-term inhibition of net NO3 − uptake, either by NH4 + per se (Ullrich 1987) or by a product of its assimilation (Flores et al. 1980), and/or by inactivation or repression of NR (Guerrero et al. 1981; MacKown et al. 1982). In this article, we summarize our investigations on NO3 − and NO2 − utilization by sunflower plants (Helianthus annum L. cv Peredovic).