NITROGEN ASSIMILATION IN ROOTS AND LEAVES OF GREEN BEAN PLANTS: DEFICIENCY VS TOXICITY OF NITROGEN

Abstract

Plant growth depends, among other factors, of an adequate supply of nitrogen to produce amino acids, proteins, nucleic acids and other cell components needed for plant development. One of the factors influencing the enzyme regulation system responsible for N assimilation, is the nutritional status of N in the plant; hence the objective of this research was to study this physiological process under a range of N rates varying from deficiency to toxicity. Nitrogen was applied in the nutrient solution in the form of NH₄NO₃, at increasing N doses N1=1.5 mM, N2=3.0 mM, N3=6.0 mM, N4=12.0 mM, N5=18.0 mM, N6=24.0 mM of N. Nitrogen deficiency (N1 and N2) produced a significant reduction of N assimilation in roots and leaves. The enzymatic activities of NR, NiR, GS, GOGAT and PEPCasa were lower than in N treatment (N3) which corresponded to the optimum rate. The lower N assimilation in treatments N1 and N2 caused minimum concentrations of N-organic compounds (amino acids, proteins and organic N) and reduced plant biomass compared to N3. On the other hand, N toxicity (N4, N5 and N6) was characterized for higher concentrations of NO₃- and NH₄+ in roots and leaves, but without having a negative effect on N assimilation; these treatments showed the maximum enzymatic activity and higher N-organic compounds (N6 treatment), although this was not reflected on higher biomass production. We concluded that N toxicity had a higher adverse effect on biomass production than N-deficiency.