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Abstract: Nitrogen (N) fertilization in citrus orchards is prone to significant environmental losses. Managing nutrients for sustainable development and optimizing nitrogen use efficiency (NUE) are key tasks in modern agriculture. NUE depends on N uptake and assimilation. In addition to intrinsic fertilizer use efficiency, we evaluated plant growth, nitrate reductase (NRase) activity, and the nutritional status of citrus plants fertilized with different N sources [calcium nitrate (CN), ammonium nitrate (AN), and ammonium sulfate (AS)]. After 240 days, plants supplied with either CN or AN exhibited higher biomass production and N accumulation in the leaves than those fertilized with AS. The supply of ammonium-containing sources (AN and AS) reduced the substrate pH, in which those with AS exhibited lower N partitioning to the leaves, as well as reduced the absorption and accumulation of magnesium and phosphorus. NRase activity was higher in plants supplied with CN or AN compared to AS, and CN fertilization provided the greatest NUE. The data demonstrated that citrus plants fertilized with sources containing nitrate increased the NUE from 10 to 25% and consequently had greater growth compared to those fed with AS.

Increasing the nitrogen use efficiency of crop plants is a result of the improvement in the mechanisms involved in nitrogen uptake, translocation, assimilation and remobilization. In this study, tobacco plants with high accumulation of proline caused by the insertion of the VaP5CSF129A mutant gene under control of the constitutive promoter CaMV 35S was used as a model to investigate the effect of overexpression of the transgene in the uptake and utilization of N. For this, an experiment was carried out in a completely randomized design, in a 2x5 factorial arrangement, with two genotypes (transgenic event and untransformed control) and five levels of nitrogen fertilization (25, 50, 100, 200 and 400 kg N ha-?), cultivated under greenhouse conditions. Plants were evaluated for height, number of leaves, dry mass, free proline content, nitrogen content, total protein content and use of the applied N (efficiency of absorption - EAbs, efficiency of leaf utilization - EUtil and efficiency of plant use - EUN). The transgenic plants with high proline accumulation were less efficient than the control plants in terms of nitrogen absorption (with lower N levels in root, leaves and whole plant), which resulted in lower EAbs. There was no difference between the transgenic and control genotypes for EUtil. However, the transgenic plants presented lower EUN in comparison to the control plants. Our data showed that transgenic tobacco plants with high proline accumulation do not have potential for direct use in agricultural systems, since the alteration of the biochemical balance between the metabolisms of nitrogen and carbon did not result in plants with higher EUN. [ABSTRACT FROM AUTHOR]