Title : Absorption of macronutrients influenced by nitrogen fertilization in Anacardium othonianum seedlings
The levels of nitrogen available to plants can affect plant structure and physiology, as well as alter the absorption of other essential nutrients. Among the essential nutrients for plant growth, nitrogen is one of the most required. Knowing the requirement of N by the plant is fundamental to the practices of mineral nutrition, allowing gains in plant development, because this nutrient contributes to the synthesis of various organic compounds, such as amino acids, proteins, enzymes and nucleic acids. In order to know the nutritional uptake in Anacardium othonianum Rizz. seedlings kept under different concentrations of N, a greenhouse experiment was installed in a hydroponic system. The treatments were seven doses of nitrogen (0.0, 2.5, 5.0, 7.5, 10.0, 12.5 and 15.0 mmoL L-1) in the nutrient solution of Hoagland and Arnon during 120 days after transplanting the seedlings. The nutritional contents of N, P, K, Ca, Mg and S in leaves, stem and root were determined. The N and P contents in the stem were not influenced by the N doses of the solution, reaching a mean value of 3.64 g kg-1 and 1.47 g kg-1 respectively. The maximum N values in the leaves were 3.16 g kg-1 and in the roots 5.29 g kg-1, at the estimated doses of 9.7 and 14.4 mmoL L-1 of N. The maximum content of nitrogen was found in the roots. This is due to the fact that this is the plant organ required directly for the absorption of the N present in the solution, the higher N uptake by the roots was observed in the plants submitted to the dose of 14.4 mmoL L-1. At doses higher than that, the decrease in N uptake by the roots was verified. Under the absence of N in the solution (0 mmoL L-1), the highest values were obtained for the contents of K, 2.54, 1.75 and 1.90 g kg-1 in leaves, stems and roots, respectively. The increase of N doses in the nutrient solution decreased the contents of K until the doses of 11.9, 9.1 and 13.0 mmoL L-1 of N for leaves, stems and roots, this was also verified with the Ca content in the leaves, since the highest accumulation of this nutrient was 0.75 g kg-1 in dose of 0 mmoL L-1 of N. The highest leaf Mg content was also observed in the treatment with absence of N of the solution, the estimated value being 0.391 g kg-1. According to the increase in N availability, there was a decrease in the Ca and Mg leaf contents. It was concluded that the dose of 9.7 mmoL L-1 of N was the one that best stimulated the nitrogen content in the leaves, while the total absence of N stimulated the foliar availability of K, Ca and Mg in plants of A. othonianum, demonstrated the use of such ions in plant metabolism, in an attempt to supply the absence of nitrogen.