Nitrogen and sulphur in Marandu grass: relationship between supply and concentration in leaf tissues

In order to better assess the nutritional status of forage grasses there has been an increasing interest in studying the nutrient concentrations present in different parts of plant shoot. The aim of this work was to evaluate the nitrogen and sulphur concentrations in leaf tissues of Brachiaria brizantha cv. Marandu grown in nutrient solutions with combined rates of both nitrogen and sulphur. An experiment was conducted in a greenhouse during the Spring season in Piracicaba-SP, Brazil, using ground quartz as substrate. A fractionated 5² factorial design was used, with 13 combinations of nitrogen and sulphur rates in the solutions, in mg L-1, as follows: 14 and 3.2; 14 and 32; 14 and 80; 126 and 12.8; 126 and 64; 210 and 3.2; 210 and 32; 210 and 80; 336 and 12.8; 336 and 64; 462 and 3.2; 462 and 32, and 462 and 80. The experimental units were set in a randomized block design, with four replications. Plants were harvested twice. Emerging leaves, recently expanded lamina and mature lamina were sampled separately at each harvest time. In these three types of tissues the nitrogen concentration was closely related to the nitrogen supply in the solution, whereas the sulphur concentration, N:S ratio and chlorophyll meter (SPAD) readings depended upon the supply of both nitrogen and sulphur. Mature lamina did not relate well to the changing in sulphur supply. To properly assess the nitrogen and sulphur concentrations, N:S ratio and SPAD readings for the Marandu grass, the two recently expanded lamina were the best tissues to indicate the availability of both nitrogen and sulphur in the nutrient solution