Nitrogen metabolism of two tropical forage grass species: nitrogen availability × cultivars

The understanding about N metabolism in roots and/or shoots of forage grasses should help determine how some cultivars use N more efficiently. This study was designed to determine whether two Brachiaria brizantha cultivars and two Panicum maximum cultivars differ in their N metabolism. Two cultivars of Panicum maximum and two cultivars of Brachiaria brizantha were investigated under the supply of low and high nitrogen rates in a greenhouse experiment. Some characteristics such as dry mass production, total nitrogen, nitrate and ammonium concentrations and contents; nitrate reductase (NR) and glutamine synthetase (GS) activities; and total free amino acid concentrations in shoots and roots were measured. The results showed that Panicum cultivars showed higher NR activity and GS activity in diagnostic leaves (DL) than Brachiaria cultivars. Brachiaria cultivars showed the highest ammonium concentrations in roots at low N supply and Panicum cultivars at high N supply. The total amino acid concentrations in DL were highest in Panicum and root total amino acid concentrations were highest in Brachiaria. Panicum and Brachiaria cultivars differed in their nitrogen metabolism with varying N supply. The results indicate that at low nitrogen supply to the pasture, Brachiaria brizantha cultivar Piatã may be an option to use in less intensive production environments, instead of high dry mass production. For those pastures with high nitrogen availability, the cultivars Mombaça and Aruana of Panicum maximum were clearly superior to Piatã, which suggests priority in the selection of these cultivars to be used in more intensive production systems that apply more nitrogen