The work was carried out at the Universidade Federal de Santa Maria, Rio Grande do Sul State, Brazil, from October to December 2004. Three simultaneous experiments were carried out, each one with one Mentha species (M. arvensis, M. x gracilis and M. x piperita var. citrata) in four hydroponic solutions with different K concentrations (276; 414; 552 and 690 mg L-1 ) and plant ages. The experimental design was of randomized complete blocks, with five replications, the treatments showing a factorial for each species: 4x6, 4x7 and 4x8, four K concentrations and plant ages being determined weekly starting 27 days after planting date. The height of plants; foliar area; fresh and dry phytomass of leaves and stems; leaf:stem relation; and the content of essential oil in fresh leaves were evaluated. The increase of K concentrations in the hydroponic solutions affected negatively the growth and accumulation of phytomass in mint plants, but allowed an increase in the content of essential oil per plant. In all consecutive harvesting dates an increase in the growth of the three species of plants were observed. M. arvensis and M. x piperita var. citrata accumulated more stem than leaf phytomass, the reverse occurring with M. x gracilis, which also presented a higher equivalent production of oil per ha, in the lowest K concentration. A good growth of Mentha under hydroponic cultivation was observed under K concentrations of 276 and 414 mg L-1.
Mentha arvensis; Mentha x gracilis; Mentha x piperita var. citrata; yield; aromatic oil
