The growth of four heathland species, two grasses (D. flexuosa, M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix), was tested in solution culture at pH 4.0 with 2 mol m(-3) N, varying the NO3-/NH4+ ratio up to 40% nitrate. In addition, measurements of NRA, plant chemical composition, and biomass allocation were carried out on a complete NO3-/NH4+ replacement series up to 100% nitrate. With the exception of M. caerulea, the partial replacement of NH4+ by NO3- tended to enhance the plant's growth rate when compared to NH4+ only. In contrast to the other species, D. flexuosa showed a very flexible response in biomass allocation: a gradual increase in the root weight ratio (RWR) with NO3- increasing from 0 to 100%. In the presence of NH4+, grasses reduced nitrate in the shoot only; roots did not become involved in the reduction of nitrate until zero ambient NH4+. The dwarf shrubs, being species that assimilate N exclusively in their roots, displayed an enhanced root NRA in the presence of nitrate; in contrast to the steady increase with increasing NO3- in Calluna roots, enzyme activity in Erica roots followed a rather irregular pattern. Free nitrate accumulated in the tissues of grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO4- liproportion of absorbed ammonium highly preferentially (relative uptake ratio for NO3-
Original languageEnglish
JournalJournal of Experimental Botany
Journal publication date1995

ID: 387639