UPTAKE AND ASSIMILATION OF NITRATE AND AMMONIUM BY AN EVERGREEN FYNBOS SHRUB SPECIESPROTEA REPENSL. (PROTEACEAE)

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Abstract
Summary: The uptake and assimilation of15N supplied either as NO3−or NH4+has been studied over a 9‐d period inProtea repens, a member of the Fynbos (South African Mediterranean‐type vegetation) to establish the preferred N source for plant growth and development. Low uptake and assimilation rates were found for both NO3−and NH4+feeding (4.8 and 5.9/μg15N g−1fresh mass d−1, respectively) and total15N absorbed over 9 d was greater in the NH4+‐fed plants than in NO3+‐fed ones. The form of N supplied did not cause significant changes in the total N content of the plants over the feeding period and no alteration in total N distribution patterns was evident, the shoot to root nitrogen ratio being 7:3.Distribution of15N between the soluble N fraction and the bound fraction within root and shoot was similar for both NO3+and NH4+‐fed plants. In both treatments approximately 70 % of the15N was found in the root, mainly in the bound fraction.Protea repensappears to have adapted to the conditions prevalent in the later stages of Fynbos succession, not by showing an absolute requirement for a particular inorganic N form, but by an ability to absorb and assimilate small quantities of both NO3−and NH4+, which satisfy the low N demand of this slow growing, evergreen species.