Intraclonal translocation of ammonium and nitrate nitrogen in Carex bigelowii Torr. ex Schwein. using 15N and nitrate reductase assays

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Abstract
Carex bigelowii Torr. ex Schwein. forms extensive rhizome systems consisting of numerous long-lived tiller generations with only the roots and the rhizome remaining alive, and only few tillers with above-ground shoots. The hypothesis that benefits to the young photosynthetic tillers are provided by the non-photosynthetic tiller generations through the uptake and translocation of nutrients was tested by tracing the movement of nitrogen within clones growing in the field in the Swedish Lapland. Application of nitrate to individual roots of 3- to 7-year-old tillers induced nitrate reductase activity (NRA) in the youngest tiller generations after 2-3 days. The subsequent application of 15N-nitrate and 15N-ammonium to indivdiual roots of old tiller generations showed that both ions were taken up by these old roots, but ammonium-N was taken up in greater quantities. The 15N was mainly translocated distally to the younger tillers, but some proximal translocation was also detected which was greater for ammonium-N. Nitrate-N was translocated at a higher rate than ammonium-N, resulting in a greater accumulation of nitrate-N in the youngest tiller generation during the 2 days of the experiment. The independent NRA and 15N methods confirm (a) the continuing resource acquistion by old nonphotosynthetic tillers; (b) a long period of ramet interdependence extending over at least 7 years; (c) a division of labour between ramet generations and (d) the abilities of C. bigelowii clones to assimilate both ammonium and nitrate nitrogen. These aspects of clonal integration have important consequences for the fitness of C. bigelowii in its cold and nutrient-poor environments.