THE EFFECT OF NITROGEN-SOURCE ON GROWTH, NITROGEN ECONOMY AND RESPIRATION OF 2 HIGH ARCTIC PLANT-SPECIES DIFFERING IN RELATIVE GROWTH-RATE

1. The effect of NH4+ and NO3- on growth, nitrogen economy and respiration rates in two high arctic plant species differing in relative growth rate (RGR; Oxyria digyna and Dryas integrifolia), were investigated under controlled conditions. 2. Dryas integrifolia, which is typically found in exposed, nutrient-poor habitats in the field, exhibited similarly low RGR values on the two nitrogen sources, with nitrogen treatment having no effect on its partitioning of biomass. In contrast, the species characteristic of more sheltered, fertile habitats, 0. digyna, grew faster on NO3- than NH4+ over much of the experimental period, due a greater net assimilation rate (NAR) and nitrogen productivity (NP). 3. Nitrogen treatment had no effect on the leaf area ratio (or its components) of either species. However, growth of O. digyna on NO3- resulted in thicker leaves and leaves with a lower dry matter content than in the NH4+-grown plants. Root dry matter content was also lower in the NO3- grown plants of both species. 4. Respiration rates were greater in both the shoots and roots of O. digyna grown on NH4+ than on NO3-, due entirely to higher rates of cytochrome pathway activity. However, neither the rate of carbon loss by respiration per se, or differences in the efficiency of respiration, could explain the differences in NAR and RGR in 0. digyna grown on NO3- and NH4+. 5. The faster growth of 0. digyna (RGR up to 165 mg g-1 day-1), relative to D. integrifolia (RGR up to 41 mg g-1 day-1), was caused largely by its greater investment in photosynthetic tissues and its higher efficiency of nitrogen use. Differences in the total rate and efficiency of respiration could not account for the differences in RGR between 0. digyna and D. integrifolia. 6. We concluded that both species were capable of utilizing NO3-, despite typically growing on NH4+-dominated soils in the field.