Nitrogen uptake and translocation by Chara

The potential for above-ground and below-ground uptake and subsequent internal translocation of ammonium (NH4+) and nitrate (NO3-) by the macroalga Chara spp. was investigated. In a two compartment experimental set-up separating above-ground and below-ground algal parts, the charophytes were exposed to various combinations of N-15-labelled NH4+ and NO3-. Uptake in one compartment and translocation to the other were measured. Chara spp. was able to take up and translocate nitrogen between below-ground and above-ground parts. Uptake of (NH4+)-N-15 in rhizoids was two-fold higher than that of (NO3-)-N-15, indicating a preferential uptake of (NH4+)-N-15. Translocation after 5 days was always less in the direction from above-ground to below-ground parts (on average 2% of total N-15 uptake), than in the below-ground to above-ground direction (on average 29%). Translocation occurred when the ratio of N-15-atomic percentage in the algal material in the exposed compartment roughly exceeded 2%, and was thus more determined by the internal gradient in the N-15 content than by the nature of the N source (either NH4+ or NO3-). Translocation of N-15 from the below-ground to the above-ground compartment also occurred when the charophytes were exposed to high concentrations of either NO3- or NH4+ in the above-ground compartment. The results of this study are supportive for a mechanism with preferential uptake of NH4+ over NO3-, and subsequent passive diffusion between cells as the dominant transport mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.