Nitrogen uptake and translocation by Chara

The potential for above-ground and below-ground uptake and subsequent internal translocation of ammonium (NH 4 +) and nitrate (NO 3 −) 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 15 N -labelled NH 4 + and NO 3 −. 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 15 NH 4 + in rhizoids was two-fold higher than that of 15 NO 3 − , indicating a preferential uptake of 15 NH 4 + . Translocation after 5 days was always less in the direction from above-ground to below-ground parts (on average 2% of total 15 N uptake), than in the below-ground to above-ground direction (on average 29%). Translocation occurred when the ratio of 15 N -atomic percentage in the algal material in the exposed compartment roughly exceeded 2%, and was thus more determined by the internal gradient in the 15 N content than by the nature of the N source (either NH 4 + or NO 3 −). Translocation of 15 N from the below-ground to the above-ground compartment also occurred when the charophytes were exposed to high concentrations of either NO 3 − or NH 4 + in the above-ground compartment. The results of this study are supportive for a mechanism with preferential uptake of NH 4 + over NO 3 −, and subsequent passive diffusion between cells as the dominant transport mechanism.