Effects of rhizospheric bicarbonate on net nitrate uptake and partitioningbetween the main nitrate utilising processes in Populus canescens and Sambucus nigra

Increased levels of rhizospheric dissolved inorganic carbon have repeatedly been demonstrated to enhance plant growth by up to 80%, although carbon fr om dark fixation accounts for only 1-3% of total plant carbon gain. This st udy, therefore, aimed at investigating the effects of bicarbonate on nitrat e uptake, assimilation and translocation to shoots. Clonal saplings of popl ar (Populus canescens (Ait.) Sm.) and elder (Sambucus nigra L.) were grown hydroponically for 35 days in a nutrient solution containing 0, 0.5 and 1 m M bicarbonate and 2 mM nitrate as the sole nitrogen source at pH 7.0. Net n itrate uptake, root nitrate accumulation and reduction, and export of nitro genous solutes to shoots were measured after incubating plants with (1)5N-l abelled nitrate for 24 h. Net nitrate uptake increased non-significantly in plant species (19-61% compared to control plants) in response to 1 mM bica rbonate. Root nitrate reduction and nitrogen export to shoots increased by 80 and 95% and 15 and 44% in poplar and elder, respectively. With enhanced root zone bicarbonate, both species also exhibited a marked shift between t he main nitrate utilising processes. Poplar plants increasingly utilised ni trate via nitrate reduction (73-88% of net nitrate uptake), whereas the pro portions of export (20-9%) and storage in roots (7-3%) declined as plants w ere exposed to 1 mM external bicarbonate. On the other hand, elder plants e xhibited a significant increase of root nitrate reduction (44-66%) and root nitrate accumulation (6-25%). Nitrate translocation to elder shoots decrea sed from 50 to 8% of net nitrate uptake. The improved supply of nitrogen to shoots did not translate into a significant stimulation of growth, relativ e growth rates increased by only 16% in poplar saplings and by 7% in elder plants.