Effects of rhizospheric bicarbonate on net nitrate uptake and partitioning between 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 from dark fixation accounts for only 1-3% of total plant carbon gain. This study, therefore, aimed at investigating the effects of bicarbonate on nitrate uptake, assimilation and translocation to shoots. Clonal saplings of poplar (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 mM bicarbonate and 2 mM nitrate as the sole nitrogen source at pH 7.0. Net nitrate uptake, root nitrate accumulation and reduction, and export of nitrogenous solutes to shoots were measured after incubating plants with 15N-labelled nitrate for 24 h. Net nitrate uptake increased non-significantly in plant species (19-61% compared to control plants) in response to 1 mM bicarbonate. 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 the main nitrate utilising processes. Poplar plants increasingly utilised nitrate via nitrate reduction (73-88% of net nitrate uptake), whereas the proportions of export (20-9%) and storage in roots (7-3%) declined as plants were exposed to 1 mM external bicarbonate. On the other hand, elder plants exhibited a significant increase of root nitrate reduction (44-66%) and root nitrate accumulation (6-25%). Nitrate translocation to elder shoots decreased from 50 to 8% of net nitrate uptake. The improved supply of nitrogen to shoots did not translate into a significant stimulation of growth, relative growth rates increased by only 16% in poplar saplings and by 7% in elder plants.