Hypoxia Affects Nitrogen Uptake and Distribution in Young Poplar (Populus × canescens) Trees

The present study with young poplar trees aimed at characterizing the effect of O(2) shortage in the soil on net uptake of NO(3) (-) and NH(4) (+) and the spatial distribution of the N taken up. Moreover, we assessed biomass increment as well as N status of the trees affected by O(2) deficiency. For this purpose, an experiment was conducted in which hydroponically grown young poplar trees were exposed to hypoxic and normoxic (control) conditions for 14 days. (15)N-labelled NO(3) (-) and NH(4) (+) were used to elucidate N uptake and distribution of currently absorbed N and N allocation rates in the plants. Whereas shoot biomass was not affected by soil O(2) deficiency, it significantly reduced root biomass and, consequently, the root-to-shoot ratio. Uptake of NO(3) (-) but not of NH(4) (+) by the roots of the trees was severely impaired by hypoxia. As a consequence of reduced N uptake, the N content of all poplar tissues was significantly diminished. Under normoxic control conditions, the spatial distribution of currently absorbed N and N allocation rates differed depending on the N source. Whereas NO(3) (-) derived N was mainly transported to the younger parts of the shoot, particularly to the developing and young mature leaves, N derived from NH(4) (+) was preferentially allocated to older parts of the shoot, mainly to wood and bark. Soil O(2) deficiency enhanced this differential allocation pattern. From these results we assume that NO(3) (-) was assimilated in developing tissues and preferentially used to maintain growth and ensure plant survival under hypoxia, whereas NH(4) (+) based N was used for biosynthesis of storage proteins in bark and wood of the trees. Still, further studies are needed to understand the mechanistic basis as well as the eco-physiological advantages of such differential allocation patterns.