Tree Physiology Advance Access published March 3, 2014
Tree Physiology 00, 1–9 doi:10.1093/treephys/tpu005
Research paper
Nitrogen uptake over entire root systems of tree seedlings B.J. Hawkins1, S. Robbins and R.B. Porter
Received September 27, 2013; accepted January 10, 2014; handling Editor Torgny Näsholm
Uptake of nitrogen (N) by sequential root regions in six tree species was measured in roots of 16- to 26-month-old seedlings at 50 and 1500 µM NH4NO3 concentration, at the cell level using oscillating microelectrodes and at the root region level using enriched 15N application. Our objective was to determine the root regions making the greatest contribution to total N uptake in each species as measured by the two contrasting techniques. White and condensed tannin zones were the regions with the smallest surface area in all species, but these zones often had the highest net flux of NH4+ and NO3−. For most species, little variation was found among root regions in N flux calculated using a 15N mass balance approach, but where significant differences existed, high N flux was observed in white, cork or woody zones. When N fluxes measured by each of the two methods were multiplied by the estimated surface area or biomass of each root region, the effect of root region size had the greatest influence on regional N uptake. Root regions of greatest overall N uptake were the cork and woody zones, on average. Total N uptake may thus be greatest in older regions of tree seedling roots, despite low rates of uptake per unit area. Keywords: nitrogen isotopes, non-invasive microtest technology, nutrient uptake, root physiology.
Introduction The ability to accurately measure rates of nitrogen (N) uptake by plant roots is of value to researchers in many areas of plant biology. Nitrogen is the essential mineral nutrient required in the greatest amounts, yet its availability is often limited in natural and agricultural soils. Many techniques are used to measure N uptake, directly and indirectly, at levels from the whole root to the cell. Our laboratory has utilized oscillating microelectrodes to measure NH4+ and NO3− flux at the scale of a few cells, and also stable N isotopes to measure N uptake by the whole root. We wished to utilize these contrasting techniques to assess the relative contribution to total N uptake made by different root regions of tree seedlings. Tree root anatomy and morphology are very different from well-studied crop plants, and change dramatically along the length of the root (Peterson et al. 1999). Non-mycorrhizal, actively growing roots of Pinus banksiana Lamb. have a white zone (WZ), proximal to the root cap, comprised of a living
c ortex, endodermis with passage cells, pericycle and vascular tissue. The living cortex allows for efficient ion uptake (Peterson et al. 1999); however, in trees, white roots typically represent
Tree Physiology 00, 1–9 doi:10.1093/treephys/tpu005
Research paper
Nitrogen uptake over entire root systems of tree seedlings B.J. Hawkins1, S. Robbins and R.B. Porter
Received September 27, 2013; accepted January 10, 2014; handling Editor Torgny Näsholm
Uptake of nitrogen (N) by sequential root regions in six tree species was measured in roots of 16- to 26-month-old seedlings at 50 and 1500 µM NH4NO3 concentration, at the cell level using oscillating microelectrodes and at the root region level using enriched 15N application. Our objective was to determine the root regions making the greatest contribution to total N uptake in each species as measured by the two contrasting techniques. White and condensed tannin zones were the regions with the smallest surface area in all species, but these zones often had the highest net flux of NH4+ and NO3−. For most species, little variation was found among root regions in N flux calculated using a 15N mass balance approach, but where significant differences existed, high N flux was observed in white, cork or woody zones. When N fluxes measured by each of the two methods were multiplied by the estimated surface area or biomass of each root region, the effect of root region size had the greatest influence on regional N uptake. Root regions of greatest overall N uptake were the cork and woody zones, on average. Total N uptake may thus be greatest in older regions of tree seedling roots, despite low rates of uptake per unit area. Keywords: nitrogen isotopes, non-invasive microtest technology, nutrient uptake, root physiology.
Introduction The ability to accurately measure rates of nitrogen (N) uptake by plant roots is of value to researchers in many areas of plant biology. Nitrogen is the essential mineral nutrient required in the greatest amounts, yet its availability is often limited in natural and agricultural soils. Many techniques are used to measure N uptake, directly and indirectly, at levels from the whole root to the cell. Our laboratory has utilized oscillating microelectrodes to measure NH4+ and NO3− flux at the scale of a few cells, and also stable N isotopes to measure N uptake by the whole root. We wished to utilize these contrasting techniques to assess the relative contribution to total N uptake made by different root regions of tree seedlings. Tree root anatomy and morphology are very different from well-studied crop plants, and change dramatically along the length of the root (Peterson et al. 1999). Non-mycorrhizal, actively growing roots of Pinus banksiana Lamb. have a white zone (WZ), proximal to the root cap, comprised of a living
c ortex, endodermis with passage cells, pericycle and vascular tissue. The living cortex allows for efficient ion uptake (Peterson et al. 1999); however, in trees, white roots typically represent
