Biomedical subjects

A J Bloom

Publications and source records attributed to A J Bloom.

4 recordsLinked to original sources

PubMed2001

Wheat leaves emit nitrous oxide during nitrate assimilation.

Nitrous oxide (N(2)O) is a key atmospheric greenhouse gas that contributes to global climatic change through radiative warming and depletion of stratospheric ozone. In this report, N(2)O flux was monitored simultaneously with photosynthetic CO(2) and O(2) exchanges from intact canopies of 12 wheat seedlings. The rates of N(2)O-N emitted ranged from <2 pmol x m(-2) x s(-1) when NH(4)(+) was the N source, to 25.6 +/- 1.7 pmol x m(-2) x s(-1) (mean +/- SE, n = 13) when the N source was shifted to NO(3)(-). Such fluxes are among the smallest reported for any trace gas emitted by a higher plant. Leaf N(2)O emissions were correlated with leaf nitrate assimilation activity, as measured by using the assimilation quotient, the ratio of CO(2) assimilated to O(2) evolved. (15)N isotopic signatures on N(2)O emitted from leaves supported direct N(2)O production by plant NO(3)(-) assimilation and not N(2)O produced by microorganisms on root surfaces and emitted in the transpiration stream. In vitro production of N(2)O by both intact chloroplasts and nitrite reductase, but not by nitrate reductase, indicated that N(2)O produced by leaves occurred during photoassimilation of NO(2)(-) in the chloroplast. Given the large quantities of NO(3)(-) assimilated by plants in the terrestrial biosphere, these observations suggest that formation of N(2)O during NO(2)(-) photoassimilation could be an important global biogenic N(2)O source.

Ecology↗

PubMed2001

United kingdoms.

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Animals↗

PubMed1998

Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations.

We examined the hypothesis that elevated CO2 concentration would increase NO3- absorption and assimilation using intact wheat canopies (Triticum aestivum cv. Veery 10). Nitrate consumption, the sum of plant absorption and nitrogen loss, was continuously monitored for 23 d following germination under two CO2 concentrations (360 and 1000 micromol mol-1 CO2) and two root zone NO3- concentrations (100 and 1000 mmol m3 NO3-). The plants were grown at high density (1780 m-2) in a 28 m3 controlled environment chamber using solution culture techniques. Wheat responded to 1000 micromol mol-1 CO2 by increasing carbon allocation to root biomass production. Elevated CO2 also increased root zone NO3- consumption, but most of this increase did not result in higher biomass nitrogen. Rather, nitrogen loss accounted for the greatest part of the difference in NO3- consumption between the elevated and ambient [CO2] treatments. The total amount of NO3(-)-N absorbed by roots or the amount of NO3(-)-N assimilated per unit area did not significantly differ between elevated and ambient [CO2] treatments. Instead, specific leaf organic nitrogen content declined, and NO3- accumulated in canopies growing under 1000 micromol mol-1 CO2. Our results indicated that 1000 micromol mol-1 CO2 diminished NO3- assimilation. If NO3- assimilation were impaired by high [CO2], then this offers an explanation for why organic nitrogen contents are often observed to decline in elevated [CO2] environments.

Absorption↗

PubMed1996

Nitrogen dynamics in plant growth systems.

The predominant nitrogen source for the plants in closed environmental systems is the mineral nitrogen (i.e., nitrate and/or ammonium) in the nutrient medium. The following focuses on the processes through which plants obtain nitrate and ammonium from the rhizosphere and on the influences that each form has upon plant performance. Most plant species can sustain full growth at nitrate or ammonium concentrations that are over two orders of magnitude lower than those provided in most plant growth systems. Under the high concentrations (mM) normally used, root nitrogen absorption is downregulated: a) both the affinity and capacity of the transport systems for ammonium or nitrate are diminished, b) efflux of either ion becomes a significant percentage of influx, and c) root growth is inhibited. High concentrations also promote accumulation of ammonium or borate in plant tissues to potentially deleterious levels and foster microbial outbreaks. Several lines of evidence argue that roots in natural soils are normally exposed to lower concentrations (micromoles) of nitrate or ammonium: models of root nutrient absorption indicate that roots deplete rhizosphere nitrate and ammonium to such levels; the high-affinity transport systems for nitrate and ammonium have optimal control in this range; and root growth and development is maximized under such conditions. The high-affinity transport systems are distinct for nitrate and ammonium. In general, the affinity of the nitrate system for nitrate is less than the ammonium system for ammonium. Nitrate absorption is induced by the presence of ammonium or nitrate. Roots most rapidly absorb nitrate in the zone where root hairs emerge and ammonium in the zone of division near the apex. Nitrate absorption tends to alkalinize the rhizosphere, whereas ammonium absorption acidifies the rhizosphere. The energy requirements for absorption and assimilation of nitrate are several fold higher than those of ammonium. Root growth and elongation are more extensive when ammonium is provided as the sole nitrogen source, perhaps as a consequence of the lower energy requirements or the increased rhizosphere acidity.

Absorption↗