Marie Mattsson

INFLUENCE OF NITROGEN NUTRITION AND METABOLISM ON AMMONIA VOLATILIZATION IN PLANTS

Barley (Hordeum vulgare L. cv. Golf) was grown in solution culture with controlled nitrogen availability in order to study the influence of nitrogen nutrition on ammonia emission from the leaves. Ammonia emission measured in cuvettes connected to an automatic NH3 monitor was close to zero for nitrate grown plants but increased to 0.9–1.3 nmol NH3 m-2 leaf area s-1 after 3–5 days of ammonium nutrition. Increasing concentrations from 0.5 to 10 mM NH4+ in the root medium increased NH3 emission from the shoots, root glutamine synthetase activity and NH4+ concentrations in apoplast, xylem sap and bulk tissue, while apoplastic pH values decreased.Inhibition of glutamine synthetase in nitrate grown barley plants by addition of 1 mM methionine sulfoximine (MSO) to the root medium caused ammonia emission to increase 5 to 10-fold after 2–3 hours. At the same time shoot tissue ammonium concentrations started to increase. Addition of an inhibitor of photorespiration, 1 mM pyrid-2-yl hydroxymethane sulfonate (HPMS) reduced this increase in ammonia emission showing a relation between NH3 emission and photorespiration.Oil seed rape (Brassica napus L. cv. Global) plants grown at 3 different nitogen levels (2N, 4N and 7N) in a sand/soil mixture showed increasing NH3 compensation points with increasing N level. This increase was highly correlated with increasing NH4+ concentrations in the leaf apoplast and total leaf tissue. The NH3 compensation points could be succesfully predicted on basis of the pH and NH4+ concentration in the leaf apoplast.

Quantification of ammonia exchange between agricultural cropland and the atmosphere: Measurements over two complete growth cycles of oilseed rape, wheat, barley and pea

The exchange of ammonia between the atmosphere and the canopy of barley, wheat, oilseed rape and pea crops was studied over two growing seasons by use of a modified aerodynamic gradient technique in which passive horizontal flux samplers were applied with a wind profile in gradient configuration. The crop foliage was a net source of NH3 to the atmosphere, with NH3 emissions on a seasonal basis between 1 and 5 kg NH3–N ha−1. The amount of NH3 lost constituted between 1 and 4% of the applied nitrogen and between 1 and 4% of the actual amount of nitrogen present in the mature shoots. The volatile NH3 losses depended on seasonal variations in climatic conditions affecting the growth and nitrogen economy of the crops and increased under conditions with excessive N absorption by roots and a high N concentration in the foliage. The accumulated NH3 loss was positively correlated with the above-ground crop N content at anthesis, but not with that at final maturity. There were no indications that NH3 emissions were larger under conditions unfavourable for nitrogen remobilization from vegetative plant parts (low N harvest index). Nevertheless, a distinct peak in NH3 emission occurred during senescence. It is concluded that crops in many areas will represent a significant input of ammonia to the atmosphere and that NH3 losses may become large enough to significantly affect crop N budgets.

Physiological regulation of plant-atmosphere ammonia exchange.

Plants have a compensation point for NH3 which ranges from 0.1 to 20 nmol mol-1, and may be several-fold higher or lower than naturally occurring atmospheric NH3 concentrations. This implies that NH3 fluxes over vegetated surfaces are bi-directional and that ammonia exchange with the atmosphere in many cases contributes significantly to the nitrogen economy of vegetation. Physiological regulation of plant–atmosphere NH3 fluxes is mediated via processes involved in nitrogen uptake, transport and metabolism. A rapid turnover of NH3+ in plant leaves leads to the establishment of a finite NH3+ concentration in the leaf apoplastic solution. This concentration determines, together with that of H+, the size of the NH3 compensation point. Barley and oilseed rape plants with access to NH3+ in the root medium have higher apoplastic NH3+ concentrations than plants absorbing NO3-. Furthermore, the apoplastic NH3+ concentration increases with the external NH3+ concentration. Inhibition of GS leads to a rapid and substantial increase in apoplastic NH3+ and barley mutants with reduced GS activity have higher apoplastic NH3+ than wild-type plants. Increasing rates of photorespiration do not affect the steady-state NH3+ or H+ concentration in tissue or apoplast of oilseed rape, indicating that the NH3+ produced is assimilated efficiently. Nevertheless, NH3 emission increases due to a temperature-mediated displacement of the chemical equilibrium between gaseous and aqueous NH3 in the apoplast. Sugarbeet plants grown with NO3- seem to be temporarily C-limited in the light due to a repression of respiration. As a consequence, the activity of chloroplastic GS declines during the day causing a major part of NH3+ liberated in photorespiration to be assimilated during darkness when 2-oxoglutarate is supplied in high rates by respiration.

Dynamic and Steady-State Responses of Inorganic Nitrogen Pools and NH3 Exchange in Leaves of Lolium perenne and Bromus erectus to Changes in Root Nitrogen Supply

Short- and long-term responses of inorganic N pools and plant-atmosphere NH3 exchange to changes in external N supply were investigated in 11-week-old plants of two grass species,Lolium perenne and Bromus erectus, characteristic of N-rich and N-poor grassland ecosystems, respectively. A switch of root N source from NO3 − to NH4 + caused within 3 h a 3- to 6-fold increase in leaf apoplastic NH4 + concentration and a simultaneous decrease in apoplastic pH of about 0.4 pH units in both species. The concentration of total extractable leaf tissue NH4 + also increased two to three times within 3 h after the switch. Removal of exogenous NH4 + caused the apoplastic NH4 + concentration to decline back to the original level within 24 h, whereas the leaf tissue NH4 +concentration decreased more slowly and did not reach the original level in 48 h. After growing for 5 weeks with a steady-state supply of NO3 − or NH4 +, L. perenne were in all cases larger, contained more N, and utilized the absorbed N more efficiently for growth than B. erectus, whereas the two species behaved oppositely with respect to tissue concentrations of NO3 −, NH4 +, and total N. Ammonia compensation points were higher for B. erectus than for L. perenne and were in both species higher for NH4 +- than for NO3 −-grown plants. Steady-state levels of apoplastic NH4 +, tissue NH4 +, and NH3 emission were significantly correlated. It is concluded that leaf apoplastic NH4 + is a highly dynamic pool, closely reflecting changes in the external N supply. This rapid response may constitute a signaling system coordinating leaf N metabolism with the actual N uptake by the roots and the external N availability.

Photorespiratory NH4 + Production in Leaves of Wild-Type and Glutamine Synthetase 2 Antisense Oilseed Rape

Exposure of oilseed rape (Brassica napus) plants to increasing leaf temperatures between 15°C and 25°C increased photorespiratory NH4 + production from 0.7 to 3.5 μmol m−2 s−1. Despite the 5-fold increase in the rate of NH4 + production, the NH4 + concentration in root and leaf tissue water and xylem sap dropped significantly, whereas that in the leaf apoplastic fluid remained constant. The in vitro activity of glutamine synthetase (GS) in both leaves and roots also increased with temperature and in all cases substantially exceeded the observed rates of photorespiratory NH4 + production. The surplus of GS in oilseed rape plants was confirmed using GS2 antisense plants with 50% to 75% lower in vitro leaf GS activity than in the wild type. Despite the substantial reduction in GS activity, there was no tendency for antisense plants to have higher tissue NH4 + concentrations than wild-type plants and no overall correlation between GS activity and tissue NH4 + concentration was observed. Antisense plants exposed to leaf temperatures increasing from 14°C to 27°C or to a trifold increase in the O2 to CO2 ratio did not show any change in steady-state leaf tissue NH4 + concentration or in NH3emission to the atmosphere. The antisense plants also had similar leaf tissue concentrations of glutamine, glycine, and serine as the wild type, whereas glutamate increased by 38%. It is concluded that photorespiration does not control tissue or apoplastic levels of NH4 + in oilseed rape leaves and, as a consequence, that photorespiration does not exert a direct control on leaf atmosphere NH3 fluxes.

Leaf-Atmosphere NH3 Exchange in Barley Mutants with Reduced Activities of Glutamine Synthetase

Mutants of barley (Hordeum vulgare L. cv Maris Mink) with 47 or 66% of the glutamine synthetase (GS) activity of the wild type were used for studies of NH3 exchange with the atmosphere. Under normal light and temperature conditions, tissue NH4+ concentrations were higher in the two mutants compared with wild-type plants, and this was accompanied by higher NH3 emission from the leaves. The emission of NH3 increased with increasing leaf temperatures in both wild-type and mutant plants, but the increase was much more pronounced in the mutants. Similar results were found when the light intensity (photosynthetic photon flux density) was increased. Compensation points for NH3 were estimated by exposing intact shoots to 10 nmol NH3 mol-1 air under conditions with increasing temperatures until the plants started to emit NH3. Referenced to 25[deg]C, the compensation points were 5.0 nmol mol-1 for wild-type plants, 8.3 nmol mol-1 for 47% GS mutants, and 11.8 nmol mol-1 for 66% GS mutants. Compensation points for NH3 in single, nonsenescent leaves were estimated on the basis of apoplastic pH and NH4+ concentrations. These values were 0.75, 3.46, and 7.72 nmol mol-1 for wild type, 47% GS mutants, and 66% GS mutants, respectively. The 66% GS mutant always showed higher tissue NH4+ concentrations, NH3 emission rates, and NH3 compensation points compared with the 47% GS mutant, indicating that NH4+ release was curtailed by some kind of compensatory mechanism in plants with only 47% GS activity.

Influence of nitrogen and sulphur form on manganese acquisition by barley (Hordeum vulgare)

The influence of various nitrogen (N) and sulphur (S) forms on the uptake of manganese (Mn) in young spring barley (Hordeum vulgare L cv Golf) plants was examined in both a hydroponic system and in a soil-based system. The soil was a typical Danish Mn-deficient soil viz. a sandy loam soil developed on old marine sediments. Plants growing in solution culture with NO3− as the only N source had a higher Mn uptake than plants receiving mixtures of NO3− and NH4+. These findings were opposite to the results obtained in the soil-based experiments, where plants fertilized with NO3− as the only N source accumulated much less Mn than plants fertilized with NH4+. Combining the results of these experiments confirmed that NH4+ acted as a powerful antagonist to Mn2+ during uptake but that this antagonistic effect was more than compensated for by the influence of NH4+ in reducing plant-unavailable Mn(IV) to plant-available Mn(II) in the soil. Furthermore the soil experiments showed that fertilizers containing sulphur in the form of reduced S (thiosulphate) had a strong mobilizing effect on Mn, and enabled the plants to accumulate large amounts of Mn in the biomass compared with oxidized S (sulphate). Thus, fertilization with thiosulphate may be very effective in alleviating Mn-deficiency in soils developed on old marine sediments where Mn availability is limiting plant growth.

Ammonia exchange between plants and the atmosphere: Effects of ammonium supply to the roots, dark-induced senescence and reduced GS activity

AbstractAmmonia exchange with the atmosphere was studied in barley (Hordeum vulgare L. cv. Golf) grown in nutrient solution. Ammonia emission from the leaves was evident when NH4+ was taken up by the roots or when the plants had been subjected to darkness for 3 to 7 days. Also NH4+ concentrations in shoot and root tissues increased with these treatments while the activity of the ammonium assimilating enzyme glutamine synthetase (GS) increased in the roots with increasing NH4+ concentrations supplied to the medium and decreased in both shoot and root after 3 days of dark-induced senescence.Barley mutant plants (cv. Maris Mink) with only 66 or 47% of normal GS activity showed higher tissue NH4+ concentrations, higher NH3 emission and a greater sensitivity to increased temperature than wild type barley plants. The 66% GS mutant always showed higher NH3 emission compared to plants with the lowest GS activity (47%). probably due to a mechanism preventing tissue NH4+ concentrations from increasing too much. Apo...

Regulation of Ammonium Distribution in Plants

Ammonium is constantly generated from a variety of processes in plant nitrogen metabolism. Re-fixation of the liberated ammonium is catalysed by the enzyme glutamine synthetase (GS). Cytosolic GS isoforms dominate in the roots, while a chloroplastic GS isoform is the quantitatively most important in green leaves. Despite the central role of NH 4 + as an intermediate in plant nitrogen metabolism very little is known about the processes regulation NH 4 + distribution between cell organelles and plant organs.

Business modelling in farm-based biogas production: towards network-level business models and stakeholder business cases for sustainability

Farm-based biogas production is a promising renewable energy technology with the potential for creating sustainable economic, environmental, and social value. However, Swedish farmers engaged in this activity struggle to turn a profit because of high-investment costs and severe price competition with fossil fuels. One way to address this situation is to re-organize the activity by innovating the business model (BM) towards sustainability. In this study, a team of researchers took an action research approach that proposed solutions for the financial difficulties at a farm cooperative that intended to develop its farm-based biogas production. Two participatory workshops (including researchers, producers, students, and consultants) were conducted using the sustainable business-modelling tool called the Flourishing Business Canvas (FBC). Based on the 215 ideas developed in the workshops, five sustainable BM prototypes were created. These five prototypes form the basis of an approach for initiating the development of a network-level BM for sustainability that highlights its superiority over a single-firm BM. The network-level BM’s main advantage in the farm-based biogas context is its strong focus on stakeholder collaboration that supports the development of a stakeholder business case for sustainability. Overall, this study highlights the usefulness of the network concept in the practice of sustainable BM development. Collaborative business modelling for developing network-level BMs that address environmental and social problems for and with stakeholders can be an effective way to increase long-term financial profit and promote the growth of a firm, a network, or an industry.