| 1. |
- Husted, Sören, et al.
(författare)
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Photorespiratory NH4+ Production in Leaves of Wild-Type and Glutamine Synthetase 2 Antisense Oilseed Rape
- 2002
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Ingår i: Plant Physiology. - Rockville : American Society of Plant Biologists. - 0032-0889 .- 1532-2548. ; 130:2, s. 989-998
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Tidskriftsartikel (refereegranskat)abstract
- 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 NH3 emission 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.
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| 2. |
- Mattsson, Marie, 1960-, et al.
(författare)
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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
- 2002
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Ingår i: Plant Physiology. - Rockville, Md. : American Society of Plant Physiologists. - 0032-0889 .- 1532-2548. ; 128:2, s. 742-750
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Mattsson, Marie, 1960-, et al.
(författare)
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Senescence-induced changes in apoplastic and bulk tissue ammonia concentrations of ryegrass leaves
- 2003
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Ingår i: New Phytologist. - Oxford : Blackwell publishing. - 0028-646X .- 1469-8137. ; 160:3, s. 489-499
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Tidskriftsartikel (refereegranskat)abstract
- Apoplastic and bulk tissue concentrations of NH4+ and H+ were measured during senescence of intact (attached) and excised ryegrass (Lolium perenne) leaves differing in nitrogen and carbon status. The potential for NH3 emission from the senescing leaves was estimated on the basis of the ratio between [NH4+] and [H+], designated the Γ-value, in apoplastic solution and bulk tissue.Attached leaves with visual symptoms of senescence showed two to three times higher [NH4+] and 0.5–1 unit lower pH in both apoplastic solution and bulk tissue extracts compared with green leaves. The Γ-values were, in all cases, low in attached leaves, ranging from 20 to 300 in the apoplastic solution and 500–900 in the bulk tissue.In excised leaves with high nitrogen status and low C : N ratio (≈ 10), apoplastic [NH4+] increased from around 40 µm to 2 mm after senescence in darkness for 4–9 d. Bulk tissue water [NH4+] increased in the same period to > 30 mm. Apoplastic Γ-values were in all cases < 1000, while bulk tissue Γ-values increased dramatically and reached more than 60 000 in high-nitrogen leaves.Ammonia compensation points predicted on the basis of apoplastic [NH4+] and pH in senescing leaves with high-nitrogen status reached 6–8 nmol mol-1 air. Consequently, senescing leaves may constitute a significant source of atmospheric NH3.
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| 4. |
- Schjoerring, Jan K., et al.
(författare)
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The regulation of ammonium translocation in plants
- 2002
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Ingår i: Journal of Experimental Botany. - : Oxford University Press. - 0022-0957 .- 1460-2431. ; 53:370, s. 883-890
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Tidskriftsartikel (refereegranskat)abstract
- Much controversy exists about whether or not NH+4 is translocated in the xylem from roots to shoots. In this paper it is shown that such translocation can indeed take place, but that interference from other metabolites such as amino acids and amines may give rise to large uncertainties about the magnitude of xylem NH+4 concentrations. Elimination of interference requires sample stabilization by, for instance, formic acid or methanol. Subsequent quantification of NH+4 should be done by the OPA-fluorometric method at neutral pH with 2-mercaptoethanol as the reducing agent since this method is sensitive and reliable. Colorimetric methods based on the Berthelot reaction should never be used, as they are prone to give erroneous results. Significant concentrations of NH+4, exceeding 1 mM, were measured in both xylem sap and leaf apoplastic solution of oilseed rape and tomato plants growing with NO-3 as the sole N source. When NO-3 was replaced by NH+4, xylem sap NH+4 concentrations increased with increasing external concentrations and with time of exposure to NH+4. Up to 11% of the translocated N was constituted by NH+4. Glutamine synthetase (GS) incorporates NH+4 into glutamine, but root GS activity and expression were repressed when high levels of NH+4 were supplied. Ammonium concentrations measured in xylem sap sampled just above the stem base were highly correlated with NH+4 concentrations in apoplastic solution from the leaves. Young leaves tended to have higher apoplastic NH+4 concentrations than older non-senescing leaves. The flux of NH+4 (concentration multiplied by transpirational water flow) increased with temperature despite a decline in xylem NH+4 concentration. Retrieval of leaf apoplastic NH+4 involves both high and low affinity transporters in the plasma membrane of mesophyll cells. Current knowledge about these transporters and their regulation is discussed.
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