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Outstanding Lobelia dortmanna in iron armortissue concentrations of N and P are stimulated by a small organicenrichment (e.g., from 0.3 to 0.6% of dry weight) due to highermineralization, a stronger organic enrichment (2.4%) stresses theplants and leads to loss of chlorophyll, poor growth and die off 4 .Organic enrichment is accompanied by deprivation of oxygen poolsand lower oxygen penetration depths (e.g., from 35 to 5 mm) in thesediments signaling that insufficient oxygen supply to the root tipsis a main reason for the reduction of root length and plant performance.4 Quantity and degradability of the organic matter input willobviously determine the severity of plant stress. 6Other rosette species such as Littorella uniflora and Isoetes lacustrisrespond similarly but are less sensitive to sediment anoxia presumablydue to lower radial oxygen loss from the roots. Littorella produces amixture of thin, short roots and thick, longer and probably lesssusceptible roots. Both species tolerate organic sediments and in deepcalm water you find Isoetes lacustris anchored with just the stem invery soft organic sediments, but with no roots and an obvious riskof vegetation loss during storms or release of methane bubbles fromthe sediments.Iron Plaques as a Protection?Wetland plants and probably most aquatic plants have continuouslayers of suberin or lignin just below the root surface as barriers toradial oxygen loss to the sediment. 6,11 In Phragmites australis thebarrier becomes stronger when the roots are exposed to anoxia andsmall fatty acids from anaerobic fermentation. 5 There is no sign thatthis diffusion barrier is present or inducible in Lobelia roots, whilethe response of other rosette species is unknown. 9 If the diffusionbarrier turns out to be inducible, it will impede root uptake of CO 2,but because sediment CO 2concentrations are elevated by organicenrichment 4 it could match a certain reduction of root permeabilityand still maintain the CO 2influx for photosynthesis.Figure 1. Radial oxygen loss measured with platinum sleeve electrodes alongthe length of roots in an anoxic medium. The basal part of the roots was incontact with atmospheric air. Lobelia dortmanna had either none or thickiron coatings of the root surface (0.09 ± 0.05 and 30 ± 3 mmol Fe m -2 rootsurface respectively), 9 while Phragmites australis 5 and the seagrass, Zosteramarina (Unpublished) were without iron coatings.For Lobelia, alteration of bio-geochemical processes accompanyingorganic enrichment of the sediment can offer additional protectionto radial oxygen loss from the roots and this mechanism has hithertobeen overlooked. As Lobelia’s roots become shorter by oxygen depletionof sediments, 1–2 mm thick coatings of oxidized iron precipitateon the root surfaces 4,10 (Fig. 2). Mobile dissolved Fe 2+ is producedfrom insoluble Fe 3+ by degradation of organic matter in the anoxicsediment and Fe 2+ is re-precipitated as Fe 3+ -oxyhydroxides at theroot surface when it meets the outward oxygen flux. Iron coatings onaquatic plant roots in organically rich sediments are common and theconsequences should be general.Figure 2. Roots become shorter and iron coatings thicker on Lobelia roots exposed from left to right to increasing levels of organic matter (from 0.2 to 3.4% of dry weight) and potential oxygen consumption rates (from 1.1 to 24.0 μg O 2 ml -1 sediment h -1 ) for 16 weeks. 10www.landesbioscience.com Plant Signaling & Behavior 883
Outstanding Lobelia dortmanna in iron armorPaper 5Iron coatings on Lobelia roots (30 mmol m -2 ) reduce 10-foldoxygen permeability of the root wall and two-fold radial oxygen loss(Fig. 1). Therefore, oxygen concentrations rise within the roots andcan better sustain root respiration including the distal meristem. 9While iron coatings only lead to c. 15% higher internal oxygenconcentrations in the root tips at low respiration rates at low temperatures,9 the influence will be stronger at higher respiration rates andat reduced oxygen supply from the leaves in the dark.Therefore, we anticipate that iron coatings should offer the strongestprotection to root anoxia and other toxic effects during warmsummer nights when oxygen consumption rates in the plants and thesediments peak, hence, no oxygen is produced by photosynthesis andoxygen supply from the bottom waters is reduced by falling oxygenconcentrations.Oxidized iron may offer additional protection by increasingthe diffusive resistance to toxic solutes across the root surfaces andproviding an oxidation capacity slowing the ingress of sulfide andreduced metals from the sediment. 5,9 Iron coatings formed byoxygen release from the roots during the day can, thereby, offerprotection during the night when oxygen release diminishes or isreversed. 1 Likewise, iron coatings formed during sunny days canoffer protection during dark days.Studies of marine sediments have clearly demonstrated howsurface pools of oxidized iron formed in winter and spring whenbottom waters are rich in oxygen can protect the anoxic bottomlayer during summer against the release of toxic sulfides from deepersediments. 12To test the protective effect of oxidized iron on or close to rootsurfaces independent of the influence of sediment anoxia and oxygensupply from the water, we shall expose Lobelia and other freshwaterspecies to controlled crossed gradients of degradable organic matterand iron in the sediments and set levels of dissolved oxygen in thewater and evaluate the resulting morphological and functional plantresponses.Iron and other important redox constituents vary extensively inamount and input by groundwater seepage through sediments fromdifferent lakes and wetlands as do the presence and well-being of thevegetation. Thus, if oxidized iron is generally ecological significantfor plant survival it will also be important for plant distribution andhistorical changes of the vegetation. Work is under way to evaluatethe importance of light availability and sediment contents of organicmatter and iron for the survival of the Red-listed rosette vegetation inthe diminishing numbers of oligotrophic lakes worldwide. 13References1. Pedersen O, Sand-Jensen K, Revsbech NP. Diel pulses of O 2 and CO 2 in sandy lake sedimentsinhabited by Lobelia dortmanna. Ecology 1995; 76:1536-45.2. Sand-Jensen K, Pedersen O, Binzer T, Borum J. Contrasting oxygen dynamics in the freshwaterisoetid Lobelia dortmanna and the marine seagrass Zostera marina. Ann Bot 2005;96:613-23.3. Justin SHFW, Armstrong W. The anatomical characteristics of roots and plant-response tosoil flooding. New Phytol 1987; 106:465-95.4. Sand-Jensen K, Borum J, Binzer T. Oxygen stress and reduced growth of Lobelia dortmannain sandy lake sediments subject to organic enrichment. Freshwater Biol 2005; 50:1034-48.5. Armstrong J, Armstrong W. Rice and Phragmites: effects of organic acids on growth, rootpermeability and radial oxygen loss to the rhizosphere. Am J Bot 2001; 88:1359-70.6. Colmer TD. Long-distance transport of gases in plants: a perspective on internal aerationand radial oxygen loss from roots. Plant Cell Environm 2003; 26:17-36.7. Sand-Jensen K, Prahl C. Oxygen exchange with the lacunae and across leaves and roots ofthe submerged vascular macrophyte, Lobelia dortmanna L. New Phytol 1982; 91:103-20.8. Sand-Jensen K, Prahl C, Stokholm H. Oxygen release from roots of submerged aquaticmacrophytes. Oikos 1982; 50:1034-48.9. Møller CL, Sand-Jensen K. Iron plaques improve the oxygen supply to root meristems ofthe freshwater plant, Lobelia dortmanna. New Phytol 2008; 179:848—56.10. Raun AL. Sediment organic matter influences growth and survival of submerged plants. MS.Thesis, Freshwater Biological Laboratory 2008; University of Copenhagen.11. Soukup A, Armstrong W, Schreiber L, Franke R, Votrubova O. Apoplastic barriers to radialoxygen loss and solute penetration: a chemical and functional comparison of the exodermisof two wetland species, Phragmites australis and Glyceria maxima. New Phytol 2007;173:264-78.12. Azzoni R, Giordani G, Viaroli P. Iron-sulphur-phosphorus interactions: implications forsediment buffering capacity in a Mediterranean eutrophic lagoon (Sacca di Goro, Italy).Hydrobiologia 2005; 550:131-48.13. Sand-Jensen K, Riis T, Vestergaard O, Larsen S. Macrophyte decline in Danish lakes andstreams over the past 100 years. J Ecol 2000; 88:1030-40.884 89Plant Signaling & Behavior 2008; Vol. 3 Issue 10
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Oxygen dynamics and plant-sedimenti
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PrefaceThe content of this thesis i
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AbstractAbstract● Isoetids occupy
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IntroductionIntroductionOligotrophi
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Introductionwater (Wigand et al. 19
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IntroductionSand-Jensen K, Prahl C,
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Thesis summaryThesis summaryAdditio
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Thesis summaryFig. 4. Chlorophyll c
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Thesis summaryascribed to lack of A
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Paper 1Oxygen, carbon and iron dyna
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Paper 1extensive sediment volume by
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Paper 1method of Andersen (1976) an
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Paper 1Table 3. Retention of added
Page 37 and 38: Paper 1recalcitrant organic pools a
Page 39: Paper 2High sensitivity of Lobelia
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Page 48 and 49: NewPaper 2Phytologist Research 327(
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Page 52 and 53: NewPaper 2Phytologist Research 331c
Page 55 and 56: Paper 3Higher gas permeability of l
Page 57 and 58: Paper 3Here, we perform a parallel
Page 59 and 60: Paper 3Fig. 1. O 2 penetration dept
Page 61 and 62: Paper 3Table 2. O 2 flux across lea
Page 63 and 64: Paper 3Fig. 4. Leaf chlorophyll of
Page 65 and 66: Paper 3high internal concentrations
Page 67 and 68: Paper 3Colmer TD 2003. Long-distanc
Page 69: Paper 4Organic enrichment of sedime
Page 72 and 73: Paper 4Surveys on aquatic plants re
Page 74 and 75: Paper 4ensure mixing and air satura
Page 76 and 77: Paper 4ResultsFig 1. O 2 -penetrati
Page 78 and 79: Paper 4Table 2. Individual leaf bio
Page 80 and 81: Paper 4Fig. 4. Root density (○) a
Page 82 and 83: Paper 4hours every night although l
Page 84 and 85: Paper 4Møller CL, Sand-Jensen K. 2
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Page 93 and 94: Planterødders overlevelse ivanddæ
Page 95 and 96: Paper 6Figur 3. Aflejringer af lign
Page 97: Paper 6Vi har sat planterødders ve
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