Oxygen dynamics and plant-sediment interactions in isoetid ...

NewPaper 2Phytologist Research 321when sediments become reduced (Colmer, 2003). Lobeliadoes not form root tissue barriers because it requires CO 2for photosynthesis from the sediment (Møller & Sand-Jensen, 2008). As a result, roots of Lobelia and other isoetidspecies become shorter when grown in sediments with highoxygen demand (Sand-Jensen et al., 2005a; Raun et al.,2010), but shorter roots also have smaller root surfacesfor absorption of mineral nutrients. Solute transport andmineral nutrition can be further compromised by O 2deprivation of roots and symbiotic root fungi because ofcessation of ATP production by oxidative phosphorylationand thus a severe energy deficit (Colmer & Flowers, 2008).Anaerobic carbohydrate catabolism provides some ATPduring anoxia, albeit only 3–35% of the rate of energyproduction in aerobic cells (Gibbs & Greenway, 2003;Greenway & Gibbs, 2003), but takes a higher toll on carbohydrates.Lobelia could be particularly sensitive to anoxiabecause of the observed low photosynthesis and carbohydrateproduction, high proportions by weight of nonphotosyntheticstem and root tissue and low capability of root tissueto speed up anaerobic fermentation and ATP production(Sand-Jensen & Søndergaard, 1979; Nielsen & Sand-Jensen,1989; Smits et al.,1990).O 2 availability is the main determinant of sediment andplant processes. We therefore made a special effort to measureO 2 continuously during light and dark periods in lakewater, sediments and leaf lacunae to establish if, where andfor how long hypoxia or anoxia occurred in laboratory andfield populations subjected to gradually increasing organicenrichment and O 2 consumption rates. Because Lobeliagrows slowly and sediment processes respond slowly, wefollowed the responses for up to 194 d after organic enrichmentto make sure that plant stress was fully expressed andrecovery was possible.Materials and MethodsLaboratory experiments with organic enrichmentSix intact sediment turfs were collected in mid-Octoberfrom shallow water in a homogeneous Lobelia dortmannapopulation in oligotrophic Lake Värsjö, south-west Sweden.The low-organic sandy turfs (0.60 ± 0.05% (mean ± SD)organic matter of sediment DW) were 17 cm long and15 cm wide and had a sediment depth of 12–14 cm thatensured intact root systems of c. 20 plants. Turfs werebrought fully submerged to the laboratory for experiments.One turf was left as a control, while gradually longer rodshapeddry pellets (5 mm in diameter, 1–15 mm long) ofcommercially available pasture grass (Dlg, Copenhagen,Denmark) were added to the other five turfs forming a gradientof added labile organic matter equivalent to 0 (control),0.1, 0.2, 0.4, 0.8 and 1.6% organic matter of DW. Organicpellets contained (as a percentage of DW) 91 ± 1% (n =4)organic matter, 46.9 ± 0.4% organic carbon (C), 2.3 ±0.15% total nitrogen (TN) and 0.27 ± 0.01% total phosphorus(TP). This composition is equivalent to weightproportions of 156 C, 8.5 N and 1.0 P, which is richer innutrients relative to C and richer in P relative to N thanLobelia tissue (Sand-Jensen et al., 2005a). Pellets wereinserted with pincers at 4 cm sediment depth with a fixedhorizontal distance of 2 cm equivalent to 71 pellets per turf.Pincers were stuck into control sediments to keep physicaldisturbance constant. Previous experiments have documentedthat aerobic O 2 consumption rates of surfacesediments increase linearly with addition of this type oforganic matter, generating an increasing O 2 stress on plantroots (Raun et al., 2010). The turf specimens were incubatedat 14–16°C in 80 l aquaria with a 12 : 12 h light : darkcycle at an irradiance of 110 lmol photons m )2 s )1 (photosyntheticallyactive radiation, PAR) and exposed to the samewater to eliminate possible effects of elevated carbon andnutrient availability in the water resulting from release fromenriched sediments. The water was renewed every secondweek to keep nutrient concentrations low and avoid algalgrowth and it atmospheric air was bubbled slowly through itto ensure mixing and air saturation. Because large watervolumes were needed we used filtered water from nearbyLake Esrum diluted 20 times with demineralized water toobtain a chemical composition closely resembling that ofLake Värsjö. After dilution mean values were: 0.13 mMinorganic carbon (DIC), 0.3 lM plant available P (ortho-P), 0.4 lMNO ) 3 and 0.3 lMNH + 4 .Pore-water chemistry and sediment characteristicsRepeated pore-water samples from 1, 2, 4, 6 and 8 cm depthwere extracted from three representative sites distributedevenly across the turf area at the end of the 194-d-long experimentand analysed for dissolved Fe 2+ , DIC, pH, NH + 4 andortho-P. Pore-water was sampled by inserting thin capillaryglass tubes (1 mm) in the sediment, keeping the upper endabove the water. Capillary forces and pressure differenceslowly and steadily filled the capillary tube with water fromthe desired sediment depth within a few minutes. Quantitiesof 50–100 ll of pore-water were withdrawn with a glasssyringe from each of five capillary tubes at a certain sedimentsite to yield an integrated sample for 0–8 cm depth withminimum air contact and these were analysed immediately.Reduced Fe 2+ was measured spectrophotometrically accordingto the phenanthrolin method (Eaton et al., 1995). DICwas determined by injecting a minute pore-water volumeinto 3% HNO 3 in a bubble chamber purged with N 2 gascarrying evolved CO 2 into an Infrared gas analyzer (IRGA,ADC-225-MK3, Hoddesdon, UK) as previously described(Vermaat & Sand-Jensen, 1987). pH was measured by aflat-membrane pH electrode (LoT403-M8-S7 ⁄ 120, MettlerToledo, Greifensee, Switzerland) positioned close to a glassÓ 2010 The AuthorsNew Phytologist Ó 2010 New Phytologist Trust42New Phytologist (2011) 190: 320–331www.newphytologist.com