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

NewPaper 2Phytologist Research 325Table 2 Maximum net photosynthesis (NP), chlorophyll (chl), total phosphorus (TP) and total nitrogen (TN) content of Lobelia leaves inrelation to DW, sediment organic content and pore-water concentrations of Fe 2+ and dissolved inorganic carbon (DIC) at 4 cm depth after80 d of experiments with sandy sediments receiving 0 and 0.8% labile organic matter in Lake Värsjö from 10 July to 1 OctoberTreatment Photosynthesis Leaf contentSedimentcompositionPore waterOrganic matteradded (%)NP (lmolO 2 g )1 DW h )1 )Chl (mgg )1 DW)TP (mgg )1 DW)TN (mgg )1 DW)Organiccontent (%) Fe 2+ (lM) DIC (mM)0.0 282 ± 18 2.47 ± 0.34 1.62 ± 0.26 23.2 ± 4.32 0.66 ± 0.10 16.0 ± 5.61 1.03 ± 0.540.8 64 ± 17 0.99 ± 0.15 0.89 ± 0.02 11.3 ± 2.21 0.81 ± 0.03 1204 ± 381 4.78 ± 0.95Mean ± SD, n =3.(a)(b)(c)they rapidly went anoxic and stayed anoxic for several hoursin the dark. Daytime O 2 in leaf lacunae of Lobelia on sedimentsenriched with 1.6% organic matter never exceeded20 kPa after 40 d into the experiment and O 2 disappearedmore rapidly in the dark than in the 0.4% organic treatment.The duration of leaf anoxia during the 12 h dark periodwas significantly positively correlated to % organic matteradded (Spearman’s r, P < 0.01, joint analysis of all measurements;Fig. 4). As root morphology changed anddegradation of organic matter progressed towards the endof the experiment, the anoxic period in leaf lacunae in thedark declined to 1.8 h on control sediments and to 5.9 and6.6 h on sediments enriched with 0.4 and 1.6% organicmatter, respectively (Fig. 4). The decline of the anoxicperiod with time of the experiment was systematic in alltreatments, although not statistically significant.Field experiments with Lobelia populations in lateAugust under the same temperatures, a 13 : 11 h light :dark cycle and higher daytime irradiance displayed the samediurnal O 2 course in leaf lacunae and sediment pore waterat 1 cm depth as in laboratory experiments (Fig. 5). Thus,O 2 in the sediment reached 20–23 kPa (close to 100%saturation) late in the afternoon and vanished for 1–6 h latein the night. In the leaf lacunae, O 2 peaked at 22–31 kPalate in the afternoon and vanished for 1–3 h late in thenight.Fig. 2 Diurnal changes of O 2 partial pressure (PO 2 ) in leaf lacunaeof Lobelia (solid line), sediment pore water at 10 mm depth (dashedline) and water phase (dotted line) in laboratory experiments in thecontrol (a), 0.4% (b) and 1.6% (c) treatments after 4, 9 and 7 d ofenrichment, respectively. The diurnal trace started with a shift from12 h light to 12 h darkness followed by a shift back to 12 h light.Values are single measurements.Chlorophyll content and photosynthesisChlorophyll content and photosynthesis at saturating lightand CO 2 changed in concert across organic enrichments andover time in laboratory experiments (Fig. 6). Chlorophyllcontent and maximum photosynthesis were highest and atapproximately the same level among organic enrichments18 d into the experiment. A small increase of both chlorophyllcontent and photosynthesis sometimes occurred atlow organic enrichment (0.1 and 0.2%); however, whenomitting the control treatment from the dataset, both variablessubsequently declined significantly (linear regression,Ó 2010 The AuthorsNew Phytologist Ó 2010 New Phytologist Trust46New Phytologist (2011) 190: 320–331www.newphytologist.com