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

Paper 3high internal concentrations in the leaf lacunaeto the low air-equilibrium CO 2 concentrations inthe lake water. Because CO 2 formation isextremely low in pristine, oligotrophic Lobelialakesit is essential to maintain intimateexchange between the plant and the sedimentboth during photosynthesis and respiration andminimize CO 2 losses to the lake water becauseCO 2 concentrations needed to saturatephotosynthesis (~ 3000 µM) or just tocompensate respiration (~ 60 µM) are muchhigher than air-saturated levels (~ 15 µM; Sand-Jensen 1987, Pedersen et al. 1995, Winkel &Borum 2009). High permeability of Littorellaleaves could result in much greater CO 2 loss tothe lake water in the dark, but Littorellapossesses a special physiological feature whichallows temporary incorporation of leaf CO 2 byPEP-carboxylase into malate in the dark, anddecarboxylation and CO 2 incorporation vianormal C-3 photosynthesis in the following lightperiod, thus lowering plant-mediated CO 2release from the sediment via the aerenchyma(Madsen 1985). These CO 2 fluxes through theplants have not been quantified as yet.Secondly, the diffusion barrier onLobelia leaves effectively reduces evaporationand ensures survival of leaves when theyregularly become exposed to the air followingdrawdown of the water table during summer inthe seepage lakes (Pedersen & Sand-Jensen1992). Most submerged aquatic plants,including Littorella with 12-fold fasterevaporation rates than Lobelia, dry out upon airexposure and these species either die or survive,as in the case of Littorella, by producing newaerial, less permeable leaves with cuticle andfunctional stomata (Nielsen et al. 1991). Lobeliadoes not need to invest in a new set of leavesfollowing alternating submergence and airexposure which is costly and perhaps impossibleconsidering its low intrinsic growth rate andvery nutrient-poor habitat. In contrast, Littorellafaces this extra cost, but also has a fasterintrinsic growth rate and prefers to inhabitnutrient-richer sediments (Sand-Jensen &Søndergaard 1978, 1979). Thus, particularstructural and physiological adaptations fulfillseveral purposes and need to be viewed inregard to the entire plant life and the complexityof environmental conditions.Organic additions had more profoundand lasting effects on sediment chemistry inlaboratory than in field experiments. The muchfaster loss of organic matter and produced DIC,NH 4 + and Fe +2 in the field can be explained byhigher field temperatures promoting degradationrates, longer days with higher irradiancesstimulating oxygenation and, thereby, organicdegradation and, very likely, better physicalexchange by pressure waves or groundwaterflow between pore-water and lake water.Anoxic stress, nutrient supply and plantperformanceLobelia experienced extensive anoxia in bothleaves and roots by organic enrichment, whereasLittorella leaves always remained oxic andcould supply O 2 to the roots. More widespreadanoxia in Lobelia will impede effective ATPformation by oxidative phosphorylation, exhaustenergy resources (Greenway & Gibbs 2003a,b)and, in turn, restrict ion uptake and transportfrom roots to shoots (Colmer & Flowers 2008;Møller & Sand-Jensen 2011). Catabolicprocesses to sustain the photosyntheticapparatus in the leaves can also be impededbecause less metabolic energy and fewernutrients are available. This may account for thedecline of nutrients, chlorophyll and65