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

Paper 3Fig. 5. Net photosynthesis at light and CO 2 saturation (y)as a function of leaf chlorophyll content x) of Lobeliadortmanna (upper panel) and Littorella uniflora (lowerpanel). Measurements were made with leaves fromcontrol sediments (open symbols) and sedimentsreceiving increasing amounts of labile organic matter(0.1-1.6% DW; gradually darker symbols).Littorella sediments, whereas Lobelia sedimentswent anoxic. The higher oxygenation ofLittorella sediments can be explained by thehigher biomass, photosynthesis and darktransport of O 2 from the lake water through theplant to the sediment. High O 2 permeability ofLittorella’s leaf surfaces can account for thesubstantial O 2 content (10 kPa) in the leaflacunae in the dark despite anoxic sediments,whereas Lobelia’s leaf surfaces are almostimpermeable and lacunae go anoxic in the dark.The higher resistance to O 2 release from leafthan root surfaces of Lobelia accords with thepredominant (90-100%) release of O 2 from rootsurfaces during photosynthesis, whereasLittorella releases more O 2 from leaves (72%)than roots (28 %; Sand-Jensen et al. 1982, Sand-Jensen & Prahl 1982). Thus, whereas Lobeliaturns anoxic entirely when in anoxic sediments,Littorella’s leaves remain oxic and rootscontinue to receive O 2 from the lake waterthrough leaf surfaces and intra-plant downwardtransport. The longest Littorella roots,nonetheless, face O 2 deficiency on highlyreducing sediments according to observed FeSprecipitates at the root tips and the roots growshorter to ensure sufficient downward O 2transport to the active meristmatic root zone(Colmer 2003, Raun et al. 2010). Lobelia ismuch more susceptible to sediment anoxia andreduces root development earlier and moreprofoundly than Littorella upon organicenrichment of the sediment (Fig. 4, see alsoRaun et al. 2010).The almost gas impermeable leaf surfaceof Lobelia is responsible for extensive anoxia inall plant tissue once the O 2 supply from thesediment vanishes. A few hours of anoxia late atnight is a natural recurring phenomenon even onnutrient-poor, low-organic sandy sedimentsduring summer because the species has almostno O 2 uptake from the water (Møller & Sand-Jensen 2011). Even a modest increase of O 2consumption in the sediments due to supply oflabile organic matter, therefore, increases theduration of night anoxia in Lobelia in contrast toLittorella leaves which remain permanentlyoxic. It is unlikely that the leaf anatomy ofLobelia has been selected to optimize O 2dynamics because it strongly increases the riskof anoxia and several associated stress reactions.Thus, low leaf permeability must havealternative advantages in order to have becomeselected.Firstly, the gas impermeable Lobelialeaves reduces the passive loss of CO 2 from the64