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

Paper 4hours every night although low O 2 tension hasbeen shown to prevent AMF development (LeTacon et al. 1983).Plant growth, stress and nutritionPlants were clearly stressed by high organicaddition and this was always coupled with lowerAMF colonization. In experiments with intactsediment turfs, P levels in leaves tended todecrease at the same organic addition (0.4%)causing AMF colonization to drop (Fig. 2; Table3). In contrast, Littorella was able to grow andmaintain unaltered nutrient and chlorophylllevels and biomasses with very low AMFcolonization at low organic enrichments (0.2%in the colonization experiment) as plants withhigh AMF colonization in control sediments(Table 1). There is, therefore, no clear evidenceof AMF being responsible for the observedstress. It has been shown for the isoetid Isoetesalpinus that root anoxia stops translocation ofphotosynthates from leaves to roots (Sorrell2004) which can lead to root malfunction anddecreased plant nutrition (Møller & Sand-Jensen2011) and this is the most plausible reason forthe observed stress. Furthermore, under thesesediment conditions shading of the isoetids byfaster growing plants or filamentous algae islikely to occur (Sand-Jensen 2000; Arts 2002).It is, therefore, more reasonable to regard AMFas an advantage under very nutrient-poorconditions whereas transport of photosynthatesto AMF under more nutrient-rich sedimentconditions can be spared because plants cansustain sufficient nutrient uptake without thepresence of AMF (Smith & Read 2008).This investigation showed that isoetidswith high AMF colonization have extensivehyphal networks increasing sediment contactand surface area for nutrient uptake in nutrientpoorsediments. AMF colonization in plantsfrom mixed populations in Värsjö was similar tothat in long-term laboratory experiments withmono-specific populations in Littorella andLobelia turfs collected in the field. Littorellahad higher root colonization and a higher AMFcolonization per leaf weight than Lobelia and isknown to have two times higher growth rateunder field conditions (Sand-Jensen &Søndergaard 1978; Bosten & Adams 1989).Thus, Littorella needs a higher nutrient uptakeper biomass than Lobelia. Higher colonizationof Littorella roots being replaced more rapidlythan Lobelia roots requires higher colonizationrates. This higher colonization rate of AMFcould be supported by higher downward O 2supply to Littorella roots and sediments duringthe night whereas Lobelia even at moderatetemperatures (≈16 o C) under natural conditionsexperiences anoxia in the roots (Møller & Sand-Jensen 2011a). Furthermore, higherphotosynthesis of Littorella than Lobelia couldpromote higher organic carbon supplies to thesymbionts.AcknowledgementsWe thank Anubias (France) and Jan OlePedersen for providing non-mycorrhizalLittorella uniflora and Helene Rasmussen forprocessing and counting hyphal lengths insediment samples. We thank The Willum KannFoundation for financial support to this studythrough The Centre of Excellence for Researchon Lake Restoration (CLEAR).References82