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

Thesis summaryexperiment (15 o C), speeding up decompositionprocesses and respiration in both plants andsediment, hence, increased temperature isprobably an extra stress factor. The ability tomaintain oxic conditions in plant tissue canexplain the broader occurrence of Littorella inlakes undergoing eutrophication. Hence,Littorella only needs sufficient light to sustaincarbon production to cover aerobic respirationof the plant tissue. In eutrophic lakes Lobeliawill suffer from both decreased production andlow energy output under anaerobic respirationwhich could be the main reason for loss ofLobelia from many former growth sites.Although widespread anoxia occurred inLobelia at summer temperatures, O 2 diffusionacross leaves may be sufficient to satisfy therespiratory demand during winter when snowcovered ice prevents photosynthesis and lowtemperature slow down respiration.Paper 4 focus on occurrence of arbuscularmycorrhizal fungi associations in isoetids andsediments and the responses to organicenrichment. Mixed field populations were usedto compare AMF colonization of both specieswhile experiments with intact sediment turfs ofLobelia or Littorella with indigenous AMFassociations were used to investigate responseof AMF colonization and hyphal density insediment to organic enrichment. Furthermore,experiments with non-colonized Littorellaintroduced to sediments with AMF and differentorganic additions were performed to investigatecolonization under more eutrophic conditions.In nutrient poor sediments Littorellahave higher AMF colonization than Lobeliawhen growing in mixed populations undernatural conditions. This result is consistent withthe two times higher growth rate of LittorellaFig 5. Root length colonized by AM fungi (%) in rootsof Lobelia dortmanna (upper panel) and Littorellauniflora (lower panel) after experimentation withaddition of labile organic matter to sediments.Colonization frequencies of roots produced during theexperiment (○) and roots most likely established priorexperimentation (●) are presented. Values are means ±SD.than Lobelia. i.e., more nutrients are required tosustain higher growth. The investigation alsoshowed that hyphal density in the aquaticsediments were high and comparable withterrestrial systems, with hyphal surfacesexceeding the area of roots several times andscavenging a much larger sediment volume.Hyphal density was well correlated to colonizedroot length in the sediments.Addition of organic matter to sedimentsdecreased colonization of roots of plants alreadycolonized by AMF and was mainly caused bydecreased colonization of roots produced whilegrowing in more reduced sediments (Fig 5).Almost no colonization occurred in initiallynon-colonized Littorella at low additions whileAMF colonized roots in low-organic controlsediments. The stress of Lobelia and Littorellaobserved at high organic additions could not be22