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

Thesis summaryFig. 4. Chlorophyll content (a) and maximum netphotosynthesis (b) of Lobelia leaves after increasingaddition of labile organic matter (% of sediment DW).Single measurements were made after 18 d (○) and 59 d(●) and triplicate measurements (± SD) after 194 d (□) ofexperiments in the laboratory.The long anoxic periods in plants duringthe night is the most plausible reason for theobserved plant stress since low yieldinganaerobic respiration can deplete Lobelia ofcarbon resulting in insufficient carbon supply toroots. This will cause root malfunction andosmotic stress constraining transfer of nutrientsto leaves. However, Lobelia can cope withanoxia which is a recurring phenomenon innatural populations during the summer.Paper 3 addresses differences in plantmorphology and response to organic addition ofLobelia and Littorella. Measurements from turfexperiments in the laboratory and in-situexperiments with mixed populations were usedto evaluate effects of enrichment. O 2 loss fromleaf surfaces of both species to hypoxic waterwas used to investigate leaf gas permeability.Addition of organic matter dramaticallydecreased photosynthetic rates of Lobelia whileLittorella was able to cope with additions inlaboratory experiments. This is partly explainedby higher plant density in Littorella turfs, butmarked differences were observed between O 2dynamics in Lobelia and Littorella where O 2concentration in leaves was unaffected inLittorella regardless of O 2 availability insediments while Lobelia faced long anoxicperiods during nighttime. Littorella was able tomaintain nutrient levels and chlorophyll contentin leaves at higher organic additions thanLobelia. The observed difference in O 2dynamics is the result of 13-16 times higher gaspermeability of Littorella leaves compared toLobelia (Table 1).In-situ measurements confirmed thatLobelia was more subjected to stress thanLittorella when mixed populations weresubjected to organic enrichments, hence,experiencing exactly the same conditions sincethe higher oxygenation capacity of Littorellaalso benefits Lobelia in mixed populations. Inthis experiment Littorella was more stressedthan in the laboratory experiments. The higherresponse of both species over the much shorterexperiment (90 days) could be due to highertemperatures (Mean 20.6 o C, maximum 35 o C) inthe in-situ experiment, then the laboratoryTable 1. O 2 flux across leaf surface to hypoxic water withbasal leaf lacunae in air contact and evaporation to drystill air of Lobelia dortmanna and Littorella uniflora.SpeciesTemperature( o C)Flux(n mol O 2 m -2 s -1 )Lobelia 5 30 ± 7 a15 48 ± 3 aLittorella 5 482 ± 199 b15 608 ± 144 bValues are means ± SD of 3 (O 2 flux) or 10 (Evaporation)replicates. Different letters show significant differences(2-way or 1-way ANOVA).21