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

NewPaper 2Phytologist Research 327(a)(a)(b)(b)(c)Fig. 6 Chlorophyll content (a) and maximum net photosynthesis (b)of Lobelia leaves after increasing addition of labile organic matter(% of sediment DW). Single measurements were made after 18 d(open circles) and 59 d (closed circles) and triplicate measurements(± SD) after 194 d (squares) of experiments in the laboratory.DiscussionFig. 5 Diurnal changes of O 2 partial pressure measured at 10 mmdepth in sandy sediments next to two different plants (a, solidline), in leaf lacunae of two plants (b, solid line) and in the waterphase (a and b, dotted lines) in Lobelia populations in Lake Värsjöin late August. Irradiance (photosynthetically active radiation,400–700 nm; c, solid line) and water temperature (c, dashed line)are also shown.Tissue TP remained close to the common critical thresholdfor maximum biomass yield of submerged macrophytes incontrol sediments and in sediments enriched with only 0.1and 0.2% organic matter (Fig. 7, Table 2). However,depletion of TP was more extreme and concentrationsdropped below common critical threshold for maximumgrowth rate, maximum biomass yield and even minimumthreshold to sustain growth of submerged macrophytes at0.4–1.6% organic enrichment (Fig. 7, Table 2). Depletionof TN in leaf tissue occurred later during the experimentand at higher additions of organic matter, and TN concentrationsonly dropped below the critical threshold formaximum biomass yield at the highest organic treatmenttowards the end of experiments.O 2 dynamics in sediments and plantsLaboratory experiments reflected the natural processes inthe field. We observed the same profound O 2 fluctuationsfrom above air saturation in the light to anoxia in the darkin the sandy sediments and also the same plant responses inthe two instances. It was surprising, however, that leaf lacunaetracked O 2 conditions in the sediment pore water soclosely and that they went anoxic in the dark shortly afterO 2 had disappeared from the sediment. In a Novemberexperiment at 4°C in the same Lobelia population, O 2remained in leaf lacunae (9 kPa), root lacunae (6 kPa) andin the sediment (3 kPa) in the dark because plant and sedimentrespiration is reduced at low winter temperature(Sand-Jensen et al., 2005b). Leaf and root anoxia is, however,a recurring phenomenon during summer nights infield populations of Lobelia despite 100% air saturation inlake water and large retention capacity for O 2 in the extensiveair lacunae (Sand-Jensen & Prahl, 1982). The impededgas exchange across leaf surfaces as a result of very lowpermeability (Sand-Jensen & Prahl, 1982; Pedersen &Sand-Jensen, 1992) implies that air lacunae and plant tissuego anoxic only 1–2 h after O 2 has disappeared from theÓ 2010 The AuthorsNew Phytologist Ó 2010 New Phytologist Trust48New Phytologist (2011) 190: 320–331www.newphytologist.com