Isoetid ecology
| Adaptations of submerged Lobelia dortmanna L. to aerial life form:
morphology, carbon sources and oxygen dynamics |
| Diel pulses of O2 and CO2 in sandy lake sediments inhabited by
Lobelia dortmanna |
Lobelia dortmanna L. is a vascular macrophyte of the isoetid life form which grows in sandy sediments of nutrient-poor lakes and may become exposed to the air during summer months. Both land and water forms of Lobelia have short stiff leaves which lack stomata and are covered by a thick cuticle providing high resistance to gas exchange and water loss. Incessant large air lacunae run between leaf and root tips. With the leaves exposed to atmospheric air, both life forms absorb virtually all CO2 by the roots from the CO2-rich sediments (ca 30 times air level) and release a substantial proportion of the O2 produced during photosynthesis. The thick cuticle and lack of stomata maintain high CO2 concentrations (ca 23 times air level) for photosynthesis within the leaf lacunae. Release of O2 via the roots and low O2 consumption rates in the sandy sediments result in the presence of dissolved O2 in the pore water and higher daytime concentrations than during night. Root oxygenation may assist in the acquisition of limiting nutrient resources in the infertile habitat. We propose that the special morphology and the predominant root exchange of CO2 and O2 evolved on land before Lobelia dortmanna became submerged.
From: Pedersen, O & Sand-Jensen, K (1992) OIKOS 65:89-96
Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation on sandy sediments in nutrient-poor softwater lakes in North America and Europe. We report here the discovery of pronounced depth profiles and pulses of dissolved O2 and CO2 in the pore-water of vegetated sediments between light and darkness. Pulses of O2 and CO2 were extensive in the upper sediment layers of high root density, because Lobelia roots absorb CO2 and release O2 during photosynthesis, but absorb O2 and release CO2 during respiration, whereas fluxes across leaf surfaces are impeded by a cuticle of low gas permeability. Release of most O2 from the roots during Lobelia photosynthesis leads to O2 concentrations close to saturation in the root zone and O2 penetration to great sediment depth because of low microbial O2 consumption rates. The CO2 concentrations in the root zone of the most nutrient-poor sediments examined here (0.37-0.71 mM) were below the levels required to saturate Lobelia photosynthesis (> 3 mM) and experimental enrichment with CO2 stimulated the development of higher O2 concentrations in the root zone. Cycles of O2, CO2 and nutrients are, therefore, intimately coupled in the Lobelia-sediment system, in contrast to terrestrial and other aquatic plants, where gas and nutrient fluxes are physically separated between leaves and roots and their respective surrounding media. The release of O2 from the roots should ensure efficient aerobic degradation of organic matter, lead to oxidized forms of Fe, Mn, and N and can supply O2 to mycorrhiza fungi and an aerobic fauna associated with the roots.
From: Pedersen, O, Sand-Jensen, K & Revsbech, NP (1995) ECOLOGY 76:1536-1545
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