ADAPTATIONS OF SUBMERGED LOBELIA-DORTMANNA TO AERIAL LIFE FORM - MORPHOLOGY, CARBON-SOURCES AND OXYGEN DYNAMICS

Lobelia dortmanna 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.