Physiological Adaptation and Plant Distribution along a Steep Hydrological Gradient.

Plant species often separate strongly along steep environmental gradients. Our objective was to study how coupling between plant physiology and environmental conditions shapes vegetation characteristics along a distinct hydrological gradient. We therefore investigated species photosynthesis in air and under water within a limited area from dry-as-dust to complete submergence in a nutrient-poor limestone habitat on Öland's Alvar, Sweden. We found structural and physiological adaptations of species to endure water limitation at the dry end (e.g., moss cushions and CAM-metabolism) and diffusive carbon limitation (e.g., bicarbonate use) at the submerged end of the gradient. As anticipated, mean photosynthesis in air increased 18-fold from the species-poor assembly of cushion-mosses and Sedum CAM-species on mm-thin limestone pavements to the species-rich assembly of C-3 terrestrial plants in deeper and wetter soils. A GLM-model indicated that 90% of the variation in species richness could be explained by a positive effect of soil depth, a negative effect of the duration of water cover and their interaction. In water, mean photosynthesis was highest among aquatic species, low among Sedum species and cushion mosses, and negligible among C-3 terrestrial plants. While aquatic species dried out in air, drought-resistant small species were probably competitively excluded from the more suitable terrestrial habitats on deeper soils with moderate flooding by taller species of high photosynthetic capability. In conclusion, the clear distribution of species along the steep hydrological gradient reflects distinct structural and physiological adaptations, environmental filtering and interspecific competition.

High sensitivity of Lobelia dortmanna to sediment oxygen depletion following organic enrichment.

• Lobelia dortmanna thrives in oligotrophic, softwater lakes thanks to O(2) and CO(2) exchange across roots and uptake of sediment nutrients. We hypothesize that low gas permeability of leaves constrains Lobelia to pristine habitats because plants go anoxic in the dark if O(2) vanishes from sediments. • We added organic matter to sediments and followed O(2) dynamics in plants and sediments using microelectrodes. To investigate plant stress, nutrient content and photosynthetic capacity of leaves were measured. • Small additions of organic matter triggered O(2) depletion and accumulation of NH(4)(+), Fe(2+) and CO(2) in sediments. O(2) in leaf lacunae fluctuated from above air saturation in the light to anoxia late in the dark in natural sediments, but organic enrichment prolonged anoxia because of higher O(2) consumption and restricted uptake from the water. Leaf N and P dropped below minimum thresholds for cell function in enriched sediments and was accompanied by critically low chlorophyll and photosynthesis. • We propose that anoxic stress restricts ATP formation and constrains transfer of nutrients to leaves. Brief anoxia in sediments and leaf lacunae late at night is a recurring summer phenomenon in Lobelia populations, but increased input of organic matter prolongs anoxia and reduces survival.

Iron plaques improve the oxygen supply to root meristems of the freshwater plant, Lobelia dortmanna.

* High radial oxygen loss (ROL) from roots of aquatic plants to reduced sediments is thought to deplete the roots of oxygen and restrict the distribution of those species unable to form a barrier to oxygen loss. Metal precipitates with high iron content (Fe-plaques) frequently form on roots of aquatic plants and could create such a diffusion barrier, thereby diverting a larger proportion of downward oxygen transport to the root meristems. * To investigate whether Fe-plaques form a barrier to oxygen loss, ROL and internal oxygen concentrations were measured along the length of roots of the freshwater plant Lobelia dortmanna using platinum sleeve electrodes and Clark-type microelectrodes. * Measurements showed that ROL was indeed lower from roots with Fe-plaques than roots without plaques and that ROL declined gradually with thicker iron coating on roots. The low ROL was caused by low diffusion coefficients through root walls with Fe-plaques resulting in higher internal oxygen concentrations in the root lacunae. * By diverting a larger proportion of downward oxygen transport to root meristems in L. dortmanna, the presence of Fe-plaques should diminish root anoxia and improve survival in reduced sediments.

Outstanding Lobelia dortmanna in iron armor.

Lobelia dortmanna leads a group of small, highly-valued rosette species that grow on coarse, nutrient-poor soils in temperate soft-water lakes. They acquire most CO(2) for photosynthesis by root uptake and efficient gas transport in large air channels to the leaves. Lobelia is the only species that releases virtually all photosynthetic oxygen from the roots and generates profound day-night changes in oxygen and CO(2) in the sediment pore-water. While oxygen release from roots stimulates decomposition and supports VA-mycorrhiza fungi, the ready gas exchange presents a risk of insufficient oxygen supply to the distal root meristems as sediments accumulate organic matter from lake pollution. So the plant with the greatest oxygen release from roots is also the most sensitive to oxygen depletion in sediments and it dies or losses anchorage by shortening the roots from 10 to 2 cm at even modest contents (2.4%) of degradable organic matter. Coatings of oxidized iron on roots in organically enriched sediments reduce radial oxygen loss and, thereby, increase internal concentrations and supply of oxygen to root tips. Oxidized iron is also a redox buffer which may prevent the ingress of sulfides and other reduced toxic solutes during nights. Controlled experiments are under way to test if iron enrichment can help survival of rosette species threatened by lake pollution or whether removal of organic surface sediments is required.