Diurnal courses of net photosynthesis and photosystem II quantum efficiency of submerged Lagarosiphon major under natural light conditions

An optode device for net-photosynthesis measurements, based on oxygen-depending quenching of fluorescence from O₂-specific sensors, and PAM fluorometry have been used to study diurnal courses of net-photosynthesis and the F_v/F_m ratio of the submerged plant Lagarosiphon major. Plants were pre-cultivated and studied in large mesocosm flow-through outdoor tanks under 50% and 80% shade cloth, respectively. Growth under the different shade cloths resulted in similar light compensation points (∼20 μmol photons m⁻² s⁻¹), but strongly different light saturation levels, with about 150 μmol m⁻² s⁻¹ for plants grown under 80% shade cloth and about 350 μmol m⁻² s⁻¹ for plants grown under 50% shade cloth. Plants under both growth conditions showed a transient reduction of the maximum F_v/F_m value in the afternoon (down to 70% of the morning control values under 80% shade cloth and down to 85% under 50% shade cloth), which was not accompanied by a reduction of the net photosynthetic rate. This indicated that the fluorescence parameter F_v/F_m must not be a reliable indicator of the rate of photosynthesis under all conditions. The new photo-optical device became evidenced as a valuable tool not only for laboratory experiments, but also for field studies of gas exchange of submerged plants.     

The influence of water level on the growth and photosynthesis of Hydrocotyle ranunculoides L.fil.

Floating Pennywort (Hydrocotyle ranunculoides L.fil.), a native to North America and naturalized in Central and South America, is an invasive aquatic weed in western Europe and several other regions worldwide. H. ranunculoides settles primarily in stagnant to slow-flowing waters (e.g. ditches, canals, rivers, lakes and ponds). The species prefers sunny and nutrient-rich sites and forms dense interwoven mats, which can quickly cover the surface of infested waters. In this study, the effect of three different water levels on growth of Floating Pennywort was investigated. Plants were cultivated on high-nutrient soils under waterlogged, semi-drained and drained conditions. Highest relative growth rates (RGR) of 0.097±0.004 g g⁻¹ dw d⁻¹ were reached under waterlogged conditions. This was significantly higher than RGR of plants cultivated semi-drained and drained. Floating Pennywort showed some phenological adaptations to drained soil conditions, including significant differences in the relative amounts of leaf, petiole and shoot biomass, whilst the relative amount of root biomass was not significantly influenced by the water level. Furthermore, Floating Pennywort reached under drained conditions lower relative water contents (RWC) of leaves, petioles and shoots, a significant shorter length of internodes, a significant lower extent of shoot porosity (POR), a lower chlorophyll content and an increased Chl_a:Chl_b ratio. In addition, maximum gas exchange of drained cultivated plants is significantly lower, due to strongly decreased leaf conductance under reduced water availability. Overall, H. ranunculoides showed ability to grow under different water levels, but performed best under waterlogged conditions.     

Growth and photosynthesis of Hydrocotyle ranunculoides L. fil. in Central Europe

Floating Pennywort (Hydrocotyle ranunculoides L. fil.) is a worldwide distributed aquatic plant. The species is native to North America and quite common also in Central and South America. In Europe, Japan and Australia it is known as an alien plant, sometimes causing serious problems for affected ecosystems and human use of water bodies. Starting from Western Europe with an eastwards directed spread, Floating Pennywort was recorded in Germany in 2004 for the first time. Since then, the species spread out and got established in western parts of Central Europe. For a definite prediction of the potential of a further spread, data about biology, in particular growth and photosynthesis are needed. Here, regeneration capacity, growth at different nutrient availabilities and photosynthesis of H. ranunculoides were investigated. In addition biomass samples were taken in the field. Results show an enormous regeneration capacity (e.g., by forming new shoots from small shoot fragments), increasing growth rates under increasing nutrient availability and a maximum increase of biomass reaching 0.132±0.008 g g⁻¹ dw d⁻¹. Dense populations of H. ranunculoides growing in ponds and oxbows were found at high nutrient content of the substrate, the biomass reaching there up to 532.4±14.2 g dw m⁻². Gas exchange analysis showed a physiological optimum of H. ranunculoides CO₂ uptake at temperatures between 25 and 35 °C and high photon flux densities (PPFD) above 800 μmol photons m⁻² s⁻¹. In comparison, native Hydrocotyle vulgaris showed an optimum of net photosynthesis at 20–30 °C and a light saturation of CO₂ gas exchange at 350 μmol photons m⁻² s⁻¹.     

Internal transport of alien and native plants by geese and ducks: an experimental study

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Chlorophyll fluorometry sheds light on the role of desiccation resistance for vegetative overland dispersal of aquatic plants

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Acclimation of photosynthesis to supersaturated CO2 in aquatic plant bicarbonate users

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Low light acclimated submerged freshwater plants show a pronounced sensitivity to increasing irradiances

The high light sensitivity of three submerged aquatic freshwater plant species, Egeria densa, Elodea nuttallii and Myriophyllum heterophyllum, which have been cultivated at a photosynthetically active radiation (PAR, 400-700 nm) of 70 μmol photons m⁻² s⁻¹, was studied by means of chlorophyll fluorescence and pigment analyses. Exposure of plants to 100, 300, 600 and 1000 μmol photons m⁻² s⁻¹ PAR for up to 360 min induced a strong reduction of the F_v/F_m ratio, indicating a pronounced inactivation of PSII even at the lowest PAR applied. These changes were accompanied by a reduction of the chlorophyll content to about 60-70% of control values at the highest PAR. Rapidly inducible photoprotective mechanisms were not affected, as derived from the rapid generation of pH-dependent energy dissipation under these conditions. At PAR higher than 100 μmol photons m⁻² s⁻¹, however, the primary quinone acceptor of photosystem II, Q_A, was reduced to about 80% and the effective quantum yield of photosystem II, Φ_PSII, dropped to values of about 10%, indicating a high reduction state of the photosynthetic electron transport chain. These data support the notion that the three aquatic macrophytes have a very low capacity for the acclimation to higher light intensities.     

From first reports to successful control: a plea for improved management of alien aquatic plant species in Germany

Alien aquatic plant species can strongly affect all types of freshwater ecosystems. Their number has more than doubled between 1980 and 2009 in Germany, and currently 27 are known and their number is still increasing. Eleven have been classified as invasive, but only four are managed yet, mainly by weed cutting. Most of the alien aquatic plant species were probably introduced as aquarium and pond waste. Despite this fact, 18 of the 27 known alien species are traded as ornamentals for aquaria or garden ponds in German shops. Alien species can most successfully be controlled when their management starts as soon as possible after their introduction. In Germany, the delay between first records and start of management actions seems too long for successful control. The public awareness of alien aquatic plants and problems they can cause in Germany is still limited despite a number of recent projects. At present, Black lists are developed that help nature conservationists, stakeholders and politicians to select those alien species for which prevention measures should be implemented. These, however, are not legally binding and laws regulating trade in Black listed plant species are strongly needed to reduce their impact on the environment and economy.     

Interactive effects of nitrate concentrations and carbon dioxide on the stoichiometry,biomass allocation and growth rate of submerged aquatic plants

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