Low‐molecular weight carbohydrates modulate dormancy and are required for post‐germination growth in turions of Spirodela polyrhiza

The aquatic duckweed Spirodela polyrhiza propagates itself vegetatively by forming turions – bud‐like perennation organs – in the autumn, which spend the winter on the bottom of ponds and then germinate in the following spring and proliferate on the water surface. Newly formed turions usually require a period of cold after‐ripening and light to germinate effectively, but an ample supply of exogenous sugar can lead to germination even in the dark and independent of after‐ripening. The results of the present study indicate that the availability of readily metabolised carbohydrates is a determining factor for turion germination. Freshly harvested turions do not contain soluble, low‐molecular weight carbohydrates at a level sufficient to allow germination to take place, but after‐ripened turions do. Augmentation of the soluble carbohydrate content during after‐ripening derives from gradual breakdown of reserve starch of the turions. The long time required for any germination to be observed in turions incubated in darkness and the limited frequency of germination in the dark (about 50% of turion population), even with an ample external sugar, supply emphasise that both after‐ripening and light are essential for ensuring rapid germination and subsequent frond proliferation at an ecologically appropriate time. The carbohydrate supply required for rapid proliferation of the fronds produced at germination is provided by the rapid light‐induced breakdown of turion reserve starch.     

Growing duckweed in swine wastewater for nutrient recovery and biomass production

Spirodela oligorrhiza, a promising duckweed identified in previous studies, was examined under different cropping conditions for nutrient recovery from swine wastewater and biomass production. To prevent algae bloom during the start-up of a duckweed system, inoculating 60% of the water surface with duckweed fronds was required. In the growing season, the duckweed system was capable of removing 83.7% and 89.4% of total nitrogen (TN) and total phosphorus (TP) respectively from 6% swine lagoon water in eight weeks at a harvest frequency of twice a week. The total biomass harvested was 5.30 times that of the starting amount. In winter, nutrients could still be substantially removed in spite of the limited duckweed growth, which was probably attributed to the improved protein accumulation of duckweed plants and the nutrient uptake by the attached biofilm (algae and bacteria) on duckweed and walls of the system.     

Low levels of intraspecific genetic variation at a rapidly evolving chloroplast dna locus in North American duckweeds (Lemnaceae)

Although most previous studies on chloroplast (cp) DNA variation in plants have concentrated on systematics and evolution above the species level, intraspecific variation in cpDNA is common and has provided useful insights into population-level evolutionary processes. Polymerase chain reaction methods were used to examine restriction site and sequence variation in the chloroplast rpLI6 gene within and among populations of duckweed species (Spirodela and Lemna) from the southern and eastern United States. To our knowledge, the rpL16 region has not previously been used to investigate cpDNA variation in nature. While considerable restriction site and sequence variation were detected among species, no variation was found within populations of either of the two species (S. punctata and L. minor) selected for sequence analysis, and S. punctata showed no interpopulational variation. Two cpDNA haplotypes were identified in L. minor, with one haplotype restricted to three sites in Louisiana and the other found in all other populations sampled. This paucity of variation cannot be readily explained as the result of a low mutation rate. In general, group II introns appear to be subject to very little functional constraint, and extensive sequence differences have been found between species in the chloroplast rpL16 intron in particular. However, factors such as historical range expansions and contractions, founding effects, fluctuations in local population size, and natural selection may play a role in reducing cpDNA sequence variability in these species.     

Bioremoval of lead from water using Lemna minor

This study examined the ability of duckweed (Lemna minor) to remove soluble lead from water. The duckweed was obtained from the Devils Lake wastewater treatment plant in North Dakota. The viable aquatic plants were exposed to a single dose of lead (from Pb(NO₃)₂) at a concentration of 5.0 mg/l for a time period of 21 days. Lead concentrations were measured in the water daily and in the biomass at the conclusion of the experiment. All measurements were done in triplicate and performed in accordance with standard methods. These data were used to calculate the removal efficiency with respect to time, and to provide the necessary empirical constants to model the removal behavior. Viable biomass removed 85–90% of the lead, viable duckweed previously exposed to lead removed 70–80% of the lead, non-viable biomass (control group) removed 60–75% of the lead, and there was no removal in the ‘no-biomass' control group. Based on these results we conclude that the viable biomass is effective in removing lead present at sub-lethal levels.