Influence of the Nitrate-N to Ammonium-N Ratio on Relative Growth Rate and Crude Protein Content in the Duckweeds Lemna minor and Wolffiella hyalina

In order to produce protein-rich duckweed for human and animal consumption, a stable cultivation process, including an optimal nutrient supply for each species, must be implemented. Modified nutrient media, based on the N-medium for duckweed cultivation, were tested on the relative growth rate (RGR) and crude protein content (CPC) of Lemna minor and Wolffiella hyalina, as well as the decrease of nitrate-N and ammonium-N in the media. Five different nitrate-N to ammonium-N molar ratios were diluted to 10% and 50% of the original N-medium concentration. The media mainly consisted of agricultural fertilizers. A ratio of 75% nitrate-N and 25% ammonium-N, with a dilution of 50%, yielded the best results for both species. Based on the dry weight (DW), L. minor achieved a RGR of 0.23 ± 0.009 d-1 and a CPC of 37.8 ± 0.42%, while W. hyalina's maximum RGR was 0.22 ± 0.017 d-1, with a CPC of 43.9 ± 0.34%. The relative protein yield per week and m2 was highest at this ratio and dilution, as well as the ammonium-N decrease in the corresponding medium. These results could be implemented in duckweed research and applications if a high protein content or protein yield is the aim.

Duckweed biomarkers for identifying toxic water contaminants?

Surface or ground waters can be contaminated with numerous toxic substances. The duckweeds Lemna minor and Lemna gibba are widely used for assaying waterborne toxicity to higher plants in terms of growth inhibition and photosynthetic pigment reduction. These tests cannot, however, in themselves determine the nature of the agents responsible for toxicity. Morphological, developmental, physiological, biochemical, and genetic responses of duckweeds to exposure to toxic water contaminants constitute biomarkers of toxic effect. In principle, the very detection of these biomarkers should enable the contaminants having elicited them (and being responsible for the toxicity) to be identified. However, in practice, this is severely compromised by insufficient specificity of biomarkers for their corresponding toxicants and by the lack of documentation of biomarker/toxin relationships. The present contribution illustrates the difficulties of using known water contaminant-related duckweed biomarkers to identify toxins, and discusses possibilities for achieving this goal.

The Spirodela polyrhiza genome reveals insights into its neotenous reduction fast growth and aquatic lifestyle

The subfamily of the Lemnoideae belongs to a different order than other monocotyledonous species that have been sequenced and comprises aquatic plants that grow rapidly on the water surface. Here we select Spirodela polyrhiza for whole-genome sequencing. We show that Spirodela has a genome with no signs of recent retrotranspositions but signatures of two ancient whole-genome duplications, possibly 95 million years ago (mya), older than those in Arabidopsis and rice. Its genome has only 19,623 predicted protein-coding genes, which is 28% less than the dicotyledonous Arabidopsis thaliana and 50% less than monocotyledonous rice. We propose that at least in part, the neotenous reduction of these aquatic plants is based on readjusted copy numbers of promoters and repressors of the juvenile-to-adult transition. The Spirodela genome, along with its unique biology and physiology, will stimulate new insights into environmental adaptation, ecology, evolution and plant development, and will be instrumental for future bioenergy applications.

Spirodela, or duckweed, is a basal monocotyledonous plant with both pharmaceutical and commercial value. Here, the authors sequence the genome of Spirodela polyrhiza, suggesting its genome has evolved by neotenous reduction and clonal propagation, and provide a platform for future comparative genomic studies in angiosperms.

Assessment, validation and deployment strategy of a two-barcode protocol for facile genotyping of duckweed species

Lemnaceae, commonly called duckweeds, comprise a diverse group of floating aquatic plants that have previously been classified into 37 species based on morphological and physiological criteria. In addition to their unique evolutionary position among angiosperms and their applications in biomonitoring, the potential of duckweeds as a novel sustainable crop for fuel and feed has recently increased interest in the study of their biodiversity and systematics. However, due to their small size and abbreviated structure, accurate typing of duckweeds based on morphology can be challenging. In the past decade, attempts to employ molecular barcoding techniques for species assignment have produced promising results; however, they have yet to be codified into a simple and quantitative protocol. A study that compiles and compares the barcode sequences within all known species of this family would help to establish the fidelity and limits of this DNA-based approach. In this work, we compared the level of conservation between over 100 strains of duckweed for two intergenic barcode sequences derived from the plastid genome. By using over 300 sequences publicly available in the NCBI database, we determined the utility of each of these two barcodes for duckweed species identification. Through sequencing of these barcodes from additional accessions, 30 of the 37 known species of duckweed could be identified with varying levels of confidence using this approach. From our analyses using this reference dataset, we also confirmed two instances where mis-assignment of species has likely occurred. Potential strategies for further improving the scope of this technology are discussed.

Turion formation in Spirodela polyrhiza: the environmental signals that induce the developmental process in nature

The formation of turions of Spirodela polyrhiza is induced by a large number of environmental signals, already investigated under laboratory conditions. To get more close-to-nature experimental conditions, chemical composition and temperature of the water were measured during the growing season in 2002 and 2003 in a pond near Jena (50 degrees 52'N, 11 degrees 42'E). Whereas the concentrations of nitrate and sulphate (both in the millimolar range) remained fairly constant that of phosphate decreased from approximately 13 microM at the beginning of the season to 2 microM at the time of onset of turion formation (17 August in 2002, 21 July 2003). This concentration was used in experiments under controlled conditions together with the other outdoor data (day temperature, lower night temperature and photoperiod) in subsequent experiments to investigate their role in the induction of turion formation. The concentration of the nutrient media were kept constant. The following conclusions were drawn. (1) Low phosphate concentration appears to be the decisive factor in inducing turion formation. Growing fronds take up phosphate, and turion formation is then induced towards the end of the season. (2) Lower temperatures during the day (18 vs 25 degrees C) and especially during the night (18 vs 15 degrees C) evidently enhance the effect of the turion-inducing factor phosphate by increasing the yield. (3) At much higher anthropogenic phosphate concentrations low temperature takes over the function of inducing turion formation. (4) Whereas much lower concentrations act directly to induce the formation of turions regardless of the season.

[Changes in vectors of endogenously generated ion currents in light-induced germination of turions of Spirodela polyrrhiza]

Nondormant turions of Spirodela polyrhiza (L.) Schleiden were utilized to investigate endogenous ion currents in light-induced germination and early growing processes of higher plants. A small outward current was detected at the ventral side of the turions near the pocket containing the most developed sprout primordium. After a light pulse, the direction of the endogenous current changed from outward to inward. These ion currents are most likely conditioned by unspecific diffusion of cations (probably H+) inside the cell. Three-day-old sprouts of Spirodela showed the highest inward current near the sprout base which decreases toward its edge. Newly formed sprouts demonstrated a strong gravity effect (bending), which was preceded by a lowering of the Z-component of vectors close to the sprout base after a change of the turion fixation.

Multiple effects of chromate on the photosynthetic apparatus of Spirodela polyrhiza as probed by OJIP chlorophyll a fluorescence measurements

Chromate (Cr) decreases the growth of Spirodela polyrhiza. The fronds lost their pigments. The O2 evolution was also decreased. The Cr effect was found to be dose dependent. The toxic effects of Cr have further been studied on the photosynthetic activity of Spirodela polyrhiza by means of the chlorophyll a (Chl a) fluorescence transient O-J-I-P. The Chl a fluorescence transients were recorded in vivo with high time resolution and analyzed according to the JIP-test which can quantify the photosystem II behavior. Cr treated plants show a decrease in yield for primary photochemistry, phi Po. The performance index of PSII, PIABS, which is the combination of the indexes of three independent parameters, (1) the total number of active reaction centers per absorption (RC/ABS), (2) yield of primary photochemistry (phi Po) and (3) efficiency with which a trapped exciton can move an electron into the electron transport chain (psi 0), decreased due to Cr treatment. Chromate sensitivity varies within plant populations. In summary Cr affects several targets of PSII. More specifically, the main targets of Cr, according to the JIP-test, can be listed as a decrease in the number of active reaction centers and damage to the oxygen-evolving complex.

Light-induced starch degradation in non-dormant turions of Spirodela polyrhiza

Continuous red light controls starch degradation in turions of Spirodela polyrhiza [Dölger, K., U. K. Tirlapur and K.-J. Appenroth [1997] Photochem. Photobiol. 66, 126-127 (1997)]. This light could be replaced by repeated red light pulses with the reciprocity law fulfilled over a large range of fluence rates. The effect of red light pulses repeated every 24 or 12 h for 6 days was reversible by subsequent far-red light pulses. In contrast, hourly applied red pulses were irreversible by far-red light. This discrepancy was explained by showing the starch degradation activity of far-red pulses themselves. The investigated process was categorized as a phytochrome low fluence response with an unusual property: requirement of light treatment for several days. A partial fulfillment of this requirement was obtained with a red pulse followed by a dark period and a 24 h continuous irradiation. These results suggest the existence of two separate steps in the process of starch degradation in turions: formation of a sprout (= sink) during the pulse-induced germination, and starch degradation in the storage tissue (= source) induced by the second light treatment.

Kinetics of chromium(V) formation and reduction in fronds of the duckweed Spirodela polyrhiza--a low frequency EPR study

The uptake of chromate by the duckweed Spirodela polyrhiza was investigated with atomic absorption spectroscopy and the reduction of Cr(VI) to Cr(V) was measured using low frequency EPR spectroscopy. The biphasic kinetics of the uptake was fitted to parameters of a proposed kinetic model. Another model was developed to simulate chromate reduction. The first step of chromate reduction was found to be much faster than the uptake of Cr(VI) from the free space. Most probably, this step occurs already in the cell wall or on the cell membrane surface. Further reduction of Cr(V) to Cr(III) was estimated to be slower. The disappearance of the Cr(V) signal, following transfer of the plants into a Cr-free solution, lasted several tens of hours; the kinetics was mono- or biexponential depending on the length of Cr loading. The rate constants for Cr reduction in living plants were determined for the first time.

Regulation of transcript level and synthesis of nitrate reductase by phytochrome and nitrate in turions of Spirodela polyrhiza (L.) Schleiden

Control by light and nitrate of the appearance of nitrate reductase (NR; EC 1.6.6.1) in the turions of Spirodela polyrhiza (L.) Schieiden was investigated during the pre-germination phase, i.e. up to 120 h after the transfer from after-ripening to germination conditions. Turions are particularily suited for this type of coaction study since control of nitrate uptake by light and long-distance transport of nitrate do not play a role. Control of NR gene expression was mainly studied between 72 and 120 h after transfer to germination conditions (phase II of the pre-germination process). It was found that the effect of light on NR appearance is exerted via phytochrome. The light effect on enzyme synthesis was only observed in the presence of nitrate. On the other hand, in darkness, the NR level decreased to almost zero even in the presence of an optimum supply of nitrate. It is concluded that in phase II a coaction of light (phytochrome) and nitrate is required to bring about NR synthesis. However, when nitrate was applied to turions - following a dark incubation without nitrate - the turions responded to nitrate even in darkness with strong NR synthesis. This response was augmented by light. The highest transcript levels were observed in the presence of both factors, light and nitrate. As a single factor, light was more effective in stimulating the transcript level than nitrate. However, no correlation between the transcript level and the rate of enzyme synthesis was observed. This is consistent with the previous conclusion (Schuster et al. 1988, Planta 174, 426-432) that in higher plants control at the transcript level is only coarse and does not determine quantitatively the output at the level of enzyme protein.

Photophysiology of turion germination in Spirodela polyrhiza (L.) Schleiden VIII. Calcium dependence and β-amylase activity

Red-light-induced germination (via phytochrome) of turions of Spirodela polyrhiza (L.) Schleiden, strain SJ, showed an absolute requirement for exogenously applied calcium (Appenroth et al. 1990b, Biochem. Physiol. Pflanzen 186, 209-219). With calcium step-up experiments (from 0.9 μM to 1 mM Ca(2+)) the influence of Ca(2+) on the escape from far-red-light reversibility was investigated. Delaying the time of Ca(2+) application after the red light pulse resulted in an increased reversibility. This confirmed that the calcium-requiring phase is within the phytochrome-dependent period. Ten days after a red light pulse was applied the activity of β-amylase (EC 3.2.1.2) was increased about tenfold. The starting point was similar in the germination and enzymatic responses, but half-maximal response times according to the Mitscherlich function (2.5 d and 6.9 d, respectively) and times of saturation (about 6 d and 9 d, respectively) were different. Increase of β-amylase activity was completely inhibited by irradiation with far-red light (demonstrating phytochrome as the photoreceptor), as well as by inhibitors of translation (cycloheximide, puromycin) and transcription (cordycepin). Once formed, the active enzyme protein was stable for at least 4 d after cycloheximide application. After application of cordycepin a small amount of translation activity was detected until complete inhibition was reached after 2 d. This phytochrome-modulated, de-novo synthesis of β-amylase was only slightly dependent on exogenous Ca(2+). Therefore, it can be concluded that the phytochrome-mediated signal-transduction chain branches either in its reversible part or as a consequence of diversity of the primary phytochrome reaction.

Is there a third photoreceptor involved in the control of chloroplast movements in mougeotia?

The photometric method was used to test a possibility proposed recently that a new photoreceptor with maximum activity at 620 nm is involved in mediating chloroplast rotation in Mougeotia (Z Lechowski, J Bialczyk [1988] Plant Physiol 88: 189-193). The hypothesis was tested under conditions of continuous dichromatic unilateral or mutually perpendicular irradiation with red light of wavelengths 620 or 660 (680) nanometers and far-red. When the red light was polarized parallel to the long cell axis, chloroplast response could be monitored by changing the direction of far-red irradiation. The level of the response obtained with red and far-red applied from the same direction depended on far-red intensity: at higher fluence rates the maximum response was shifted to longer wavelengths of red light. A high fluence rate of far-red inhibited the response. The absorption coefficients of Mougeotia chloroplasts were measured for the studied wave-lengths using the microphotometric method. Possible impact of absorption by the chloroplast on photoreception has been discussed. Current and previous results can be interpreted in terms of phytochrome action and do not support the involvement of the hypothetical 620 nanometer photoreceptor.