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Heterophyllous aquatic plants produce aerial (i.e., floating and terrestrial) and submerged leaves—the latter lack stomata—while homophyllous plants contain only submerged leaves, and cannot survive on land. To identify whether differences in morphogenetic potential and/or physiological stress responses are responsible for variation in phenotypic plasticity between two plants types, responses to abscisic acid (ABA) and salinity stress were compared between the closely related, but ecologically diverse pondweeds, Potamogeton wrightii (heterophyllous) and P. perfoliatus (homophyllous). The ABA-treated (1 or 10 μM) P. wrightii plants exhibited heterophylly and produced leaves with stomata. The obligate submerged P. perfoliatus plants were able to produce stomata on their leaves, but there were no changes to leaf shape, and stomatal production occurred only at a high ABA concentration (10 μM). Under salinity stress conditions, only P. wrightii leaves formed stomata. Additionally, the expression of stress-responsive NCED genes, which encode a key enzyme in ABA biosynthesis, was consistently up-regulated in P. wrightii , but only temporarily in P. perfoliatus . The observed species-specific gene expression patterns may be responsible for the induction or suppression of stomatal production during exposure to salinity stress. These results suggest that the two Potamogeton species have an innate morphogenetic ability to form stomata, but the actual production of stomata depends on ABA-mediated stress responses specific to each species and habitat.

Harvesting is a direct and simple artificial method to regulate submerged macrophyte communities. However, there is still a gap in whether the succession of different seasonal submerged macrophytes can be achieved by harvesting. The morphological, physiological index, competition intensity of Hydrilla verticillata ( H. verticillata ) and underwater light under different harvesting intensities of Potamogeton crispus ( P. crispus ) was studied by a controlled experiment. Under moderate harvesting intensity (harvest 30% and 45% of plant height) of P. crispus , H. verticillata was the least inhibited, resulting in accumulating more Chl a and higher plant height. The underwater light created by moderate-intensity harvesting was favorable for H. verticillata to compete with P. crispus . Thus, moderate-intensity harvesting of P. crispus can promote the growth of H. verticillata . In practice, moderate harvesting intensity of P. crispus can be used to create conditions for the growth of summer species to achieve seasonal succession of submerged macrophytes and maintain the clear-water state of the lake.

Thorough understanding of biodiversity is a fundamental prerequisite for biological research. A lack of taxonomic knowledge and species misidentifications are particularly critical for conservation. Here we present an example of Potamogeton floridanus, the Florida Pondweed, an endangered taxon endemic to a small area in the Florida panhandle, whose taxonomic status remained controversial for more than a century, and all previous attempts to elucidate its identity have failed. We applied molecular approaches to tackle the origin of the mysterious taxon and supplemented them with morphological and anatomical investigations of both historical herbarium collections and plants recently collected in the type area for a comprehensive taxonomic reassessment. Sequencing of two nuclear ribosomal markers and one chloroplast non-coding spacer resulted in the surprising discovery that P. floridanus is a hybrid of P. pulcher and P. oakesianus, with the former being the maternal parent. The hybrid colony is currently geographically isolated from the distribution range of P. oakesianus. We show that previous molecular analyses have failed to reveal its hybrid identity due to inadequate nuclear DNA sequence editing. This is an example how the uncritical use of automized sequence reads can hamper molecular species identifications and also affect phylogenetic tree construction and interpretation. This unique hybrid taxon, P. ×floridanus, adds another case study to the debate on hybrid protection; consequences for its conservation are discussed.

Aquatic plants play an important role in maintaining the health of water environment in nature. Studies have shown that linear alkylbenzene sulfonate (LAS), a type of omnipresent pollutant, can cause toxic damage to aquatic plants. In the present research, we studied the physiological and growth response of submerged plant Potamogeton perfoliatus L. to different concentrations of LAS (0.1, 1.0, 10.0, 20.0, and 50.0 mg l −1 ). The results showed that LAS is toxic to P. perfoliatus , and the toxicity is dose-dependent. Only slightly reversible oxidative damages were observed in the physiological parameters of P. perfoliatus when P. perfoliatus was exposed to lower LAS doses (< 10 mg l −1 ): soluble sugar, soluble protein, H 2 O 2 , and malondialdehyde (MDA) content in P. perfoliatus increased significantly at 0.1 mg l −1 and then returned to normal levels at 1.0 mg l −1 . Antioxidant enzymes were activated before the LAS concentration reached 10 mg l −1 , and the activities of superoxide dismutase (SOD), catalase (CAT), and photosynthesis pigment content declined significantly when the concentration of LAS exceeded 10 mg l −1 . In addition, at higher concentrations (20–50 mg l −1 ) of LAS, dry weight and fresh weight of P. perfoliatus showed significant declines. The results indicate that LAS above 10 mg l −1 can cause serious physiological and growth damage to P. perfoliatus .

Biogeographic barriers for freshwater biota can be effective at various spatial scales. At the largest spatial scale, freshwater organisms can become genetically isolated by their high mountain ranges, vast deserts, and inability to cross oceans. Isolation by distance of aquatic plants is expected to be stronger across than alongside mountain ridges whereas the heterogeneity of habitats among populations and temporary droughts may influence connectivity and hamper dispersal. Suitable aquatic plant habitats became reduced, even for the widespread submerged Potamogeton pectinatus L. (also named Stuckenia pectinata) giving structure to various aquatic habitats. We compared the level of genetic diversity in a heterogeneous series of aquatic habitats across Iran and tested their differentiation over distances and across mountain ranges (Alborz and Zagros) and desert zones (Kavir), with values obtained from temperate region populations. The diversity of aquatic ecosystems across and along large geographic barriers provided a unique ecological situation within Iran. P. pectinatus were considered from thirty-six sites across Iran at direct flight distances ranging from 20 to 1,200 km. Nine microsatellite loci revealed a very high number of alleles over all sites. A PCoA, NJT clustering and STRUCTURE analysis revealed a separate grouping of individuals of southeastern Iranian sites and was confirmed by their different nuclear ITS and cpDNA haplotypes thereby indicating an evolutionary significant unit (ESU). At the level of populations, a positive correlation between allelic differentiation Dest with geographic distance was found. Individual-based STRUCTURE analysis over 36 sites showed 7 genetic clusters. FST and RST values for ten populations reached 0.343 and 0.521, respectively thereby indicating that allele length differences are more important and contain evolutionary information. Overall, higher levels of diversity and a stronger differentiation was revealed among Iranian P. pectinatus than previously observed for temperate European regions, due to regional differences across mountain ranges over long distances.

• Background and Aims Molecular-based studies of thermotolerance have rarely been performed on wild plants, although this trait is critical for summer survival. Here, we focused on thermotolerance and expression of heat shock transcription factor A2 (HSFA2) and its putative target gene (chloroplast-localized small heat shock protein, CP-sHSP) in two allied aquatic species of the genus Potamogeton (pondweeds) that differ in survival on land. • Methods The degree of thermotolerance was examined using a chlorophyll bioassay to assess heat injury in plants cultivated under non-and heat-acclimation conditions. Potamogeton HSFA2 and CP-sHSP genes were identified and their heat-induction was quantified by real-time PCR. • Key Results The inhibition of chlorophyll accumulation after heat stress showed that Potamogeton malaianus had a higher basal thermotolerance and developed acquired thermotolerance, whereas Potamogeton perfoliatus was heat sensitive and unable to acquire thermotolerance. We found two duplicated HSFA2 and CP-sHSP genes in each species. These genes were induced by heat shock in P. malaianus, while one HSFA2a gene was not induced in P. perfoliatus. In non-heat-acclimated plants, transcript levels of HSFA2 and CP-sHSP were transiently elevated after heat shock. In heat-acclimated plants, transcripts were continuously induced during sublethal heat shock in P. malaianus, but not in P. perfoliatus. Instead, the minimum threshold temperature for heat induction of the CP-sHSP genes was elevated in P. perfoliatus. • Conclusions Our comparative study of thermotolerance showed that heat acclimation leads to species-specific changes in heat response. The development of acquired thermotolerance is beneficial for survival at extreme temperatures. However, the loss of acquired thermotolerance and plasticity in the minimum threshold temperature of heat response may be favourable for plants growing in moderate habitats with limited daily and seasonal temperature fluctuations.

Ammonium pulse attributed to runoff of urban surface and agriculture following heavy rain is common in inland aquatic systems and can cause profoundly effects on the growth of macrophytes, especially when combined with low light. In this study, three patterns of NH4-N pulse (differing in magnitude and frequency) were applied to examine their effects on the growth of three submersed macrophytes, namely, Myriophyllum spicatum, Potamogeton maackianus, and Vallisneria natans, in terms of biomass, height, branch/ramet number, root length, leaf number, and total branch length under high and low light. Results showed that NH4-N pulse caused negative effects on the biomass of the submerged macrphytes even on the 13th day after releasing NH4-N pulse. The negative effects on M. spicatum were significantly greater than that on V. natans and P. maackianus. The effects of NH4-N pulse on specific species depended on the ammonium loading patterns. The negative effects of NH4-N pulse on P. maackianus were the strongest at high loading with low frequency, and on V. natans at moderate loading with moderate frequency. For M. spicatum, no significant differences were found among the three NH4-N pulse patterns. Low light availability did not significantly aggregate the negative effects of NH4-N pulse on the growth of the submersed macrophytes. Our study contributes to revealing the roles of NH4-N pulse on the growth of aquatic plants and its species specific effects on the dynamics of submerged macrophytes in lakes.

Pondweed (Potamogeton distinctus) turions can elongate in the absence of O₂. Alcoholic fermentation serves to produce energy for anoxic elongation via the breakdown of starch stored in cells. However, the mechanism of cell growth during anoxic elongation is not fully understood. Changes in pH, H⁺ equivalent and lactate content of the incubation medium were measured during anoxic elongation. The effects of fusicoccin (FC), indole-3-acetic acid (IAA), vanadate, erythrosine B and K⁺ channel blockers on anoxic elongation were examined. Cytoplasmic pH and vacuolar pH were measured by ³¹P nuclear magnetic resonance (NMR) spectroscopy. Acidification of the incubation medium occurred during anoxic elongation. The contribution of CO₂ and lactic acid was not sufficient to explain the acidification. FC and IAA enhanced the elongation of stem segments. Vanadate and erythrosine B inhibited anoxic elongation. Acid growth of notched segments was observed. The activity of plasma membrane H⁺-ATPase extracted from pondweed turions was increased slightly in anoxic conditions, but that from pea epicotyls sensitive to anoxic conditions was decreased by incubation in anoxic conditions. Both the cytoplasmic pH and vacuolar pH of pondweed turion cells chased by ³²P NMR spectroscopy were stabilized during a short period < 3 h after anoxic conditions. We propose that the enhancement of H⁺ extrusion by anoxic conditions induces acidification in the apoplast and may contribute to the stabilization of pH in the cytoplasm.

In the aquatic genus Stuckenia , the wide geographic range of S . pectinata and S . filiformis make them suited for examination of topographic and climatic effects on plant evolution. Using nuclear ITS sequence and ten chloroplast sequences, we conducted comparative phylogeographical analyses to investigate their distribution regions and hybrid zones in China and compare their phylogeographical patterns and demographical histories. These two species were allopatric in China. S . filiformis occurred only on the Qinghai-Tibet Plateau (QTP), whereas S . pectinata occupied a wide range of habitats. These two species formed hybrid zones on the northeastern edge of QTP. Most of the genetic variance of S . filiformis was between the southern and eastern groups on the QTP, showing a significant phylogeographic structure. The geographical isolations caused by the Nyenchen Tanglha Mountains and the Tanggula Mountains promoted intraspecific diversification of alpine plants on the QTP. This study revealed the lack of phylogeographic structure in S . pectinata , due to the continued gene flow among its distribution regions. The ecological niche modeling showed that the distribution ranges of these two herbaceous species did not contract too much during the glacial period.

Potamogeton is one of the most difficult groups to clarify in aquatic plants, which has an extensive range of interspecific morphological and ecological diversity. Internal transcribed spacer (ITS) is prevalent for phylogenetic analysis in plants. However, most researches demonstrate that ITS has a high percentage of homoplasy in phylogenetic datasets. In this study, eighteen materials were collected in Potamogeton from China and incongruence was shown between the rbcL and ITS phylogenies. To solve the discrepancy, we employed a novel barcode PHYB to improve resolution and accuracy of the phylogenetic relationships. The PHYB phylogeny successfully resolved the incongruence between the rbcL and ITS phylogenies. In addition, six hybrids were confirmed using PHYB, including P. compressus × P. pusillus, P. octandrus × P. oxyphyllus, P. gramineus × P. lucens, P. distinctus × P. natans, P. distinctus × P. wrightii, and S. pectinata × S. amblyophylla. Whereas, only one hybrid was identified (P. compressus × P. pusillus) by ITS, indicating that ITS homoplasy was present in Potamogeton and ITS was completely homogenized to one parental lineage. Thus, ITS might have limited utility for phylogenetic relationships in Potamogeton. It is recommended that a three-locus combination of chloroplast DNA gene, ITS and PHYB is potential to effectively reveal more robust phylogenetic relationships and species identification.