Clonal integration ameliorates the carbon accumulation capacity of a stoloniferous herb, Glechoma longituba, growing in heterogenous light conditions by facilitating nitrogen assimilation in the rhizosphere.

BACKGROUND AND AIMS: Enhanced availability of photosynthates increases nitrogen (N) mineralization and nitrification in the rhizosphere via rhizodeposition from plant roots. Under heterogeneous light conditions, photosynthates supplied by exposed ramets may promote N assimilation in the rhizosphere of shaded, connected ramets. This study was conducted to test this hypothesis. METHODS: Clonal fragments of the stoloniferous herb Glechoma longituba with two successive ramets were selected. Mother ramets were subjected to full sunlight and offspring ramets were subjected to 80 % shading, and the stolon between the two successive ramets was either severed or left intact. Measurements were taken of photosynthetic and growth parameters. The turnover of available soil N was determined together with the compostion of the rhizosphere microbial community. KEY RESULTS: The microbial community composition in the rhizosphere of shaded offspring ramets was significantly altered by clonal integration. Positive effects of clonal integration were observed on NAGase activity, net soil N mineralization rate and net soil N nitrification rate. Increased leaf N and chlorophyll content as well as leaf N allocation to the photosynthetic machinery improved the photosynthetic capability of shaded offspring ramets when the stolon was left intact. Clonal integration improved the growth performance of shaded, connected offspring ramets and whole clonal fragments without any cost to the exposed mother ramets. CONCLUSIONS: Considerable differences in microbial community composition caused by clonal integration may facilitate N assimilation in the rhizosphere of shaded offspring ramets. Increased N content in the photosynthetic machinery may allow pre-acclimation to high light conditions for shaded offspring ramets, thus promoting opportunistic light capture. In accordance with the theory of the division of labour, it is suggested that clonal integration may ameliorate the carbon assimilation capacity of clonal plants, thus improving their fitness in temporally and spatially heterogeneous habitats.

Clonal integration of stress signal induces morphological and physiological response of root within clonal network.

Clonal integration of defense or stress signal induced systemic resistance in leaf of interconnected ramets. However, similar effects of stress signal in root are poorly understood within clonal network. Clonal fragments of Centella asiaticas with first-young, second-mature, third-old and fourth-oldest ramets were used to investigate transportation or sharing of stress signal among interconnected ramets suffering from low water availability. Compared with control, oxidative stress in root of the first-young, second-mature and third-old ramets was significantly alleviated by exogenous ABA application to the fourth-oldest ramets as well as enhancement of antioxidant enzyme (SOD, POD, CAT and APX) activities and osmoregulation ability. Surface area and volume in root of the first-young ramets were significantly increased and total length in root of the third-old ramets was significantly decreased. POD activity in root of the fourth-oldest and third-old ramets was significantly enhanced by exogenous ABA application to the first-young ramets. Meanwhile, total length and surface area in root of the fourth-oldest and third-old ramets were significantly decreased. Ratio of belowground to aboveground biomass in the whole clonal fragments was significantly increased by exogenous ABA application to the fourth-oldest or first-young ramets. It is suggested that transportation or sharing of stress signal may induce systemic resistance in root of interconnected ramets. Specially, transportation or sharing of stress signal against phloem flow was observed in the experiment. Possible explanation is that rapid recovery of foliar photosynthesis in first-young ramets subjected to exogenous ABA application can partially reverse phloem flow within clonal network. Thus, our experiment provides insight into ecological implication on clonal integration of stress signal.

Effects of different fertilization methods on Lolium multiflorum Lam. growth and bacterial community in waste slag.

Waste slag has low nutrient content, so it has insufficient nutrient cycling and transformation in the soil ecosystem. There are few studies on the application of oligotrophic phosphate-solubilizing bacteria and phosphate (P) fertilizer to improve the properties of waste slags. In this study, three oligotrophic bacterial strains with P solubilizing activity, namely, Bacillus subtilis 2C (7.23 µg/mL), Bacillus subtilis 6C (4.07 µg/mL), and Bacillus safensis 2N (5.05 µg/mL), were isolated from waste slags. In the pot experiment, compared with no application of P fertilizer, inoculation of Bacillus subtilis 2C with a 50% recommended dose of P fertilizer significantly increased the available phosphorus (AP), total phosphorus (TP), and total nitrogen (TN) in slag by 33.16%, 76.70%, and 233.33%, respectively. The N, P uptake and fresh weight of Lolium multiflorum Lam. were significantly improved by 114.15%, 139.02%, and 100%, respectively. The analysis of the bacterial community showed that the application of P fertilizer decreased the diversity and richness of the bacterial community, and with the addition of phosphorus fertilizer and Bacillus subtilis 2C, the bacterial community in the slag developed towards eutrophication. Redundancy analysis (RDA) showed that the TP content in the slag was significantly correlated with the bacterial community (P = 0.001, < 0.01), followed by the TN content. This study on different P fertilizer application methods can provide some basic ideas for improving the performance of waste slag.

Combined toxic effects of perfluorooctanoic acid and microcystin-LR on submerged macrophytes and biofilms.

Perfluorooctanoic acid (PFOA) and microcystin-LR (MCLR) are pervasive pollutants in surface waters that induce significant toxic effects on aquatic organisms. However, the combined environmental risk of PFOA and MCLR remains unclear. To assess the toxic effects of PFOA and MCLR on submerged macrophytes and biofilms, Vallisneria natans was exposed to different concentrations of PFOA and MCLR (0.01, 0.1, 1.0 and 10.0 µg L-1). Vallisneria natans was sensitive to high concentrations of MCLR (10 µg L-1): plants exposed to 10 µg L-1 of MCLR measured a biomass of 3.46 g, which was significantly lower than the 8.71 g of the control group. Additionally, antagonistic interactive effects were observed in plants exposed to combined PFOA and MCLR. Exposure to these pollutants adversely affected photosynthesis of the plants and triggered peroxidation that promoted peroxidase, superoxide dismutase and catalase activities, and increased malondialdehyde and glutathione concentrations. The total chlorophyll content was lower in the highest concentration of the combined treatment group (0.443 mg g-1) than in the control group (0.534 mg g-1). Peroxidase activity increased from 662.63 U mg-1 Pr to 1193.45 U mg-1 Pr with increasing PFOA concentrations. Metabolomics indicated that the stress tolerance of Vallisneria natans was improved via altered fatty acid metabolism, hormone metabolism and carbon metabolism. Furthermore, PFOA and MCLR influenced the abundance and structure of the microbial community in the biofilms of Vallisneria natans. The increased contents of autoinducer peptide and N-acylated homoserine lactone signaling molecules indicated that these pollutants altered the formation and function of the biofilm. These results expand our understanding of the combined effects of PFOA and MCLR in aquatic ecosystems.

Effects of combined exposure of PVC and PFOA on the physiology and biochemistry of Microcystis aeruginosa.

Microplastics (MPs) and per- and polyfluoroalkyl substances (PFASs) have drawn significant attention as emerging threats to aquatic ecosystems. There are currently just a few investigations on the combined toxicity of PFAS and MP on freshwater microalgae. In this research, the combined toxicity of polyvinyl chloride (PVC) and perfluorooctanoic acid (PFOA) to Microcystis aeruginosa was investigated. The results indicated that the combination of these pollutants inhibited the growth of M. aeruginosa and promoted the synthesis and release of Microcystin-LR (MC-LR). Individual and combined exposure caused different responses to cellular oxidative stress. Under the Individual exposure of PFOA, when the concentration was greater than 20.0 mg/L, the catalase (CAT) activity increased significantly, and when it was greater than 100.0 mg/L, the malondialdehyde (MDA) content increased significantly, but there is no significant change under combined exposure. PVC and PFOA exposure also caused physical damage to the algal cells and reduced the content of extracellular polymer substances (EPS) based on analysis of cell morphology. Metabolic analysis revealed that carbohydrate metabolism and amino acid metabolism of the algae were affected. The current study offers a fresh theoretical framework for MPs and PFASs environmental risk evaluations.

Response of submerged macrophytes and biofilms to coexisting azithromycin and tetracycline: Antibiotic resistance genes removal, toxicity assessment and microbial properties.

Antibiotics, such as azithromycin (AZ), tetracycline (TC), and their related antibiotic resistance genes (ARGs), create serious ecological risks to aquatic organisms. This study examined the response mechanisms of submerged macrophytes and periphytic biofilms to a mixture of AZ and TC pollution and determined the antibiotic removal efficiencies and fate of ARGs. The results showed that the plant-biofilm system had a significant capacity for removing both single and combined antibiotics with removal efficiencies of 93.06% ∼99.80% for AZ and 73.35% ∼97.74% for TC. Higher ARG (tetA, tetC, tetW, ermF, ermX, and ermB) abundances were observed in the biofilm, and subsequent exposure to the antibiotic mixture increased the abundances of these genes. Both single and combined antibiotics triggered antioxidant stress, but antagonistic effects were induced only with mixed AZ and TC exposure. Furthermore, the antibiotics changed the structural characteristics of extracellular polysaccharides and induced alterations in the structure of the biofilm microbial community. Increased N-acylated-l-homoserine lactone confirmed alternations in microbial quorum-sensing. The results extend the understanding of the fate of antibiotics and ARGs when aquatic plants and biofilms are exposed to antibiotic mixtures, as well as the organism's response mechanisms.

Joint toxicity mechanisms of perfluorooctanoic acid and sulfadiazine on submerged macrophytes and periphytic biofilms.

Hazardous chemicals, such as perfluoroalkyl substances (PFASs) and antibiotics, coexist in aquatic environments and pose a severe threat to aquatic organisms. However, research into the toxicity of these pollutants on submerged macrophytes and their periphyton is still limited. To assess their combined toxicity, Vallisneria natans (V. natans) was exposed to perfluorooctanoic acid (PFOA) and sulfadiazine (SD) at environmental concentrations. Photosynthetic parameters such as chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids were lower in the SD exposure group, indicating that SD had a significant effect on the photosynthesis of aquatic plants. Single and combined exposures effectively induced antioxidant responses, with increases in superoxide dismutase, peroxidase activities, and ribulose-1,5-bisphosphate carboxylase concentrations, as well as malondialdehyde content. Accordingly, antagonistic toxicity was assessed between PFOA and SD. Furthermore, metabolomics revealed that V. natans improved stress tolerance through changes in enoic acid, palmitic acid, and palmitoleoyloxymyristic acid related to the fatty acid metabolism pathway responding to the coexisting pollutants. Additionally, PFOA and SD in combination induced more effects on the microbial community of biofilm. The alternation of α- and ß-D-glucopyranose polysaccharides and the increased content of autoinducer peptides and N-acylated homoserine lactones indicated that PFOA and SD changed the structure and function of biofilm. These investigations provide a broader perspective and comprehensive analysis of the responses of aquatic plants and periphyton biofilms to PFAS and antibiotics in the environment.

Single and combined toxicity effects of microplastics and perfluorooctanoic acid on submerged macrophytes and biofilms.

Microplastics (MPs) and Perfluorooctanoic acid (PFOA) have contaminated nearly all types of ecosystems, including marine, terrestrial and freshwater habitats, posing a severe threat to the ecological environment. However, their combined toxicity on aquatic organisms (e.g., macrophytes) remains unknown. This study investigated single and combined toxic effects of polypropylene (PP), polyethylene (PE), polyvinylchloride (PVC), polyethylene terephthalate (PET) and PFOA on Vallisneria natans (V. natans) and associated biofilms. Results showed that MPs and PFOA significantly affected plant growth, while the magnitude of the effect was associated with concentrations of PFOA and the types of MPs, and antagonistic effects were induced at combined MPs and PFOA exposure. In addition, antioxidant responses in plants, such as promoted activities of SOD and POD, as well as increased content of GSH and MDA, were triggered effectively by exposure to MPs and PFOA alone and in combination. Ultrastructural changes revealed the stress response of leaf cells and the damage to organelles. Moreover, single and combined exposure to MPs and PFOA altered the diversity and richness of the microbial community in the leaf biofilms. These results indicated that the coexistence of MPs and PFOA can induce effective defense mechanisms of V. natans and change the associated biofilms at given concentrations in the aquatic ecosystems.

Immobilization of cadmium by mercapto-functionalized palygorskite under stimulated acid rain: Stability performance and micro-ecological response.

The interaction of cadmium (Cd) pollution and acid rain stress has seriously threatened soil ecosystem and human health. However, there are still few effective amendments for the in-situ remediation in the Cd-contaminated acidified soil. In this study, the performance and mechanisms of palygorskite (PAL) and mercapto-functionalized PAL (MPAL) on Cd immobilization were investigated, and the stability as well as effects on soil micro-ecology under stimulated acid rain were also explored. Results showed that MPAL could react with Cd to form stable Cd-sulfhydryl and Cd-O complexes. The reduction of bioavailable Cd by MPAL was 121.19-164.86% higher than that by PAL. Notably, the Cd immobilization by MPAL remained stable within 90 days in which the concentrations of HOAc-extractable Cd were reduced by 18.28-25.12%, while the reducible and residual fractions were increased by 9.26-18.53% and 54.16%-479.01%, respectively. The sequential acid rain leaching demonstrated that soil after MPAL treatments had a strong H+ resistance, and the immobilized Cd showed prominent stability. In addition, activities of acid phosphatase, catalase and invertase in MPAL treated soil were significantly enhanced by 34.60%, 22.09% and 48.87%, respectively. After MPAL application, bacterial diversity was further improved with diversified sulfur metabolism biomarkers. The decreased abundance of Cd resistance genes including cadA, cadC, czcA, czcB, czcR and zipA also indicated that soil micro-ecology was improved by MPAL. These results showed that MPAL was an effective and eco-friendly amendment for the immobilization of Cd in contaminated soil.

Implication of exogenous abscisic acid (ABA) application on phytoremediation: plants grown in co-contaminated soil.

Abscisic acid (ABA) may play an important role in alleviating negative effects of heavy metal stress on growth performance of plants. A pot experiment was conducted to investigate differential effects of exogenous ABA with different concentrations (0, 20, 40, and 60 µmol/L) on heavy metal accumulation and physiological response of Cd/Zn hyperaccumulator Sedum alfredii Hance and non-hyperaccumulator Hylotelephium spectabile (Boreau) H. Ohba grown in co-contaminated soil. In the experiment, Cd, Zn, or Pb concentration in stem and leaf of H. spectabile was significantly increased by exogenous ABA application than control. However, the opposite pattern was observed for S. alfredii. With decrease of Cd concentration, Zn or Pb concentration in root of H. spectabile grown in co-contaminated soil was significantly increased by exogenous ABA application than control. Cd, Zn, or Pb concentration in root of S. alfredii was significantly increased by exogenous ABA application than control. Compared with S. alfredii, BCF and TF of Cd, Zn, or Pb for H. spectabile were significantly increased by exogenous ABA application. With negative effect on root growth, total biomass of the two species, especially H. spectabile, was significantly increased by exogenous ABA application than control. With increase of their total chlorophyll content, antioxidant capacity of the two species subjected to heavy metal stress was improved by exogenous ABA application than control. Heavy metal-induced growth inhibition was significantly alleviated by exogenous ABA application when the two species were grown in co-contaminated soil. We tentatively concluded that differential effects of exogenous ABA application on transport pathway of ions incurred different patterns of heavy metal accumulation between Cd/Zn hyperaccumulator S. alfredii and non-hyperaccumulator H. spectabile. It is suggested that compared with Cd/Zn hyperaccumulator S. alfredii, exogenous ABA application may improve heavy metal uptake in root and transport of heavy metal ions between different organs for non-hyperaccumulator H. spectabile grown in co-contaminated soil. Our results provide insight into effects of exogenous ABA application on phytoremediation of Cd-, Pb-, and Zn-co-contaminated soil.

Effects of nutrient supply on leaf stoichiometry and relative growth rate of three stoloniferous alien plants.

Different nutrient supply brings about changes in leaf stoichiometry, which may affect growth rate and primary production of plants. Invasion of alien plants is a severe threat to biodiversity and ecosystem worldwide. A pot experiment was conducted by using three stoloniferous alien plants Wedelia trilobata, Alternanther philoxeroides and Hydrocotyle vulgaris to investigate effects of nutrient supply on their leaf stoichiometry and relative growth rate. Different nitrogen or phosphorus supply was applied in the experiment (N1:1 mmol L-1, N2:4 mmol L-1, and N3:8 mmol L-1, P1:0.15 mmol L-1, P2:0.6 mmol L-1 and P3:1.2 mmol L-1). Nitrogen and phosphorus concentrations in leaves of the three alien plants significantly increased with increase of nitrogen supply. With increase of phosphorus supply, nitrogen or phosphorus concentration of leaf was complex among the three alien plants. N:P ratio in leaf of the three alien plants subjected to different levels of nutrient supply was various. A positive correlation between relative growth rate and N:P ratio of the leaf is observed in W. trilobata and A. philoxeroides suffering from N-limitation. A similar pattern was not observed in Hydrocotyle vulgaris. We tentatively concluded that correlations between relative growth rate and N: P ratio of the leaf could be affected by species as well as nutrient supply. It is suggested that human activities, invasive history, local abundance of species et al maybe play an important role in the invasion of alien plants as well as relative growth rate.

Clonal integration benefits an invader in heterogeneous environments with reciprocal patchiness of resources, but not its native congener.

Many of the world's most invasive plants are clonal, and clonal functional traits are suggested to contribute to their invasiveness. Clonal integration is one of the most important clonal functional traits, but it is still unclear whether clonal integration can benefit invasive alien clonal plants more than native ones in heterogeneous environments with reciprocal patchiness of resources and whether invasive plants show a higher capacity of division of labor than native ones in such environments. We grew connected (allowing clonal integration) and disconnected (preventing clonal integration) ramet pairs of an invasive plant Wedelia trilobata and its occurring native congener W. chinensis in the environment consisting of reciprocal patches of light and soil nutrients (i.e., a high-light but low-nutrient patch and a low-light but high-nutrient patch). Clonal integration greatly promoted the growth of the invasive species, but had no significant effect on the native one. Both invasive and native species showed division of labor in terms of morphology, biomass allocation, and/or photosynthetic physiology, but the capacity of labor division did not differ between the invasive and the native species. We conclude that in heterogeneous environments consisting of reciprocal patches of resources, which are common in nature, clonal integration can confer invasive plants a competitive advantage over natives, but this difference is not related to their capacity of labor division. This study highlights the importance of clonal integration for plants in heterogeneous environments and suggests that clonal integration can contribute to the invasion success of alien clonal plants.

Effects of Pb-, Cd-resistant bacterium Pantoea sp. on growth, heavy metal uptake and bacterial communities in oligotrophic growth substrates of Lolium multiflorum Lam.

Phosphate-solubilizing bacteria (PSB) can accelerate phytoremediation, especially in those fertilized soils. However, PSB function in oligotrophic growth substrates remains poorly studied. In this study, we isolated lead (Pb)- and cadmium (Cd)-resistant PSB from contaminated sandy soil at an abandoned lubricant plant. The isolated Pantoea sp. PP4 (PP4 hereafter) can produce organic acid and IAA (Indole-3-acetic acid) and dissolve up to 238 mg/L of inorganic phosphate Ca2(PO4)3, exhibiting biosorption capability for Pb and bioprecipitation for Pb and Cd. In the sand pot experiment, inoculation of PP4 increased the accumulation of Pb and Cd in Lolium multiflorum Lam. by 28.9% and 95.5%, respectively, and increased the available phosphorous in oligotrophic river sand by 30.8% (P < 0.05). Meanwhile, the growth of Lolium multiflorum Lam. was also stimulated, resulting in 89.2%, 57.1%, 184.6%, and 28.5% increase in fresh weight, dry weight, root length, and shoot length, respectively. NMDS analysis showed that the bacterial communities in river sand were more clustered after inoculation with PP4. These results indicated that the application of Pantoea sp. PP4 can facilitate the phytoremediation of Pb and Cd in oligotrophic growth substrates, forming a convergent bacterial community. Our findings highlighted the importance of identifying ideal PSB to improve phytoremediation efficiency in oligotrophic environments.

Soil heterogeneity in the horizontal distribution of microplastics influences productivity and species composition of plant communities.

Contamination of soils by microplastics can have profound ecological impacts on terrestrial ecosystems and has received increasing attention. However, few studies have considered the impacts of soil microplastics on plant communities and none has tested the impacts of spatial heterogeneity in the horizontal distribution of microplastics in the soil on plant communities. We grew experimental plant communities in soils with either a homogeneous or a heterogeneous distribution of each of six common microplastics, i.e., polystyrene foam (EPS), polyethylene fiber (PET), polyethylene bead (HDPE), polypropylene fiber (PP), polylactic bead (PLA) and polyamide bead (PA6). The heterogeneous treatment consisted of two soil patches without microplastics and two with a higher (0.2%) concentration of microplastics, and the homogeneous treatment consisted of four patches all with a lower (0.1%) concentration of microplastics. Thus, the total amounts of microplastics in the soils were exactly the same in the two treatments. Total and root biomass of the plant communities were significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic was PET and PP, smaller when it was PLA, but not different when it was EPS, HDPE or PA6. In the heterogeneous treatment, total and root biomass were significantly smaller in the patches with than without microplastics when the microplastic was EPS, but greater when the microplastic was PET or PP. Additionally, in the heterogeneous treatment, root biomass was significantly smaller in the patches with than without microplastics when the microplastic was HDPE, and shoot biomass was also significantly smaller when the microplastic was EPS or PET. The heterogeneous distribution of EPS in the soil significantly decreased community evenness, but the heterogeneous distribution of PET increased it. We conclude that soil heterogeneity in the horizontal distribution of microplastics can influence productivity and species composition of plant communities, but such an effect varies depending on microplastic chemical composition (types) and morphology (shapes).

Distinct responses of frond and root to increasing nutrient availability in a floating clonal plant.

Current knowledge on responses of aquatic clonal plants to resource availability is largely based on studies manipulating limited resource levels, which may have failed to capture the "big picture" for aquatic clonal plants in response to resource availability. In a greenhouse experiment, we grew the floating clonal plant Spirodela polyrhiza under ten nutrient levels (i.e., 1/64×, 1/32×, 1/16×, 1/8×, 1/4×, 1/2×, 1×, 2×, 4× and 8×full-strength Hoagland solution) and examined their responses in terms of clonal growth, morphology and biomass allocations. The responses of total biomass and number of ramets to nutrient availability were unimodal. A similar pattern was found for frond mass, frond length and frond width, even though area per frond and specific frond area fluctuated greatly in response to nutrient availability. In contrast, the responses of root mass and root length to nutrient availability were U-shaped. Moreover, S. polyrhiza invested more to roots under lower nutrient concentrations. These results suggest that nutrient availability may have distinct influences on roots and fronds of the aquatic clonal plant S. polyrhiza, resulting in a great influence on the whole S. polyrhiza population.

Different functions and expression profiles of curcin and curcin-L in Jatropha curcas L.

To date, two types of ribosome-inactivating proteins (RIPs) have been found in Jatropha curcas. One is curcin, which has been isolated from the endosperm, and the other is curcin-L, which is expressed in leaves upon stress treatment. Phylogenetic analysis of the predicted amino acid sequences of the RIPs in plants revealed that these belong to a major subfamily and are close to trichosanthin (TCS). Studies on the mRNA and protein levels showed that both curcin and curcin-L have an organ-specific expression pattern. Curcin is only expressed and accumulated in the endosperm; its expression begins in the globular embryo period and peaks during the mature embryo period. In contrast, curcin-L is only expressed in the leaves, but its expression is induced by certain conditions such as treatment with phytohormones or polyethylene glycol, exposure to high and low temperatures, and fungal infection. Analysis of the 5' flanking regions of curcin and curcin-L revealed that the 5' flanking region of curcin-L has three major inserted fragments, which are not present in the corresponding region of curcin. Comparison of characteristic cis-elements suggests the presence of several motifs that are involved in the endosperm-specific expression in the 5' flanking region of curcin, while in curcin-L some stress- and defense-responsive motifs are found to be mainly located in the three inserted fragments. Comparison of the antifungal activity of the two RIPs showed that the one of curcin-L is higher than that of curcin. Differences in the expression and activity of curcin and curcin-L suggest that these two RIPs have different functions.

Growth and Morphological Responses of Duckweed to Clonal Fragmentation, Nutrient Availability, and Population Density.

Connected ramets of aquatic clonal plants are susceptible to fragmentation by disturbance. Such clonal fragmentation may interact with nutrient availability and individual density to affect growth and morphology of aquatic clonal plants. We grew the widespread floating clonal plant Spirodela polyrhiza (duckweed) under three levels of population density (low, medium, or high), two levels of nutrient availability (low or high), and two levels of clonal fragmentation (with or without). Clonal fragmentation and high nutrients increased biomass and ramet number, but decreased frond width, frond length, and specific frond area of S. polyrhiza. Increasing population density decreased growth (biomass and ramet number) and frond and root size, and increased frond thickness of individual ramets of S. polyrhiza. The negative effect of population density on growth of S. polyrhiza was greater under high than under low nutrient availability. Furthermore, the negative effect of population density on total mass and frond mass of S. polyrhiza was greater with fragmentation than without. These results suggest that clonal fragmentation, nutrient availability and population density can interact to affect growth and morphology of clonal floating plants. Competition for nutrients and space, rather than light, may be the mechanisms underlying reduced growth of clonal floating plants. As clonal fragmentation can increase biomass and ramet production of S. polyrhiza, disturbance that potentially causes clonal fragmentation cannot be recommended as a measure to limit the spread of clonal floating plants.

Transportation or sharing of stress signals among interconnected ramets improves systemic resistance of clonal networks to water stress.

Previous studies have elucidated the mechanisms, ecological implications and constraints on transportation or sharing of defence signals among interconnected ramets of clonal plants suffering from localised herbivore damage. To our knowledge, few studies have been conducted to provide insights into the ecological implications on transportation or sharing of stress signals for clonal plants subjected to water stress. As a chemical elicitor, ABA can induce resistance response in plants suffering from water stress. A pot experiment was conducted to explore transportation or sharing of stress signals among interconnected ramets by using clonal fragments of Centella asiaticas (L.) Urban with four successive ramets (oldest, old, mature and young) subjected to low water availability (20% soil moisture contents). Compared with control, foliar oxidative stress of the old, mature and young ramets significantly decreased, and antioxidant capacity was increased when exogenous ABA was applied to the oldest ramets. Meanwhile, foliar PSII activity and chlorophyll content of the old, mature and young ramets significantly increased. Compared with control, biomass accumulation and ratio of below-ground/aboveground biomass of whole clonal fragments were significantly increased by ABA application to the oldest ramets. However, similar patterns were not observed when exogenous ABA was applied to the young ramets. Our results show that transportation or sharing of stress signals among interconnected ramets improves systemic resistance of clonal networks to water stress, which is dependent on directionality of vascular flows. Compared with the old or mature ramets, the young ramets displayed stronger resistance response (such as higher antioxidant enzymes activities and proline content, lower O2•- production rate and malondialdehyde content) to water stress as well as higher PSII activity and chlorophyll content when exogenous ABA was applied to the oldest ramets. Thus, transportation or sharing of stress signals may favour young ramets that are most valuable for growth and fitness of clonal plant subjected to environmental stress. It is suggested that transportation or sharing of stress signals among interconnected ramets may confer clonal plants with considerable benefits in adapting to spatio-temporal heterogeneous habitats.

Effects of Transgenerational Plasticity on Morphological and Physiological Properties of Stoloniferous Herb Centella asiatica Subjected to High/Low Light.

Environmentally induced transgenerational plasticity can increase success of progeny and thereby be adaptive if progeny experiences the similarly parental environment. The ecological and evolutionary significance of transgenerational plasticity in plant has been studied mainly in the context of sexual generations. A pot experiment using the stoloniferous herb Centella asiatica was conducted to investigate the effects of high/low light treatment experienced by parental ramets (F0 generation) on morphological and physiological properties of offspring ramets (F2 generation) as well as growth performance. Light environment experienced by parental ramets (F0 generation) significantly influenced petiole length, specific petiole length, internode length of stolon, leaf area, specific leaf area (SLA), leaf nitrogen and chlorophyll contents, potential maximum net photosynthetic rate (Pmax ) in offspring ramets subjected to parental or non-parental environments even after they were detached from the parental ramets. Potential maximum net photosynthetic rate (Pmax ) of offspring ramets (F2 generation) from parental ramets (F0 generation) subjected to low light treatment was significantly greater than that of offspring ramets (F2 generation) from parental ramets (F0 generation) subjected to high light treatment. Potential maximum net photosynthetic rate (Pmax ) of offspring ramets (F2 generation) subjected to parental light environment was greater than that of offspring ramets (F2 generation) subjected to non-parental light environment. The greatest biomass accumulation and total stolon length were observed in offspring ramets (F2 generation) subjected to low light treatment as parental ramets (F0 generation) experienced. When parental ramets (F0 generation) were subjected to low light treatment, biomass accumulation and total stolon length of offspring ramets (F2 generation) experiencing parental light environment were significantly greater than those of offspring ramets (F2 generation) experiencing non-parental light environment. Opposite pattern was observed in offspring ramets (F2 generation) from parental ramets subjected to high light treatment. Our work provides evidence that transgenerational plasticity through both morphological and physiological flexibility was triggered across vegetative generations for stoloniferous herb C. asiatica subjected to high/low light treatment. The transgenerational plasticity can allow offspring ramets to present adaptive phenotype early without lag time in response to the current environment. Thus, it is very important for clonal plants in adapting temporally and spatially heterogeneous habitats.

Effects of clonal integration on microbial community composition and processes in the rhizosphere of the stoloniferous herb Glechoma longituba (Nakai) Kuprian.

The effects of rhizodeposition on soil C and N availabilities lead to substantial changes of microbial community composition and processes in the rhizosphere of plants. Under heterogeneous light, photosynthates can be translocated or shared between exposed and shaded ramets by clonal integration. Clonal integration may enhance the rhizodeposition of the shaded ramets, which further influences nutrient recycling in their rhizosphere. To test the hypothesis, we conducted a pot experiment by the stoloniferous herb Glechoma longituba subjected to heterogeneous light. Microbial biomass and community composition in the rhizosphere of shaded offspring ramets, assessed by phospholipid fatty acids (PLFAs) analysis, were markedly altered by clonal integration. Clonal integration positively affected C, N availabilities, invertase and urease activities, N mineralization (Nmin) and nitrification rates (Nnitri) in the rhizosphere of shaded offspring ramets. However, an opposite pattern was also observed in phenoloxidase (POXase) and peroxidase (PODase) activities. Our results demonstrated that clonal integration facilitated N assimilation and uptake in the rhizosphere of shaded offspring ramets. The experiment provides insights into the mechanism of nutrient recycling mediated by clonal integration.