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Interactions between climate change and UV penetration in the biosphere are resulting in the exposure of plants to new combinations of UV radiation and drought. In theory, the impacts of combinations of UV and drought may be additive, synergistic or antagonistic. Lack of understanding of the impacts of combined treatments creates substantial uncertainties that hamper predictions of future ecological change. Here, we compiled information from 52 publications and analysed the relative impacts of UV and/or drought. Both UV and drought have substantial negative effects on biomass accumulation, plant height, photosynthesis, leaf area and stomatal conductance and transpiration, while increasing stress-associated symptoms such as MDA accumulation and reactive-oxygen-species content. Contents of proline, flavonoids, antioxidants and anthocyanins, associated with plant acclimation, are upregulated both under enhanced UV and drought. In plants exposed to both UV and drought, increases in plant defense responses are less-than-additive, and so are the damage and growth retardation. Less-than-additive effects were observed across field, glasshouse and growth-chamber studies, indicating similar physiological response mechanisms. Induction of a degree of cross-resistance seems the most likely interpretation of the observed less-than-additive responses. The data show that in future climates, the impacts of increases in drought exposure may be lessened by naturally high UV regimes.

Aquaculture is an important source of animal protein and a key contributor to global food security. However, aquaculture can exert a negative effect on the aquatic environment due to the release of effluents containing high nutrient levels. In integrated multi-trophic aquaculture (IMTA), the waste produced by one species is the input for another, referred to as extractive species (ES). Potential ES include plants. In the present study, it was explored whether Lemna minor can be used to remove nitrogen and phosphorus from aquaculture wastewater. A representative synthetic wastewater was designed based on the composition of aquaculture effluents found in the literature. Synthetic wastewater was found to be a suitable medium for growth of Lemna minor, and plants readily took up NH4+-N, NO3-N and PO43--P. In particular, NH4+-N concentrations rapidly decreased. The highest removal rates per square meter of water surface, calculated for NH4+-N, NO3-N and PO43--P, were, respectively, 158, 206 and 32 mg·m-2·day-1, and these rates were achieved at a plant surface density of 80%. As removal of nutrients is essentially a surface area–related process, the effect of plant density on nutrient uptake was determined. Uptake of nutrients per square meter of surface area was highest at the highest plant density. Yet, when uptake rates were calculated per square meter of water area covered by Lemna fronds, the highest removal rates were found at the lowest plant density, and this is likely to be associated with a reduced intraspecific competition. The present work enables the calculation of potential nutrient uptake by Lemna minor and lays the foundation for a more scientific approach to the design of duckweed-based aquaculture wastewater treatment systems.

Ultraviolet radiation (UVR) is an important environmental factor that can have an impact directly, or indirectly, on the health of organisms. UVR also has the potential to inactivate pathogens in surface waters. As a result, UVR can alter host-pathogen relationships. Bivalve species are threatened by various pathogens. Here, we assessed the impacts of UVR on (i) bivalves, (ii) bivalve pathogens and (iii) the bivalve host-pathogen relationship. UVR consistently impedes pathogens. However, the effect of UVR on marine animals is variable, with both positive and negative impacts. The limited available data allude to the potential to exploit natural UVR for disease management in aquaculture, but also highlight a striking knowledge gap and uncertainty relating to climate change.

This study was conducted to answer the question of whether elevated CO2 and the presence or absence of inorganic nitrogen (NH4+) affect the uptake of different forms of organic nitrogen in two dominant saltmarsh species Schoenoplectus americanus and Spartina patens. S. americanus and S. patens were grown under elevated and ambient CO2 conditions and a series of hydroponic assays were conducted using dual labelled 13C15N– glycine, glutamic acid and urea supplied in both the absence and presence of NH4+. Results show rates of glycine and urea uptake were lower under elevated CO2 conditions for both species. Ratios of 13C and 15N in S. patens roots showed that at least 68 and 79% of glycine under ambient and elevated CO2, respectively, was taken up intact. Provision of NH4+ with organic N caused organic N uptake rates to decline by up to 75% in S. americanus and up to 50% in S. patens compared with plants that only received organic N. The reduction in organic N uptake in the presence of NH4+ suggests that plants rely primarily on mineral N in the field. In addition, we can deduce that organic N uptake is not likely to supply plants with the additional N required under elevated CO2, and that the repressive effects of elevated CO2 on organic N uptake may have negative consequences for ecosystem productivity and carbon sequestration.

Lemnaceae are being exploited to remediate a variety of different wastewaters. Our aim was to remediate dairy processing waste, which is produced in large amounts, and contains valuable plant nutrients, for example, nitrate, ammonium, phosphate and iron. However, initial trials failed to establish the growth of Lemna minor on this medium. A lack of growth can be due to a lack of essential plant nutrients, or the presence of phytotoxic ingredients. In this study we show that not just nutrient concentrations, but also the ratios between them can be important in facilitating growth. Lab-scale experiments in which L. minor were grown on 100 mL of synthetic dairy industry wastewater demonstrated that the skewed Ca:Mg ratio in synthetic wastewater is a key obstacle to good growth. Experiments showed that a ratio which favors magnesium over calcium negatively affects L. minor growth and photosynthetic yield, leading to RGRs as low as 0.05 day−1. A change in this ratio to favor calcium, through the addition of calcium sulfate, leads to RGRs of 0.2–0.3 day−1. Experiments lead us to conclude that a Ca:Mg ratio of 1:1.6 or greater is necessary for L. minor growth, and therefore phytoremediation of dairy industry processing wastewater.

In vitro culturing can generate plants with a distorted morphology. Some distortions affect the plant’s survival after transfer to an ex vitro environment, while others can affect the aesthetic value. Therefore, exogenous hormones are often applied in in vitro cultures to modulate plant architecture. In this study, it was hypothesised that regulatory effects of UV-B radiation on plant morphology can be exploited under in vitro conditions, and that UV exposure will result in sturdier, less elongated plants with more branches and smaller leaves, mediated by changes in plant hormones. Plants were grown in tissue-culture containers and exposed to ~0.22 W m−2 UV-B for 8 days. Subsequently, plants were transferred to soil and monitored for a further 7 days. Results show that UV induced a marked change in architecture with a significant increase in axillary branches, and reductions in leaf area, plant height and root weight. These changes were associated with significant alterations in concentrations of hormones, including IAA, GA7, GA3 and iP–9–G. Changes in hormone concentrations suggest a regulatory, rather than a stress response to UV-B. Therefore, it is proposed that the application of UV in in vitro culture can be an innovative approach to manipulate plant architecture.

The aquatic Lemnaceae family, commonly called duckweed, comprises some of the smallest and fastest growing angiosperms known on Earth. Their tiny size, rapid growth by clonal propagation, and facile uptake of labeled compounds from the media were attractive features that made them a well-known model for plant biology from 1950 to 1990. Interest in duckweed has steadily regained momentum over the past decade, driven in part by the growing need to identify alternative plants from traditional agricultural crops that can help tackle urgent societal challenges, such as climate change and rapid population expansion. Propelled by rapid advances in genomic technologies, recent studies with duckweed again highlight the potential of these small plants to enable discoveries in diverse fields from ecology to chronobiology. Building on established community resources, duckweed is reemerging as a platform to study plant processes at the systems level and to translate knowledge gained for field deployment to address some of society’s pressing needs. This review details the anatomy, development, physiology, and molecular characteristics of the Lemnaceae to introduce them to the broader plant research community. We highlight recent research enabled by Lemnaceae to demonstrate how these plants can be used for quantitative studies of complex processes and for revealing potentially novel strategies in plant defense and genome maintenance.

Expression of cucumber (Cucumis sativus) genes encoding the phenylpropanoid and flavonoid biosynthetic enzymes phenylalanine ammonia lyase (PAL), cinnamic acid 4-hydroxylase (C4H), and chalcone synthase (CHS), was studied under control light conditions (photosynthetically active radiation, PAR) in root, stem, and leaf. Furthermore, the expression was quantified in leaves illuminated with PAR and supplemental ultraviolet-A (315–400 nm) or ultraviolet-B (280–315 nm) radiation. The expression patterns of all twelve CsPAL, three CsC4H, and three CsCHS genes were established. Among the genes regulated by UV two general expression patterns emerge. One pattern applies to genes primarily regulated by enriched UV-A illumination (pattern 1). Another pattern (pattern 2) was found for the genes regulated by enriched UV-B. Three of the pattern 2 genes (CsPAL4, CsPAL10, and CsCHS2) displayed a particular sub-pattern (pattern 2b) with transcription enriched by at least 30-fold. In contrast to the other genes studied, the promoters of the genes regulated according to pattern 2b contained a combination of a number of cis-acting regulatory elements (MREs, ACEs, and G-boxes) that may be of importance for the particularly high enhancement of expression under UV-B-containing light. The regulation of phenylpropanoid and flavonoid biosynthesis genes in cucumber resembles that of a number of other plants. However, cucumber, due to its greater size, is an attractive species for combining more detailed studies of the morphology, physiological parameters and fine regulation of spatial and temporal expression of key genes. This, in turn, can facilitate the quantitative investigation of the relationships among different promoter motifs, the expression levels of each of these three genes, and metabolite accumulation profiles.

It is widely assumed that environmental stressors contribute to the protection of habitats from invasion by alien species, and that native species are better stress-tolerators. This assumption was tested by comparing the performance of the invasive alien Lemna minuta Kunth with that of the co-generic native L. minor Linnaeus, under several environmental stressors. The effects of temperature and drought, important determinants of the distribution of Lemnaceae, on growth and photosynthesis were explored. Also, tolerance to, and accumulation of aluminium and copper were studied. Finally, tolerance to Reactive Oxygen Species (ROS) was compared by growing the plants at different concentrations of the ROS generator paraquat (methyl-viologen). The present study shows that specific stressors (such as low temperature in this study) disproportionally affect growth of alien L. minuta. Yet, in the case of three other stressors (aluminium, copper, drought), effects on biomass growth are similar for the two species, or they are even less severe on L. minuta. Remarkably, L. minuta dries out faster, and accumulates more metals than L. minor, suggesting that the former species has a greater physiological tolerance, whilst the latter species has an avoidance strategy. Thus, the current study on the role of environmental stressors in facilitating alien invasions does not support the notion that the presence of stressors impedes alien invasions, but rather shows that differences between an alien and a native species are multi-faceted, and stressor-specific.

Invasive alien species can negatively impact on newly colonised ecosystems. Thus, it is important to understand factors that facilitate invasiveness. Genetic diversity will enable a species to exploit a variety of environmental conditions. Yet, the process of dispersal to a new ecosystem will commonly create a genetic bottleneck and, hence, result in low diversity. Here we explored variability at genetic and morpho-physiological level of island of Ireland populations of alien, invasive Lemna minuta. A comparison of nine clones of L. minuta with nine clones of co-generic, native Lemna minor shows similar levels of genetic diversity across both species. Thus, the successful invasion of Ireland by L. minuta is associated with substantial, intraspecific diversity. It is hypothesised that increased biodiversity is due to repeated invasions from continental Europe, which occurred despite the geographic barriers separating the island from mainland Europe.

Summary • Class III peroxidases catalyse the oxidative crosslinking of UV-absorbing phenolics. The effect of changes in the activity of phenol oxidising peroxidases (EC 1.11.1.7) on UV-tolerance in Nicotiana tabacum plants has been determined. • The UV-sensitivity of transgenic N. tabacum lines, altered in their peroxidase expression pattern, was studied by measuring radiation effects on photosynthetic efficiency. • Analysis of the effect of UV-radiation on the relative variable chlorophyll fluorescence showed that the SPI-2 line, which over-expresses a defence-related cationic peroxidase, is markedly UV-tolerant. By contrast, the ROPN3-line, which overexpresses a synthetic horseradish peroxidase-C gene, was found to be UV-sensitive. The increased activity of indole-3-acetic acid (IAA) inducible peroxidases in homozygous IAA-overproducing transgenic plants was also found to correlate with UV-sensitivity. • It is concluded that only specific peroxidase isozymes, through their effects on phenolic metabolism, contribute to the UV protection response. Thus, the analysis of the role of isozymes in UV-protection addresses fundamental questions of isozyme diversity and/or redundancy in relation to phenolic substrates.

Using dill (Anethum graveolens L.) as a model herb, we revealnovel associations between metabolite profile and sensory quality, by integrating non-target metabolomics with sensory data. Low night temperatures and exposure to UV-enriched light was used to modulate plant metabolism, thereby improving sensory quality. Plant age is a crucial factor associated with accumulation of dill ether and α-phellandrene, volatile compounds associated with dill flavour. However, sensory analysis showed that neither of these compounds has any strong association with dill taste. Rather, amino acids alanine, phenylalanine, glutamic acid, valine, and leucine increased in samples exposed to eustress and were positively associated with dill and sour taste. Increases in amino acids and organic acids changed the taste from lemon/grass to a more bitter/pungent dill-related taste. Our approach reveals a novel approach to establish links between effects of eustressors on sensory quality, and may be applicable to a broad range of crops., Funding Agencies:Faculty for Business, Science and Technology at Örebro University Örebro University Vice Chancellor's strategic research programme on 'Food and Health' Science Foundation Ireland S16/IA/4418

Ultraviolet (UV) light induces a stocky phenotype in many plant species. In this study, we investigate this effect with regard to specific UV wavebands (UV-A or UV-B) and the cause for this dwarfing. UV-A- or UV-B-enrichment of growth light both resulted in a smaller cucumber (Cucumis sativus L.) phenotype, exhibiting decreased stem and petiole lengths and leaf area (LA). Effects were larger in plants grown in UV-B- than in UV-A-enriched light. In plants grown in UV-A-enriched light, decreases in stem and petiole lengths were similar independent of tissue age. In the presence of UV-B radiation, stems and petioles were progressively shorter the younger the tissue. Also, plants grown under UV-A-enriched light significantly reallocated photosynthates from shoot to root and also had thicker leaves with decreased specific LA. Our data therefore imply different morphological plant regulatory mechanisms under UV-A and UV-B radiation. There was no evidence of stress in the UV-exposed plants, neither in photosynthetic parameters, total chlorophyll content, or in accumulation of damaged DNA (cyclobutane pyrimidine dimers). The abscisic acid content of the plants also was consistent with non-stress conditions. Parameters such as total leaf antioxidant activity, leaf adaxial epidermal flavonol content and foliar total UV-absorbing pigment levels revealed successful UV acclimation of the plants. Thus, the UV-induced dwarfing, which displayed different phenotypes depending on UV wavelengths, occurred in healthy cucumber plants, implying a regulatory adjustment as part of the UV acclimation processes involving UV-A and/or UV-B photoreceptors., Funding agencies:Science Foundation Ireland (S16/IA/4418)Flemish Science Foundation (FWO, grant G000515N)China Scholarship Council (CSC no. 201406320076)

Vibrio bacteria are one of the greatest threats to aquaculture, with Vibrio-associated mass mortality events occurring in all life stages worldwide. Vibrio aestuarianus is particularly problematic in Pacific oyster Crassostrea gigas cultures. A diagnostic method used to detect V. aestuarianus is quantitative (q)PCR and a confirmatory method is fluorescent in situ hybridization (FISH). These methods require specialized equipment and incur high costs. The objective of this study was to develop a novel generic (relating to a genus) PCR and DIG-labeled in situ hybridization (ISH) for the detection and confirmation of Vibrio. A PCR that detects all Vibrio spp. is particularly useful for blanket screening of hosts and environmental samples, while ISH facilitates the visual localization of bacteria. Both diagnostic methods were developed to be used synchronously with other methods that are specific to certain Vibrio spp. Primers (VibF3/VibR3) were designed to amplify a 286 bp product from the 16S ribosomal RNA gene common to all Vibrio spp. and to form the ISH probe. Confirmatory positive and negative controls were used in both methods. Results were compared with a V. aestuarianus specific qPCR protocol. Direct Sanger sequencing of PCR products, Blastn analysis and Clustal Omega alignments were used to confirm Vibrio sp. detection and assess similarity. Pathologies observed in C. gigas tissues were similar to those described in other studies. A positive ISH signal was observed in qPCR and PCR positive oysters and no signal was detected in uninfected individuals. These techniques will increase the diagnostic arsenal against this globally significant genus of bacteria.

Conspecific negative density dependence in tree species can have major consequences for structuring of communities, yet in temperate forests this phenomenon remains largely unexplored. We investigated density-dependent recruitment in the long-lived conifer yew (Taxus baccata L.), for which low levels of regeneration have been linked with reduced light availability beneath conspecific canopies and speculative autotoxic effects. We combined in situ assessment of yew regeneration in a temperate forest in southwest Ireland with ex situ pot experiments to assess whether light and/or beneath conspecific canopy conditions inhibit the germination and early growth of yew. In field experiments, recruitment was related to distance from adult conspecifics, with higher levels of regeneration found in areas not beneath yew canopies. However, reduced light availability beneath the dense yew canopy was not responsible for this pattern, with abundant seedlings found in areas of equivalent or even lower light availability. Pot experiments showed that seedling survival was highest in deep shade treatments. Experiments also demonstrated that the addition of yew needles negatively affected seedling growth, but not seed germination. Together, our results suggest that under natural forest conditions, the absence of regeneration beneath mature yew canopies, often attributed to low light availability, may at least be partly related to substances in yew canopy or leaf leachates.

UVB radiation is biologically active; in plants, it can induce a range of molecular, biochemical, morphological and developmental responses. Although much progress has been made in elucidating UVB perception and signalling pathways under controlled laboratory conditions, understanding of the adaptive, ecological role of UVB responses is still very limited. In this study, we looked at the functional role of UVR8 under outdoor light conditions, by studying growth, photosynthetic competence and accumulation of UV absorbing pigments in a mutant lacking functional UVR8 protein. It was found that the influence of UVB on morphology is restricted to summer and is independent of UVR8. In contrast, UVB had an effect on the content of UV-absorbing pigments and the maximal efficiency of photosystem II of photosynthesis in the uvr8-1 mutant throughout the year. It is concluded that the UVR8 photoreceptor plays a role throughout the year, in the temperate climate zone, even when UVB levels are relatively low.

Summary Aquatic birds can facilitate the dispersal of plants, animals, microbes and fungi. Here, we review existing literature on bird-mediated external (ectozoochorous; synonyms epizoochorous, exozoochorous) dispersal to determine the importance of birds as mobile links between aquatic environments, and to evaluate ectozoochory as a mechanism of dispersal for aquatic organisms and their propagules (collectively referred to as dispersal units). Bird-mediated ectozoochory occurs when dispersal units stick to feathers, feet and bills, become enmeshed within plumage or encapsulated within mud coating the soft parts of vector birds. The importance of waterbird-mediated ectozoochorous dispersal relates to the topical problems of increasing anthropogenic habitat fragmentation, climate change and the associated alterations in species distributions and biological invasions. Nevertheless, some aspects of the bird-mediated ectozoochorous dispersal remain understudied, and poorly understood. This review identifies two areas where further research is required. Firstly, the quantitative contribution of bird-mediated ectozoochory to overall dispersal remains unclear. Secondly, greater consideration of all the requirements necessary for successful waterbird-mediated ectozoochorous dispersal is needed. In particular, the impact of factors which influence attachment and detachment of dispersal units (e.g. dispersal unit densities, preening behaviour and the role of encapsulating mud) needs further examination. Moreover, few investigators have taken the step of examining viability of found dispersal units, and fewer still have examined tolerances of dispersal units to the ‘in-flight’ conditions experienced during transport, in order to determine probability of survival of external dispersal. Evidence presented in this review indicates that bird-mediated ectozoochorous transport of aquatic dispersal units is a frequent process, at least at a local scale, and therefore needs to be considered in the context of connectivity and gene flow between isolated aquatic environments.

Invasive alien species represent a serious worldwide threat to natural and semi-natural ecosystems. Although successful overland transport of invasive aquatic macrophytes can be facilitated by more mobile organisms or anthropogenic activity, tolerance to desiccation is likely a limiting factor. Particularly in the case of vegetative fragmentary propagules. Here we examined survival and subsequent viability (i.e. regeneration by production of new shoot or root growth) for whole plantlets of two floating (Azolla filiculoides, Lemna minuta) and stem fragments of three submerged (Elodea canadensis. E. nuttallii, Lagarosiphon major) invasive macrophytes following exposure to different desiccation regimes. Species were exposed to selected temperature (20, 27, 36 °C) and relative humidity (18, 38, 60, 85% RH) combinations for up to six hours. In general, floating plants displayed greater survival and viability than submerged species. Overall, survival and viability decreased significantly for all species with increased desiccation exposure times. In essence, increased vapour-pressure deficit significantly reduced survival of aquatic plants. Although reduced humidity rates particularly decreased survival and viability, increased temperatures were observed to bolster the impact of certain humidity treatments. In particular, when exposed to low RH (≤38%RH), little or no viability was observed after 2 h. Contrastingly, propagules kept under RH above 60% RH at 20 °C retained viability for considerably longer intervals above 4–6 h. Overall, desiccation as a biosecurity tool alone is likely inadequate to prevent the spread of fragmentary propagule stages, as even small desiccated fragments that appear to be dry may still be viable.

Aquatic macrophytes can successfully colonise and re-colonise areas separated by space and time. The mechanisms underlying such “mobility” are not well understood, but it has often been hypothesised that epizoochory (external dispersal) plays an important role. Yet, there is only limited, and mostly anecdotal, evidence concerning successful epizoochorous dispersal of aquatic macrophytes, particularly in the case of short-distance dispersal. Here we examine in situ and ex situ dispersal of aquatic macrophytes, including three invasive alien species. A high frequency of Lemna minor Linnaeus dispersal was observed in situ, and this was linked to bird-mediated epizoochory. We concluded that wind had no effect on dispersal. Similarly, in an ex situ examination Lemna minuta Kunth and Azolla filiculoides Lamarck, were found to be dispersed with a high frequency by mallard ducks (Anas platyrhynchos). No dispersal was measured for Elodea nuttalli (Planchon) H. St. John. It is concluded that short-distance or “stepping-stone” dispersal via bird-mediated epizoochory can occur with high frequencies, and therefore can play an important role in facilitating colonisation, range expansion and biological invasion of macrophytes.

Invasive alien plant species are usually characterized by a growth rate higher than their native competitors, and this higher rate can be achieved through the opportunistic use of plant nutrients. The growth of the invasive alien duckweed Lemna minuta and the co-generic native Lemna minor were compared under different conditions of nutrient availability. The two species were grown for one week under fully controlled conditions on medium containing increasing concentrations of NO3−-N, NH4+-N and PO43—P. The effects of different concentrations and ratios of Ca and Mg on growth were also determined. At the end of the experiment the Relative Growth Rate (RGR) of the plants was calculated on the basis of the biomass and number of fronds. The data highlighted that Lemna minuta outgrew L. minor under conditions of high phosphate supply while, when phosphate concentrations in the medium were low, it grew less than L. minor. However, L. minuta is not simply an opportunistic species as we observed relative tolerance to imbalances in Ca-Mg ratio in the growth medium. The different responses observed when the L. minor and L. minuta were grown at specific concentrations of P, Ca and Mg suggest that these elements are potential determinants of the invasibility of freshwaters and that they should be considered in future field studies.

Large numbers of studies have reported on the responses of plants that are exposed to a specific dose of ultraviolet-B (UV-B) radiation. However, in the natural environment UV-B is a highly dynamic variable with UV-B intensities depending on, amongst others, geographic, temporal, weather and climatic factors. Furthermore, UV-B effects on plants can potentially be modulated by other environmental variables, and vice versa. This study aimed to characterize UV-B effects on plant morphology and accumulation of UV-screening pigments within the context of an oceanic climate and to assess the potential seasonality of plant UV-B responses. Arabidopsis thaliana was grown outdoors under UV-blocking or transmitting filters. Genotypic differences in the adaptive response to UV-B were assessed at seven time-points over a 12 month period and involved the Arabidopsis accessions Ler, Col-0, and Bur-0. Strong seasonal effects were found on rosette morphology and total UV-screening pigment concentrations across the three accessions. Low temperatures were the main determinant of accumulation of UV-absorbing pigments, with no clear UV-B effect observed at any time throughout the year. There was a significant UV effect on morphology during the summer months, and this was most likely associated with stress. This study shows that UV-effects need to be analysed in the context of weather, and other co-occurring natural factors, and emphasizes the importance of a holistic, multifactorial approach for the investigation of environmentally relevant UV-effects.

Lemna minuta Kunth is an invasive, alien duckweed that is present throughout much of Europe, where it competes with native congeneric Lemna minor Linnaeus. Previously, L. minuta was found to grow faster than L. minor. The aim of this study was to determine whether the rapid growth of invasive L. minuta is based on differential light utilisation. For this purpose, the growth performance of L. minuta was compared with that of L. minor under a range of different light intensities. Both physiological and morphological parameters were determined. L. minuta showed a higher Relative Growth Rate (RGR) than L. minor when grown under medium and high intensities. Further analysis showed that, at high light intensities, L. minuta has a higher Net Assimilation Rate (NAR), and displays more photochemical quenching (qP) and a higher quantum yield (Y(II)) than L. minor. In contrast under low light intensities L. minor displayed a marginally higher RGR, due to a greater Leaf Area Ratio (LAR), and higher chlorophyll content than L. minuta. The results indicate two distinct light utilisation strategies, and reveal that the invasive species L. minuta takes more advantage from high intensity light conditions. In turn, this may influence plant distribution and, consequently, ecosystem management by relevant authorities.

A substantial number of Lemnaceae are invasive outside their natural distribution area. Lemna minuta is considered invasive in several European countries, where it can occur in the same habitat as invasive Azolla filiculoides and native Lemna minor. In this study the presence, abundance and growth rates of all three species were monitored across 24 natural ponds and in a series of mesocosms in order to explore the importance of species invasiveness and habitat invisibility. Field monitoring showed that the distribution of the three species of macrophytes is heterogeneous in space and time. However, the data show no association of nutrient or light levels with plant distribution. Indeed, using reciprocal transplanting experiments it was demonstrated that all species are able to grow in all ponds, even ponds where the species do not naturally occur. It is concluded that distribution of L. minor, L. minuta, and A. filiculoides is not limited by the prevailing physicochemical characteristics of the ponds during the summer period. Remarkably, in these experiments A. filiculoides displayed the highest RGR, and exerted a negative influence on growth rates and surface cover of L. minor and L. minuta. Despite such apparent invasiveness, A. filiculoides was relatively rare in the study area. Rather, the species most abundant was L. minor which has the lowest RGR under field conditions in summer. Therefore, this study shows that the invasiveness of the species during the summer months is not necessarily reflected in the actual distribution pattern in natural ponds. In fact, alien L. minuta and A. filiculoides are under-represented in the monitored area. It is concluded that the interaction of several factors, including growth under winter-conditions and/or dispersal after disturbances, is the major determinant of the abundance and heterogeneous distribution of L. minor, L. minuta, and A. filiculoides in the study area.

Birds as carriers of propagules are major agents in the dispersal of plants, animals, fungi and microbes. However, there is a lack of empirical data in relation to bird-mediated, epizoochorous dispersal. The microclimate found within the plumage likely plays a pivotal role in survival during flight conditions. To investigate the potential of epizoochory, we have analysed the microclimatic conditions within the plumage of mallard ducks ( Anas platyrhynchos ). Under similar ambient conditions of humidity and temperature, a sample of mallards showed a consistent microclimatic regime with variation across the body surface. The highest (mean) temperature and specific humidity occurred between feathers of the postpatagium. The lowest humidity was found between feathers of the centre back and the lowest temperature in the crissum. Observed differences in plumage depth and density, and distance from the skin, are all likely to be determining factors of microclimate condition. Specific humidity found within the plumage was on average 1.8–3.5 times greater than ambient specific humidity. Thus, the plumage can supply a microclimate buffered from that of the exterior environment. Extrapolating survival data for Lemna minor desiccation at various temperature and humidity levels to the measured plumage microclimatic conditions of living birds, survival for up to 6 h can be anticipated, especially in crissum, crural and breast plumage. The results are discussed in the context of potential long distance epizoochorous dispersal by A. platyrhynchos and similar species.

UV-B radiation affects plant sexual reproduction, but, at present, general patterns about the nature of these effects and their underlying mechanisms remain elusive. In recent years, plant UV-B research has experienced a substantial conceptual change, from a stress-dominated view towards a more regulatory perspective. With this in mind, we have surveyed the published literature on the effects of UV-B radiation on plant reproductive traits and on plant–pollinator interactions. Most floral parts are effectively UV-B protected thanks to the accumulation of UV-B absorbing compounds. However, the least protected parts, such as pollen, are sensitive to high UV-B doses. Among UV-B absorbing compounds, flavonoids have a complex role in plant reproduction, since they are essential for UV-B protection and normal pollen function, while they also modulate flower and fruit colouration, which, in turn, affects visits by pollinators and frugivores. Effects of UV-B radiation on pollination can be direct, due to UV-B effects on pollinators, or indirect, due to pollinators responding to UV-B-mediated changes in plants. In the case of annual species, our literature survey revealed that, as UV-B doses increase, there is a tendency to delay the onset of flowering and to decrease fruit and/or seed production. Yet, the present review highlights the existence of complex dose–response curves that emphasize the need to use multiple UV-B doses in future studies of UV-B-mediated flowering responses. Moreover, species, populations or even cultivars originating from geographical areas with high impact of solar radiation ( e.g. Mediterranean Basin) exhibit, in general, high protection against UV-B radiation, often showing positive responses to moderate UV-B increases. In conclusion, our survey of the literature reveals complex UV-B effects on plant reproduction. To fully comprehend these effects, integrated approaches, beyond those currently used, are required to analyze the complex mixture of direct and indirect, stimulatory and inhibitory UV-B responses.

Primary producers exposed to ultraviolet (UV) radiation accumulate a range of metabolites as part of their UV-acclimation response. These metabolites play an important role in plant and algal UV protection. We investigated whether UV protection is transferred to consumers that feed on UV-acclimated algal biomass and showed that the copepod Tigriopus brevicornis (O. F. Muller, 1776) displays increased UV tolerance when fed on UV-acclimated Enteromorpha sp. (Clorophyta). We conclude that dietary transfer of metabolites produced in UV-acclimated biomass underlies the increased UV protection of the copepods. The data emphasise the complexity of the effects of UV radiation on the rock pool ecosystem.

Vectors that underpin the natural dispersal of invasive alien species are frequently unknown. In particular, the passive dispersal (zoochory) of one organism (or propagule) by another, usually more mobile animal, remains poorly understood. Field observations of the adherence of invasive freshwater bivalves to other organisms have prompted us to assess the importance of zoochory in the spread of three prolific invaders: zebra mussel Dreissena polymorpha ; quagga mussel Dreissena bugensis ; and Asian clam Corbicula fluminea . An extensive, systematic search of the literature was conducted across multiple on-line scientific databases using various search terms and associated synonyms. In total, only five publications fully satisfied the search criteria. It appears that some fish species can internally transport viable adult D. polymorpha and C. fluminea specimens. Additionally, literature indicates that veligers and juvenile D. polymorpha can adhere to the external surfaces of waterbirds. Overall, literature suggests that zoochorous dispersal of invasive bivalves is possible, but likely a rare occurrence. However, even the establishment of a few individuals (or a single self-fertilising C. fluminea specimen) can, over-time, result in a substantial population. Here, we highlight knowledge gaps, identify realistic opportunities for data collection, and suggest management protocols to mitigate the spread of invasive alien species.

A 2-year study explored metabolic and phenotypic plasticity of sun-acclimated Vitis vinifera cv. Pinot noir leaves collected from 12 locations across a 36.69-49.98°N latitudinal gradient. Leaf morphological and biochemical parameters were analysed in the context of meteorological parameters and the latitudinal gradient. We found that leaf fresh weight and area were negatively correlated with both global and ultraviolet (UV) radiation, cumulated global radiation being a stronger correlator. Cumulative UV radiation (sumUVR) was the strongest correlator with most leaf metabolites and pigments. Leaf UV-absorbing pigments, total antioxidant capacities, and phenolic compounds increased with increasing sumUVR, whereas total carotenoids and xanthophylls decreased. Despite of this reallocation of metabolic resources from carotenoids to phenolics, an increase in xanthophyll-cycle pigments (the sum of the amounts of three xanthophylls: violaxanthin, antheraxanthin, and zeaxanthin) with increasing sumUVR indicates active, dynamic protection for the photosynthetic apparatus. In addition, increased amounts of flavonoids (quercetin glycosides) and constitutive β-carotene and α-tocopherol pools provide antioxidant protection against reactive oxygen species. However, rather than a continuum of plant acclimation responses, principal component analysis indicates clusters of metabolic states across the explored 1,500-km-long latitudinal gradient. This study emphasizes the physiological component of plant responses to latitudinal gradients and reveals the physiological plasticity that may act to complement genetic adaptations.

Key message Photosynthetic capacity of yew needles depends primarily on light environment, with little evidence that developmental stage is a determinant photosynthetic performance. However, needle morphology is mostly determined by plant size. Abstract Populations of yew Taxus baccata are often characterized by a lack of natural regeneration and stands of this species have been afforded priority conservation status within the European Union. Light availability is known to be a key factor influencing recruitment dynamics in yew. To explore potential size-related responses to light, we investigated the distribution, photosynthetic efficiency and needle morphological traits in yew across a gradient of light availability at Atlantic oak woodland in southwest Ireland. Young plants were more common in shaded areas whereas saplings and juvenile yews were distributed in areas of higher light availability. Photosynthetic capacity of yew needles was found to depend primarily on light environment, with little evidence that the developmental stage is a determinant of photosynthetic performance. Photoinhibition was evident in needles sampled from high light environments across all demographic size classes. However, larger plants also showed particularly strong needle morphological responses (needle length, width, thickness, and specific leaf area) to light levels, which were mostly lacking in seedlings. We conclude that there is a dual control of yew light responses; the efficiency of the photosynthetic light reactions is largely linked to the light environment in which the plants grew, while needle morphology is mostly determined by the size of the plants. The findings of this study highlight the importance of considering all life-history stages and multiple traits when evaluating species response to light availability.

The chemical form and content of available nitrogen (N) in salt marsh substrates varies considerably. On the western coast of Ireland, habitats designated as Ombrogenic Atlantic salt marshes were formed on ombrogenic peat substrate. The peat substrate in these systems has three times more ammonium than substrate from adjacent salt marsh habitats on sand and mud substrate. This study examined the extent to which the high concentration of ammonium in peat salt marsh substrate influences the N- assimilating enzyme activity of halophytes and the extent to which N metabolism differs between species. Specifically, this work investigated whether plants from peat salt marshes are more likely to assimilate ammonium than plants from non-peat substrates. Four halophyte plant species—Armeria maritima, Aster tripolium, Plantago maritime, and Triglochin maritime—were sampled from various saltmarsh habitats including three sites on peat substrate and three on non-peat substrate, comprising sand, mud and sand/mud. The activities of N-metabolising enzymes—glutamine synthetase (GS), glutamate synthase, glutamate dehydrogenase (GDH), and nitrate reductase (NR)—were quantified in shoot and root parts. Root GS activity in Armeria maritima and shoot GS activity in Triglochin maritima were positively correlated with increasing soil ammonium levels. Root NR activity in Aster tripolium and shoot NR activity in Plantago maritima were significantly higher in plants grown on non-peat substrates than peat substrates. The shoot : root GS activity ratio in Triglochin maritima on peat substrate was more than double the ratio on non-peat substrates. It is concluded that all species tested displayed differences in N-metabolising activities depending on the chemical form and/or concentration of N in the substrate, while three out of the four species were capable of taking advantage of the high levels of ammonium in peat substrates.

There is a need to reappraise the effects of UV-B radiation on plant morphology in light of improved mechanistic understanding of UV-B effects, particularly elucidation of the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor. We review responses at cell and organismal levels, and explore their underlying regulatory mechanisms, function in UV protection and consequences for plant fitness. UV-induced morphological changes include thicker leaves, shorter petioles, shorter stems, increased axillary branching and altered root:shoot ratios. At the cellular level, UV-B morphogenesis comprises changes in cell division, elongation and/or differentiation. However, notwithstanding substantial new knowledge of molecular, cellular and organismal UV-B responses, there remains a clear gap in our understanding of the interactions between these organizational levels, and how they control plant architecture. Furthermore, despite a broad consensus that UV-B induces relatively compact architecture, we note substantial diversity in reported phenotypes. This may relate to UV-induced morphological changes being underpinned by different mechanisms at high and low UV-B doses. It remains unproven whether UV-induced morphological changes have a protective function involving shading and decreased leaf penetration of UV-B, counterbalancing trade-offs such as decreased photosynthetic light capture and plant-competitive abilities. Future research will need to disentangle seemingly contradictory interactions occurring at the threshold UV dose where regulation and stress-induced morphogenesis overlap.

Our ability to predict and manage the spread of alien, invasive plants is limited by a lack of understanding of dispersal potential. Invasive Lemna minuta has spread within a few decennia throughout Europe. However, the mechanism by which the species continues to spread remains a matter of speculation. In this study, hypothesised epizoochorous transport of L. minuta propagules by mallard ducks was investigated. Landolt (Biosystematic investigations in the family of duckweeds (Lemnaceae) (Vol. 2), The family of Lemnaceae - a monographic study (Vol. 1), 1986, Veröffentlichungen des Geobotanischen Institutes Der Eidg. Techniasche Hochschule, Stiftung Rübel, Zürich, Switzerland) referred to desiccation as the key limitation of the "colonization capability" of Lemnaceae. Therefore, we analysed retention of viability in L. minuta kept outside the liquid growth medium. Our data show prolonged viability of L. minuta fronds inserted between the feathers of a mallard duck. Consistently, the relative humidity between feathers ranged between 65% and 90%. Taking together evidence of entanglement and retention of L. minuta between the feathers of live ducks, with retention of viability, we consider it likely that mallards contribute to L. minuta dispersal. These data have implications for the management strategy of this invasive species.

Epidemiological studies have revealed an association between a high consumption of fruit, vegetables and herbs and a lower risk of both cancer and cardiovascular diseases. This protective effect is mostly due to secondary metabolites present in plant tissues. In contrast to numerous previous UV-B studies, which have focused on the potential impacts of increased levels of solar UV-B radiation reaching the Earth's surface due to stratospheric ozone depletion, recent investigations revealed that UV-B radiation is an important regulator of plant secondary metabolism. Low, ecologically-relevant UV-B levels trigger distinct changes in the biosynthesis of secondary plant metabolites such as phenolic compounds, carotenoids and glucosinolates. Fundamental understanding of plant UV-B perception and responses opens up new opportunities for crop manipulation. Thus, targeted low dosage UV-B radiation treatments as emerging technology may be used to generate fruit, vegetables and herbs enriched with secondary plant metabolites for either fresh consumption or as a source for functional foods and nutraceuticals.

Macrophytes contribute significantly to the cycling of metals in aquatic systems, through accumulation during growth and release during herbivory or decomposition. Accumulation of high levels of metals has been extensively documented in Lemnaceae (duckweeds). However, the degree of trophic transfer of metals from Lemnaceae to secondary consumers remains poorly understood. This study demonstrates that zinc accumulated in Lemna minor is bioavailable to the herbivore consumer Gammarus pulex. Overall, the higher the zinc content of L. minor, the more zinc accumulated in G. pulex. Accumulation in G. pulex was such that mortality occurred when they were fed high zinc-containing L. minor. Yet, the percentage of consumed zinc retained by G. pulex actually decreased with higher zinc concentrations in L. minor. We hypothesise that this decrease reflects internal zinc metabolism, including a shift from soluble to covalently bound zinc in high zinc-containing L. minor. Consistently, relatively more zinc is lost through depuration when G. pulex is fed L. minor with high zinc content. The developed Lemna-Gammarus system is simple, easily manipulated, and sensitive enough for changes in plant zinc metabolism to be reflected in metal accumulation by the herbivore, and therefore suitable to study ecologically relevant metal cycling in aquatic ecosystems.

Plants respond to environmental stress by synthesizing a range of secondary metabolites for defense purposes. Here we report on the effect of chronic ultraviolet (UV) radiation on the accumulation of plant secondary metabolites in Arabidopsis thaliana leaves. In the natural environment, UV is a highly dynamic environmental parameter and therefore we hypothesized that plants are continuously readjusting levels of secondary metabolites. Our data show distinct kinetic profiles for accumulation of tocopherols, polyamines and flavonoids upon UV acclimation. The lipid-soluble antioxidant α-tocopherol accumulated fast and remained elevated. Polyamines accumulated fast and transiently. This fast response implies a role for α-tocopherol and polyamines in short-term UV response. In contrast, an additional sustained accumulation of flavonols took place. The distinct accumulation patterns of these secondary metabolites confirm that the UV acclimation process is a dynamic process, and indicates that commonly used single time-point analyses do not reveal the full extent of UV acclimation. We demonstrate that UV stimulates the accumulation of specific flavonol glycosides, i.e. kaempferol and (to a lesser extent) quercetin di- and triglycosides, all specifically rhamnosylated at position seven. All metabolites were identified by Ultra Performance Liquid Chromatography (UPLC)-coupled tandem mass spectrometry. Some of these flavonol glycosides reached steady-state levels in 34 days, while concentrations of others are still increasing after 12 days of UV exposure. A biochemical pathway for these glycosides is postulated involving 7-O-rhamnosylation for the synthesis of all eight metabolites identified. We postulate that this 7-O-rhamnosylation has an important function in UV acclimation.

Glutathione (GSH) is an important scavenger of Reactive Oxygen Species (ROS), precursor of metal chelating phytochelatins, xenobiotic defence compound and regulator of cell proliferation. Homoglutathione (hGSH) is a GSH homologue that is present in several taxa in the family of Fabaceae. It is thought that hGSH performs many of the stress-defence roles typically ascribed to GSH, yet little is known about the potential involvement of hGSH in controlling cell proliferation. Here we show that hGSH/GSH ratios vary across organs and cells and that these changes in hGSH/GSH ratio occur during dedifferentiation and/or cell cycle activation events. The use of a GSH/hGSH biosynthesis inhibitor resulted in impaired cytokinesis in isolated protoplasts, showing the critical importance of these thiol-compounds for cell division. However, exposure of isolated protoplasts to exogenous GSH accelerated cytokinesis, while exogenous hGSH was found to inhibit the same process. We conclude that GSH and hGSH have distinct functional roles in cell cycle regulation in Medicago sativa L. GSH is associated with meristemic cells, and promotes cell cycle activation and induction of somatic embryogenesis, while hGSH is associated with differentiated cells and embryo proliferation.

Ultraviolet-A radiation (UV-A: 315–400 nm) is a component of solar radiation that exerts a wide range of physiological responses in plants. Currently, field attenuation experiments are the most reliable source of information on the effects of UV-A. Common plant responses to UV-A include both inhibitory and stimulatory effects on biomass accumulation and morphology. UV-A effects on biomass accumulation can differ from those on root: shoot ratio, and distinct responses are described for different leaf tissues. Inhibitory and enhancing effects of UV-A on photosynthesis are also analysed, as well as activation of photoprotective responses, including UV-absorbing pigments. UV-A-induced leaf flavonoids are highly compound-specific and species-dependent. Many of the effects on growth and development exerted by UV-A are distinct to those triggered by UV-B and vary considerably in terms of the direction the response takes. Such differences may reflect diverse UV-perception mechanisms with multiple photoreceptors operating in the UV-A range and/or variations in the experimental approaches used. This review highlights a role that various photoreceptors (UVR8, phototropins, phytochromes and cryptochromes) may play in plant responses to UV-A when dose, wavelength and other conditions are taken into account.

Ombrogenic Atlantic salt marshes are defined as areas of halophytic, terrestrial vegetation which are periodically flooded by the tide and have a predominant underlying organic substrate comprising of wood and/or Sphagnum peat that formed under freshwater conditions. The objective of this study was to determine to what extent salt marsh plant ecology and, specifically, vegetation composition and zonation relate to this underlying substrate of organic matter (peat). A vegetation survey was carried out on nine salt marshes, three on peat substrate and two on sand, mud and sand/mud, respectively. In parallel, key edaphic variables were measured including pH, conductivity, organic content, moisture content and nutrients: ammonium, nitrate and phosphorus. Salt marshes on peat substrate are distinct. Ammonium content was twice the maximum reported in other salt marsh studies, while the vegetation composition of salt marshes on peat substrate was significantly different from that of other salt marshes. Salt marshes on peat substrate were found to be higher in species diversity and richness and characterised by a predominantly forb and rush community. However, some common salt marsh species, such as Atriplex portulacoides and Spartina anglica were absent from salt marshes on peat. Ordination analysis revealed that zonation was primarily associated with conductivity on peat substrates. In contrast, moisture plays a greater role in zonation within non-peat salt marshes. The findings confirm that the high organic matter content of ombrogenic Atlantic salt marshes is associated with distinct vegetation composition.

Exposure to UV-B radiation can cause stress in plants Solar UV-B radiation (280 315 nm) has long been recognized as being potentially damaging to living organisms. Indeed, the literature on plant UV-B radiation effects has for decades been dominated by reports on UV-B mediated stress, including growth retardation, macroscopic injuries and oxidative damage (Caldwell et al., 1994; Searles et al., 2001). These negative effects comprise damaging effects on genetic material (formation pre-mutagenic cyclobutane pyrimidine dimers [CPD] and pyrimidine [6-4] pyrimidinone dimers), photosynthetic performance (in-tandem degradation of D1-D2 core proteins of photosystem II, inactivation RUBISCO, altered stomatal function) and a range of other cellular targets (see Jansen et al., 1998; Searles et al., 2001; Jordan 2002; Rozema et al., 2005; Jenkins, 2009). In this issue Lidon et al. (2012a) review our current understanding of the deleterious effects of UV-B on photosynthesis. UV-B damage is paralleled by the formation of lipid peroxidation products such as malondialdehyde (MDA) (Hideg et al., 2003; Lidon and Ramalho, 2011) and increased oxidation of antioxidants such as glutathione (Kalbin et al., 1997), both of which reflect the oxidative character of the UV-caused stress conditions. Consistently, UV-induced ROS have been measured using EPR spin trap reporters in both leaves (Hideg and Vass, 1996) and in isolated thylakoids (Lidon et al., 2012b) exposed to high doses of UV-B.

Pronounced altitudinal and latitudinal UV-B gradients exist across the earth. Therefore, we hypothesised thatplants from different geographic origins differ in the regulation and/or magnitude of UV-protection. EightArabidopsis accessions with different geographic origins (altitude between 32 and 3016 m) were raised underPhotosynthetic Active Radiation (PAR), PAR + UV-A or PAR + UV-A + UV-B radiation for 10 days, afterwhich UV-B protection of photosynthesis was assessed by measuring the consequences of exposure to a pulseof acute UV-B. We found significant variation in UV-B protection among accessions exposed to PAR or PAR +UV-A. Yet, all accessions raised under PAR + UV-A + UV-B were well protected. Thus, differences betweenaccessions are not about UV-B protection per se, but rather about regulation of UV-B protection which variesfrom constitutive to inducible by UV-A and/or UV-B. Particularly striking are differential UV-A responses,whereby some high altitude accessions lack UV-A regulated accumulation of UV-absorbing pigments, but showa strong UV-A induced m

Chlorophyll-a fluorescence is an effective tool for measuring photosynthesis. Photosynthesis is particularly sensitive to the presence of zinc or other metals. Plant age and developmental stage strongly influence the effects of metals on the photosynthetic apparatus. However, there is little information on how this may affect toxicological assessment in plants. In this study, three Lemnaceae species were exposed to a range of zinc concentration in static, batch exposures for seven days, as detailed for Lemnaceae toxicity testing in OECD guidelines. Effects of zinc on photosynthetic performance in the three species were measured over the seven days. The maximum quantum efficiency of photosystem II, Fv/Fm, the effective quantum efficiency, Y(II), and photochemical quenching, qP, were measured in mature and young fronds, as well as along a developmental gradient within fronds. Fv/Fm and Y(II) in young, emerging Landoltia punctata fronds were more severely impacted by zinc than in developed, mature fronds. Furthermore, younger proximal sections of L. punctata fronds were more impacted than older distal frond sections. Overall, Fv/Fm and Y(II) also tended to be more affected by zinc in young, compared to mature, Lemna gibba and Lemna minor fronds. Single colony, time-point or leaf-zone analyses may not, therefore, show the full biological picture of the impact of a toxicant, while species-specific differences need also to be considered. Therefore, selection of Lemnaceae species and developmental stages of fronds should be given careful consideration when using chlorophyll-a fluorometry for assessing chemical toxicity in Lemnaceae.

Epidemiological studies have revealed an inverse association between the consumption of fruit, vegetables, and herbs and the risk of both cancer and cardiovascular disease. This protective effect is mostly due to secondary metabolites present in plant tissues. During the last decade, it has become increasingly clear that UV-B radiation is an important regulator of plant secondary metabolism. Low, ecologically-relevant UV-B levels trigger distinct changes in the accumulation of, among others, phenolic compounds, carotenoids and glucosinolates. Fundamental understanding of plant UV-B perception and responses opens up new opportunities for crop manipulation. Thus, targeted low dosage UV-B radiation treatments as emerging technology may be used to generate fruit, vegetables, and herbs enriched with secondary plant metabolites for either fresh consumption or as a source for functional foods and nutraceuticals, resulting in increased ingestion of these health-promoting substances. The UV-B induced accumulation of secondary plant metabolites is likely to have evolved as a plant defense response against harmful UV-B radiation. However, UV-B induced secondary metabolites also alter other trophic interactions, for example by altering plant herbivore resistance. Thus, UV-B driven metabolic changes in the plant's secondary metabolism have benefits for both ends of the bio-based food chain, i.e., for plants themselves as well as for humans.

Epidemiological studies have revealed an inverse association between a high consumption of vegetables and a lower risk of both cancer and cardiovascular diseases. This protective effect is mostly due to secondary plant metabolites present in plant tissues. In this context, it has become increasingly clear during the last decade that UV-B radiation is an important regulator of plant secondary metabolism. Recent studies have highlighted the regulatory properties of low, ecologically relevant UV-B levels contrary to previous studies in which UV-B radiation was exclusively regarded as a stress factor. Low-dosage UV-B applications trigger distinct changes in the plant’s secondary metabolism resulting in an accumulation of phenolic compounds such as flavonoids and glucosinolates.

Lemnaceae are currently the only freshwater plants required for regulatory toxicity testing of pesticides and other chemicals. Toxicological protocols allow for the use of different Lemnaceae species in tests. However, few studies have compared the relative sensitivity of individual duckweed species. Zinc is an essential plant nutrient but is also a common pollutant in aquatic environments and elevated levels are phytotoxic. This study shows that four species of Lemnaceae differ in their relative sensitivities to zinc sulphate, a commonly used reference chemical. Comparative zinc sensitivity, in order, from most tolerant to most sensitive was: Landoltia punctata > Lemna minor > Wolffia brasiliensis > Lemna gibba. Zinc sensitivity was also endpoint dependant. EC50 values typically increased in order of: specific biomass growth rate

Because of their sessile life style, plants have evolved the ability to adjust to environmentally harsh conditions. An important aspect of stress adaptation involves the reprogramming of the cell cycle to ensure optimal growth. The atypical E2F transcription factor DP-E2F-like 1 (E2Fe/DEL1) had been found previously to be an important regulator of the endocycle onset. Here, a novel role for E2Fe/DEL1 was identified as a transcriptional repressor of the type-II cyclobutane pyrimidine dimer-photolyase DNA repair gene PHR1. Upon ultraviolet-B (UV-B) treatment, plants knocked out for E2Fe/DEL1 had improved DNA repair abilities when compared with control plants, whereas those overexpressing it performed less well. Better DNA repair allowed E2Fe/DEL1 knockout plants to resume endoreduplication faster than control plants, contributing in this manner to UV-B radiation resistance by compensating the stress-induced reduction in cell number by ploidy-dependent cell growth. As E2Fe/DEL1 levels decreased upon UV-B treatment, we hypothesize that the coordinated transcriptional induction of PHR1 with the endoreduplication onset contributes to the adaptation of plants exposed to UV-B stress.

The rise in ultraviolet-B (UV-B) (280-315 nm) radiation levels, that is a consequence of stratospheric ozone layer depletion, has triggered extensive research on the effects of UV-B on plants. Plants raised under natural sunlight conditions are generally well protected from the potentially harmful effects of UV-B radiation. However, it is mostly unknown to which extent UV protection is constitutive and/or induced. In this study, we have analysed the role of constitutive and inducible protection responses in avoiding UV-B damage to photosystem II of photosynthesis. We have assayed the UV susceptibility of photosystem II in 224 Arabidopsis thaliana accessions from across the Northern hemisphere, and found a continuum of constitutive UV-protection levels, with some accessions being UV sensitive and others UV tolerant. Statistical analysis showed only very weak associations between constitutive UV tolerance and the geographic origin of accessions. Instead, most of the variance in constitutive UV-B protection of photosynthesis is present at the level of local Arabidopsis populations originating in the same geographic and climatic area. The variance in constitutive UV protection is, however, small compared to the amplitude of environmentally induced changes in UV protection. Thus, our data emphasise the importance of inducible responses for the protection of photosystem II against UV-B. Remarkably, the conditions that induce UV-protective responses vary; accessions from lower latitudes were found to switch-on UV defences more readily than those of higher latitudes. Such altered regulation of induction may comprise a suitable adaptation response when levels of a stressor are fluctuating in the short term, but predictable over longer periods.

Plants are sessile organisms, and consequently cannot avoid exposure to stressful environmental conditions. Exposure to mild stress conditions can induce active acclimation responses, while more severe conditions cause metabolic disruptions. A common plant acclimation response to a variety of environmental stressors is the accumulation of antioxidants and secondary metabolites. For example, ultraviolet-B (UV-B) radiation impacts on the levels of a broad range of metabolites, including phenolic, terpenoid and alkaloid compounds. Our survey of the literature reveals that the levels of some of these metabolites increase following UV-B exposure, while those of others decrease, change transiently or are differently affected by low and high UV-doses. This includes several compounds that are pharmacologically active and/or nutritionally important. We conclude that the complex patterns of stress-induced changes in plant metabolites need to be studied in more detail to determine impacts on the nutritional and pharmacological characteristics of food products. Claims that UV-B acclimated plants have nutritional benefits are currently unproven.

Ultraviolet B (UV-B) acclimation comprises complex and poorly understood changes in plant metabolism. The effects of chronic and ecologically relevant UV-B dose rates on Arabidopsis thaliana were determined. The UV-B acclimation process was studied by measuring radiation effects on morphology, physiology, biochemistry and gene expression. Chronic UV-B radiation did not affect photosynthesis or the expression of stress responsive genes, which indicated that the UV-acclimated plants were not stressed. UV-induced morphological changes in acclimated plants included decreased rosette diameter, decreased inflorescence height and increased numbers of flowering stems, indicating that chronic UV-B treatment caused a redistribution rather than a cessation of growth. Gene expression profiling indicated that UV-induced morphogenesis was associated with subtle changes in phytohormone (auxins, brassinosteroids and gibberellins) homeostasis and the cell wall. Based on the comparison of gene expression profiles, it is concluded that acclimation to low, chronic dose rates of UV-B is distinct from that to acute, stress-inducing UV-B dose rates. Hence, UV-B-induced morphogenesis is functionally uncoupled from stress responses.