Publications by authors named "Jennifer C McElwain"

35 Publications

Enhancing the productivity of ryegrass at elevated CO2 is dependent on tillering and leaf area development rather than leaf-level photosynthesis.

J Exp Bot 2021 Feb;72(5):1962-1977

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, Ireland.

Whilst a range of strategies have been proposed for enhancing crop productivity, many recent studies have focused primarily on enhancing leaf photosynthesis under current atmospheric CO2 concentrations. Given that the atmospheric CO2 concentration is likely to increase significantly in the foreseeable future, an alternative/complementary strategy might be to exploit any variability in the enhancement of growth/yield and photosynthesis at higher CO2 concentrations. To explore this, we investigated the responses of a diverse range of wild and cultivated ryegrass genotypes, with contrasting geographical origins, to ambient and elevated CO2 concentrations and examined what genetically tractable plant trait(s) might be targeted by plant breeders for future yield enhancements. We found substantial ~7-fold intraspecific variations in biomass productivity among the different genotypes at both CO2 levels, which were related primarily to differences in tillering/leaf area, with only small differences due to leaf photosynthesis. Interestingly, the ranking of genotypes in terms of their response to both CO2 concentrations was similar. However, as expected, estimates of whole-plant photosynthesis were strongly correlated with plant productivity. Our results suggest that greater yield gains under elevated CO2 are likely through the exploitation of genetic differences in tillering and leaf area rather than focusing solely on improving leaf photosynthesis.
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http://dx.doi.org/10.1093/jxb/eraa584DOI Listing
February 2021

Past climates inform our future.

Science 2020 11;370(6517)

Department of Oceanography, Texas A&M University, College Station, TX, USA.

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation-a practice that we argue should be widely adopted.
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http://dx.doi.org/10.1126/science.aay3701DOI Listing
November 2020

Carboniferous plant physiology breaks the mold.

New Phytol 2020 08 8;227(3):667-679. Epub 2020 Apr 8.

Center for Integrative Geosciences, University of Connecticut, Storrs, CT, 06269, USA.

How plants have shaped Earth surface feedbacks over geologic time is a key question in botanical and geological inquiry. Recent work has suggested that biomes during the Carboniferous Period contained plants with extraordinary physiological capacity to shape their environment, contradicting the previously dominant view that plants only began to actively moderate the Earth's surface with the rise of angiosperms during the Mesozoic Era. A recently published Viewpoint disputes this recent work, thus here, we document in detail, the mechanistic underpinnings of our modeling and illustrate the extraordinary ecophysiological nature of Carboniferous plants.
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http://dx.doi.org/10.1111/nph.16460DOI Listing
August 2020

Rising CO drives divergence in water use efficiency of evergreen and deciduous plants.

Sci Adv 2019 12 11;5(12):eaax7906. Epub 2019 Dec 11.

Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland.

Intrinsic water use efficiency (iWUE), defined as the ratio of photosynthesis to stomatal conductance, is a key variable in plant physiology and ecology. Yet, how rising atmospheric CO concentration affects iWUE at broad species and ecosystem scales is poorly understood. In a field-based study of 244 woody angiosperm species across eight biomes over the past 25 years of increasing atmospheric CO (~45 ppm), we show that iWUE in evergreen species has increased more rapidly than in deciduous species. Specifically, the difference in iWUE gain between evergreen and deciduous taxa diverges along a mean annual temperature gradient from tropical to boreal forests and follows similar observed trends in leaf functional traits such as leaf mass per area. Synthesis of multiple lines of evidence supports our findings. This study provides timely insights into the impact of Anthropocene climate change on forest ecosystems and will aid the development of next-generation trait-based vegetation models.
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http://dx.doi.org/10.1126/sciadv.aax7906DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6905860PMC
December 2019

A Novel Hypothesis for the Role of Photosynthetic Physiology in Shaping Macroevolutionary Patterns.

Plant Physiol 2019 11 4;181(3):1148-1162. Epub 2019 Sep 4.

Botany Department, School of Natural Sciences, Trinity College, Dublin 2, Ireland.

The fossil record and models of atmospheric concentrations of O and CO suggest that past shifts in plant ecological dominance often coincided with dramatic changes in Earth's atmospheric composition. This study tested the effects of past changes in atmospheric composition on the photosynthetic physiology of a limited range of early-diverging angiosperms (eight), gymnosperms (three), and ferns (two). We performed physiological measurements on all species and used the results to parameterize simulations of their photosynthetic paleophysiology using three independent modeling approaches. Unique physiological attributes were identified for the three evolutionary groups: angiosperm taxa displayed significantly higher mesophyll conductance (), yet their stomatal conductance () was lower than that of ferns. Gymnosperm taxa displayed low and , but they partially offset their significant diffusional limitations on photosynthesis through their higher maximum Rubisco carboxylation rate. Despite their high total conductance to CO, fern taxa lacked an optimized control of , which was reflected in their low intrinsic water use efficiency. Simulations of the photosynthetic physiology of ferns, angiosperms, and gymnosperms through Earth's history demonstrated that past fluctuations in O and CO concentrations may have resulted in significant shifts in the relative competitiveness of the three evolutionary groups. Although preliminary because of limited species sampling, these findings hint at a potential mechanistic basis for the observed broad temporal correlation between atmospheric change and shifts in plant evolutionary group-level richness observed in the fossil record and are presented as a framework to be tested with paleophotosynthetic proxies and through increased species sampling.
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http://dx.doi.org/10.1104/pp.19.00749DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6836816PMC
November 2019

Convergence in Maximum Stomatal Conductance of C Woody Angiosperms in Natural Ecosystems Across Bioclimatic Zones.

Front Plant Sci 2019 7;10:558. Epub 2019 May 7.

Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland.

Stomatal conductance () in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level using porometry in 218 C woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum () across bioclimatic zones and whether there was similarity in the mean of C3 woody angiosperms across ecosystem types. We also tested for differential in two broadly defined habitat groups - open-canopy and understory-subcanopy - within and across bioclimatic zones. We found strong convergence in mean of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean in open-canopy habitats (266 ± 100 mmol m s) was significantly higher than in understory-subcanopy habitats (233 ± 86 mmol m s). There was also a central tendency in the overall dataset to operate toward a of ∼250 mmol m s. We suggest that the observed convergence in mean of C3 woody angiosperms in the understory-subcanopy is due to a buffering of against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of to categorize vegetation according to habitat group.
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http://dx.doi.org/10.3389/fpls.2019.00558DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6514322PMC
May 2019

Higher species richness enhances yield stability in intensively managed grasslands with experimental disturbance.

Sci Rep 2018 10 9;8(1):15047. Epub 2018 Oct 9.

Teagasc, Environment Research Centre, Johnstown Castle, Wexford, Ireland.

Climate models predict increased frequency and severity of drought events. At an Irish and Swiss site, experimental summer droughts were applied over two successive years to grassland plots sown with one, two or four grassland species with contrasting functional traits. Mean yield and plot-to-plot variance of yield were measured across harvests during drought and after a subsequent post-drought recovery period. At both sites, there was a positive relationship between species richness and yield. Under rainfed control conditions, mean yields of four-species communities were 32% (Wexford, Ireland) and 51% (Zürich, Switzerland) higher than in monocultures. This positive relationship was also evident under drought, despite significant average yield reductions (-27% at Wexford; -21% at Zürich). Four-species communities had lower plot-to-plot variance of yield compared to monoculture or two-species communities under both rainfed and drought conditions, which demonstrates higher yield stability in four-species communities. At the Swiss but not the Irish site, a high degree of species asynchrony could be identified as a mechanism underlying increased temporal stability in four-species communities. These results indicate the high potential of multi-species grasslands as an adaptation strategy against drought events and help achieve sustainable intensification under both unperturbed and perturbed environmental conditions.
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http://dx.doi.org/10.1038/s41598-018-33262-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6177466PMC
October 2018

Paleobotany and Global Change: Important Lessons for Species to Biomes from Vegetation Responses to Past Global Change.

Annu Rev Plant Biol 2018 04;69:761-787

Botany Department, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland; email:

Human carbon use during the next century will lead to atmospheric carbon dioxide concentrations (pCO) that have been unprecedented for the past 50-100+ million years according to fossil plant-based CO estimates. The paleobotanical record of plants offers key insights into vegetation responses to past global change, including suitable analogs for Earth's climatic future. Past global warming events have resulted in transient poleward migration at rates that are equivalent to the lowest climate velocities required for current taxa to keep pace with climate change. Paleobiome reconstructions suggest that the current tundra biome is the biome most threatened by global warming. The common occurrence of paleoforests at high polar latitudes when pCO was above 500 ppm suggests that the advance of woody shrub and tree taxa into tundra environments may be inevitable. Fossil pollen studies demonstrate the resilience of wet tropical forests to global change up to 700 ppm CO, contrary to modeled predictions of the future. The paleobotanical record also demonstrates a high capacity for functional trait evolution as an additional strategy to migration and maintenance of a species' climate envelope in response to global change.
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http://dx.doi.org/10.1146/annurev-arplant-042817-040405DOI Listing
April 2018

Palaeobotany: New ways with old fossils.

Nat Plants 2017 07 31;3:17121. Epub 2017 Jul 31.

School of Biology and Environmental Science, Earth Institute, University College Dublin, Ireland.

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http://dx.doi.org/10.1038/nplants.2017.121DOI Listing
July 2017

Dynamic Carboniferous tropical forests: new views of plant function and potential for physiological forcing of climate.

New Phytol 2017 Sep 25;215(4):1333-1353. Epub 2017 Jul 25.

Center for Integrative Geosciences, University of Connecticut, Storrs, CT, 06269, USA.

Contents 1333 I. 1334 II. 1335 III. 1339 IV. 1344 V. 1347 VI. 1347 1348 1348 References 1348 SUMMARY: The Carboniferous, the time of Earth's penultimate icehouse and widespread coal formation, was dominated by extinct lineages of early-diverging vascular plants. Studies of nearest living relatives of key Carboniferous plants suggest that their physiologies and growth forms differed substantially from most types of modern vegetation, particularly forests. It remains a matter of debate precisely how differently and to what degree these long-extinct plants influenced the environment. Integrating biophysical analysis of stomatal and vascular conductivity with geochemical analysis of fossilized tissues and process-based ecosystem-scale modeling yields a dynamic and unique perspective on these paleoforests. This integrated approach indicates that key Carboniferous plants were capable of growth and transpiration rates that approach values found in extant crown-group angiosperms, differing greatly from comparatively modest rates found in their closest living relatives. Ecosystem modeling suggests that divergent stomatal conductance, leaf sizes and stem life span between dominant clades would have shifted the balance of soil-atmosphere water fluxes, and thus surface runoff flux, during repeated, climate-driven, vegetation turnovers. This synthesis highlights the importance of 'whole plant' physiological reconstruction of extinct plants and the potential of vascular plants to have influenced the Earth system hundreds of millions of years ago through vegetation-climate feedbacks.
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http://dx.doi.org/10.1111/nph.14700DOI Listing
September 2017

Paleoecology, Ploidy, Paleoatmospheric Composition, and Developmental Biology: A Review of the Multiple Uses of Fossil Stomata.

Plant Physiol 2017 06 11;174(2):650-664. Epub 2017 May 11.

Earth Institute, O'Brien Centre for Science, and School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland (J.C.M.).

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http://dx.doi.org/10.1104/pp.17.00204DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5462064PMC
June 2017

Co-ordination in Morphological Leaf Traits of Early Diverging Angiosperms Is Maintained Following Exposure to Experimental Palaeo-atmospheric Conditions of Sub-ambient O and Elevated CO.

Front Plant Sci 2016 15;7:1368. Epub 2016 Sep 15.

Earth Institute, O'Brien Centre for Science, University College DublinDublin, Ireland; School of Biology and Environmental Science, University College DublinDublin, Ireland.

In order to be successful in a given environment a plant should invest in a vein network and stomatal distribution that ensures balance between both water supply and demand. Vein density () and stomatal density (SD) have been shown to be strongly positively correlated in response to a range of environmental variables in more recently evolved plant species, but the extent of this relationship has not been confirmed in earlier diverging plant lineages. In order to examine the effect of a changing atmosphere on the relationship between and SD, five early-diverging plant species representing two different reproductive plant grades were grown for 7 months in a palaeo-treatment comprising an O:CO ratio that has occurred multiple times throughout plant evolutionary history. Results show a range of species-specific and SD responses to the palaeo-treatment, however, we show that the strong relationship between and SD under modern ambient atmospheric composition is maintained following exposure to the palaeo-treatment. This suggests strong inter-specific co-ordination between vein and stomatal traits for our study species even under relatively extreme environmental change. This co-ordination supports existing plant function proxies that use the distance between vein endings and stomata () to infer plant palaeo-physiology.
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http://dx.doi.org/10.3389/fpls.2016.01368DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5023689PMC
September 2016

Does Size Matter? Atmospheric CO2 May Be a Stronger Driver of Stomatal Closing Rate Than Stomatal Size in Taxa That Diversified under Low CO2.

Front Plant Sci 2016 24;7:1253. Epub 2016 Aug 24.

Earth Institute, Science Centre East, School of Biology and Environmental Science, University College Dublin Dublin, Ireland.

One strategy for plants to optimize stomatal function is to open and close their stomata quickly in response to environmental signals. It is generally assumed that small stomata can alter aperture faster than large stomata. We tested the hypothesis that species with small stomata close faster than species with larger stomata in response to darkness by comparing rate of stomatal closure across an evolutionary range of species including ferns, cycads, conifers, and angiosperms under controlled ambient conditions (380 ppm CO2; 20.9% O2). The two species with fastest half-closure time and the two species with slowest half-closure time had large stomata while the remaining three species had small stomata, implying that closing rate was not correlated with stomatal size in these species. Neither was response time correlated with stomatal density, phylogeny, functional group, or life strategy. Our results suggest that past atmospheric CO2 concentration during time of taxa diversification may influence stomatal response time. We show that species which last diversified under low or declining atmospheric CO2 concentration close stomata faster than species that last diversified in a high CO2 world. Low atmospheric [CO2] during taxa diversification may have placed a selection pressure on plants to accelerate stomatal closing to maintain adequate internal CO2 and optimize water use efficiency.
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http://dx.doi.org/10.3389/fpls.2016.01253DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4996050PMC
September 2016

Evolutionary trade-offs in stomatal spacing.

New Phytol 2016 06;210(4):1149-51

School of Biology and Environmental Science, The Earth Institute, O'Brien Centre for Science, University College Dublin, Belfield, Dublin 4, Ireland.

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http://dx.doi.org/10.1111/nph.13972DOI Listing
June 2016

How well do you know your growth chambers? Testing for chamber effect using plant traits.

Plant Methods 2015 22;11:44. Epub 2015 Sep 22.

School of Biology and Environmental Science, Earth Institute, O'Brien Centre for Science, University College Dublin, Belfield, Dublin 4, Ireland.

Background: Plant growth chambers provide a controlled environment to analyse the effects of environmental parameters (light, temperature, atmospheric gas composition etc.) on plant function. However, it has been shown that a 'chamber effect' may exist whereby results observed are not due to an experimental treatment but to inconspicuous differences in supposedly identical chambers. In this study, Vicia faba L. 'Aquadulce Claudia' (broad bean) plants were grown in eight walk-in chambers to establish if a chamber effect existed, and if so, what plant traits are best for detecting such an effect. A range of techniques were used to measure differences between chamber plants, including chlorophyll fluorescence measurements, gas exchange analysis, biomass, reproductive yield, anatomical traits and leaf stable carbon isotopes.

Results And Discussion: Four of the eight chambers exhibited a chamber effect. In particular, we identified two types of chamber effect which we term 'resolvable' or 'unresolved'; a resolvable chamber effect is caused by malfunctioning components of a chamber and an unresolved chamber effect is caused by unknown factors that can only be mitigated by appropriate experimental design and sufficient replication. Not all measured plant traits were able to detect a chamber effect and no single trait was capable of detecting all chamber effects. Fresh weight and flower count detected a chamber effect in three chambers, stable carbon isotopes (δ(13)C) and net rate CO2 assimilation (An) identified a chamber effect in two chambers, stomatal conductance (gs) and total performance index detected an effect only in one chamber.

Conclusion: (1) Chamber effects can be adequately detected by fresh weight measurements and flower counts on Vicia faba plants. These methods were the most effective in terms of detection and most efficient in terms of time. (2) δ(13)C, gs and An measurements help distinguish between resolvable and unresolved chamber effects. (3) Unresolved chamber effects require experimental unit replication while resolvable chamber effects require investigation, repair and retesting in advance of initiating further experiments.
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http://dx.doi.org/10.1186/s13007-015-0088-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4578792PMC
September 2015

Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution.

New Phytol 2016 Jan 31;209(1):94-103. Epub 2015 Jul 31.

School of Biological Science, University of Essex, Colchester, CO4 3SQ, UK.

Understanding the drivers of geological-scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function. Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (gmax ) and leaf vein density (Dv ) and physiological measurements including operational stomatal conductance (gop ), saturated (Asat ) and maximum (Amax ) assimilation rates were investigated for 18 extant taxa in order to improve understanding of angiosperm diversification in the Cretaceous. Our study demonstrated significant relationships between gop , gmax and Dv that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high gmax in angiosperms conferred a competitive advantage over gymnosperms by increasing the dynamic range (plasticity) of their gas exchange and expanding their ecophysiological niche space. We suggest that species with a high gmax (> 1400 mmol m(-2) s(-1) ) would have been capable of maintaining a high Amax as the atmospheric CO2 declined through the Cretaceous, whereas gymnosperms with a low gmax would experience severe photosynthetic penalty. Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high gmax , Dv and increased plasticity in gop , adds further functional insights into the mechanisms driving angiosperm speciation.
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http://dx.doi.org/10.1111/nph.13579DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5014202PMC
January 2016

Genome-wide transcriptomic analysis of the effects of sub-ambient atmospheric oxygen and elevated atmospheric carbon dioxide levels on gametophytes of the moss, Physcomitrella patens.

J Exp Bot 2015 Jul 6;66(13):4001-12. Epub 2015 May 6.

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland UCD Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland.

It is widely accepted that atmospheric O2 has played a key role in the development of life on Earth, as evident from the coincidence between the rise of atmospheric O2 concentrations in the Precambrian and biological evolution. Additionally, it has also been suggested that low atmospheric O2 is one of the major drivers for at least two of the five mass-extinction events in the Phanerozoic. At the molecular level, our understanding of the responses of plants to sub-ambient O2 concentrations is largely confined to studies of the responses of underground organs, e.g. roots to hypoxic conditions. Oxygen deprivation often results in elevated CO2 levels, particularly under waterlogged conditions, due to slower gas diffusion in water compared to air. In this study, changes in the transcriptome of gametophytes of the moss Physcomitrella patens arising from exposure to sub-ambient O2 of 13% (oxygen deprivation) and elevated CO2 (1500 ppmV) were examined to further our understanding of the responses of lower plants to changes in atmospheric gaseous composition. Microarray analyses revealed that the expression of a large number of genes was affected under elevated CO2 (814 genes) and sub-ambient O2 conditions (576 genes). Intriguingly, the expression of comparatively fewer numbers of genes (411 genes) was affected under a combination of both sub-ambient O2 and elevated CO2 condition (low O2-high CO2). Overall, the results point towards the effects of atmospheric changes in CO2 and O2 on transcriptional reprogramming, photosynthetic regulation, carbon metabolism, and stress responses.
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http://dx.doi.org/10.1093/jxb/erv197DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4473992PMC
July 2015

The severity of wheat diseases increases when plants and pathogens are acclimatized to elevated carbon dioxide.

Glob Chang Biol 2015 Jul 20;21(7):2661-2669. Epub 2015 Apr 20.

UCD Earth Institute and School of Biology and Environmental Science, College of Science, University College Dublin, Belfield, Dublin 4, Ireland.

Wheat diseases present a constant and evolving threat to food security. We have little understanding as to how increased atmospheric carbon dioxide levels will affect wheat diseases and thus the security of grain supply. Atmospheric CO exceeded the 400 ppmv benchmark in 2013 and is predicted to double or even treble by the end of the century. This study investigated the impact of both pathogen and wheat acclimation to elevated CO on the development of Fusarium head blight (FHB) and Septoria tritici blotch (STB) disease of wheat. Here, plants and pathogens were cultivated under either 390 or 780 ppmv CO for a period (two wheat generations, multiple pathogen subcultures) prior to standard disease trials. Acclimation of pathogens and the wheat cultivar Remus to elevated CO increased the severity of both STB and FHB diseases, relative to ambient conditions. The effect of CO on disease development was greater for FHB than for STB. The highest FHB disease levels and associated yield losses were recorded for elevated CO -acclimated pathogen on elevated CO -acclimated wheat. When similar FHB experiments were conducted using the disease-resistant cultivar CM82036, pathogen acclimation significantly enhanced disease levels and yield loss under elevated CO conditions, thereby indicating a reduction in the effectiveness of the defence pathways innate to this wheat cultivar. We conclude that acclimation to elevated CO over the coming decades will have a significant influence on the outcome of plant-pathogen interactions and the durability of disease resistance.
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http://dx.doi.org/10.1111/gcb.12899DOI Listing
July 2015

New methods reveal oldest known fossil epiphyllous moss: Bryiidites utahensis gen. et sp. nov. (Bryidae).

Am J Bot 2013 Dec 3;100(12):2450-7. Epub 2013 Dec 3.

Northwestern University, Department of Earth & Planetary Sciences, Technological Institute, 2145 Sheridan Road, Evanston, Illinois 60208-3130 USA.

Premise Of The Study: Epiphyllous bryophytes are a highly characteristic feature of many humid tropical forest ecosystems. In contrast to the extensive fossil record for the leaves of their host plants, the record is virtually nonexistent for the epiphylls themselves, despite a fossil record for mosses that begins in the Middle Carboniferous Period, 330 million years ago.

Methods: Epifluorescence optical microscopy, scanning electron microscopy, and atomic force microscopy were employed to investigate an intimate association between a newly discovered epiphyllous moss and a Lauraceae plant host from the middle Cretaceous.

Key Results: We describe the oldest fossil specimen of an epiphyllous moss, Bryiidites utahensis gen. et sp. nov., identified from an individual specimen only 450 µm long, situated on an approximately one millimeter square fossil leaf fragment. The moss epiphyll is exquisitely preserved as germinating spores and short-celled protonemata with transverse and oblique cross-walls closely matching those of extant epiphyllous mosses on the surface of the plant-leaf hosts.

Conclusions: The extension of the epiphyll record back to the middle Cretaceous provides fossil evidence for the appearance of epiphyllous mosses during the diversification of flowering plants, at least 95 million years ago. It also provides substantive evidence for a tropical maritime climate in central North America during the middle Cretaceous.
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http://dx.doi.org/10.3732/ajb.1300209DOI Listing
December 2013

Increased atmospheric SO₂ detected from changes in leaf physiognomy across the Triassic-Jurassic boundary interval of East Greenland.

PLoS One 2013 10;8(4):e60614. Epub 2013 Apr 10.

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, Ireland.

The Triassic-Jurassic boundary (Tr-J; ∼201 Ma) is marked by a doubling in the concentration of atmospheric CO2, rising temperatures, and ecosystem instability. This appears to have been driven by a major perturbation in the global carbon cycle due to massive volcanism in the Central Atlantic Magmatic Province. It is hypothesized that this volcanism also likely delivered sulphur dioxide (SO2) to the atmosphere. The role that SO2 may have played in leading to ecosystem instability at the time has not received much attention. To date, little direct evidence has been presented from the fossil record capable of implicating SO2 as a cause of plant extinctions at this time. In order to address this, we performed a physiognomic leaf analysis on well-preserved fossil leaves, including Ginkgoales, bennettites, and conifers from nine plant beds that span the Tr-J boundary at Astartekløft, East Greenland. The physiognomic responses of fossil taxa were compared to the leaf size and shape variations observed in nearest living equivalent taxa exposed to simulated palaeoatmospheric treatments in controlled environment chambers. The modern taxa showed a statistically significant increase in leaf roundness when fumigated with SO2. A similar increase in leaf roundness was also observed in the Tr-J fossil taxa immediately prior to a sudden decrease in their relative abundances at Astartekløft. This research reveals that increases in atmospheric SO2 can likely be traced in the fossil record by analyzing physiognomic changes in fossil leaves. A pattern of relative abundance decline following increased leaf roundness for all six fossil taxa investigated supports the hypothesis that SO2 had a significant role in Tr-J plant extinctions. This finding highlights that the role of SO2 in plant biodiversity declines across other major geological boundaries coinciding with global scale volcanism should be further explored using leaf physiognomy.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0060614PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3622679PMC
October 2013

Tracking taphonomic regimes using chemical and mechanical damage of pollen and spores: an example from the Triassic-Jurassic mass extinction.

PLoS One 2012 7;7(11):e49153. Epub 2012 Nov 7.

Department of Plant Biology, University of Illinois, Urbana, Illinois, United States of America.

The interpretation of biotic changes in the geological past relies on the assumption that samples from different time intervals represent an equivalent suite of natural sampling conditions. As a result, detailed investigations of taphonomic regimes during intervals of major biotic upheaval, such as mass extinctions, are crucial. In this paper, we have used variations in the frequency of chemical and mechanical sporomorph (pollen and spore) damage as a guide to taphonomic regimes across the Triassic-Jurassic mass extinction (Tr-J; ∼201.3 Ma) at a boundary section at Astartekløft, East Greenland. We find that the frequency of sporomorph damage is extremely variable in samples from this locality. This likely reflects a combination of taxon-specific susceptibility to damage and the mixing of sporomorphs from a mosaic of environments and taphonomic regimes. The stratigraphic interval containing evidence of plant extinction and compositional change in the source vegetation at Astartekløft is not marked by a consistent rise or fall in the frequency of sporomorph damage. This indicates that natural taphonomic regimes did not shift radically during this critical interval. We find no evidence of a consistent relationship between the taxonomic richness of sporomorph assemblages and the frequency of damage among sporomorphs at Astartekløft. This indicates that previously reported patterns of sporomorph richness across the Tr-J at this locality are likely to be robust. Taken together, our results suggest that the patterns of vegetation change at Astartekløft represent a real biological response to environmental change at the Tr-J.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0049153PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3492321PMC
May 2013

Co-ordination of physiological and morphological responses of stomata to elevated [CO2] in vascular plants.

Oecologia 2013 Jan 19;171(1):71-82. Epub 2012 Jul 19.

CNR, Istituto di Biometeorologia (IBIMET), Florence, Italy.

Plant stomata display a wide range of short-term behavioural and long-term morphological responses to atmospheric carbon dioxide concentration ([CO(2)]). The diversity of responses suggests that plants may have different strategies for controlling gas exchange, yet it is not known whether these strategies are co-ordinated in some way. Here, we test the hypothesis that there is co-ordination of physiological (via aperture change) and morphological (via stomatal density change) control of gas exchange by plants. We examined the response of stomatal conductance (G(s)) to instantaneous changes in external [CO(2)] (C(a)) in an evolutionary cross-section of vascular plants grown in atmospheres of elevated [CO(2)] (1,500 ppm) and sub-ambient [O(2)] (13.0 %) compared to control conditions (380 ppm CO(2), 20.9 % O(2)). We found that active control of stomatal aperture to [CO(2)] above current ambient levels was not restricted to angiosperms, occurring in the gymnosperms Lepidozamia peroffskyana and Nageia nagi. The angiosperm species analysed appeared to possess a greater respiratory demand for stomatal movement than gymnosperm species displaying active stomatal control. Those species with little or no control of stomatal aperture (termed passive) to C(a) were more likely to exhibit a reduction in stomatal density than species with active stomatal control when grown in atmospheres of elevated [CO(2)]. The relationship between the degree of stomatal aperture control to C(a) above ambient and the extent of any reduction in stomatal density may suggest the co-ordination of physiological and morphological responses of stomata to [CO(2)] in the optimisation of water use efficiency. This trade-off between stomatal control strategies may have developed due to selective pressures exerted by the costs associated with passive and active stomatal control.
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http://dx.doi.org/10.1007/s00442-012-2406-9DOI Listing
January 2013

Ferns: a xylem success story.

New Phytol 2011 Oct;192(2):307-10

School of Biology and Environmental Science, University College Dublin, Ireland.

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http://dx.doi.org/10.1111/j.1469-8137.2011.03865.xDOI Listing
October 2011

Stomatal control as a driver of plant evolution.

J Exp Bot 2011 May;62(8):2419-23

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.

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http://dx.doi.org/10.1093/jxb/err086DOI Listing
May 2011

The stomatal CO2 proxy does not saturate at high atmospheric CO2 concentrations: evidence from stomatal index responses of Araucariaceae conifers.

Oecologia 2011 Sep 3;167(1):11-9. Epub 2011 Apr 3.

CNR-Istituto di Biometeorologia (IBIMET), Firenze, Italy.

The inverse relationship between the number of stomata on a leaf surface and the atmospheric carbon dioxide concentration ([CO(2)]) in which the leaf developed allows plants to optimise water-use efficiency (WUE), but it also permits the use of fossil plants as proxies of palaeoatmospheric [CO(2)]. The ancient conifer family Araucariaceae is often represented in fossil floras and may act as a suitable proxy of palaeo-[CO(2)], yet little is known regarding the stomatal index (SI) responses of extant Araucariaceae to [CO(2)]. Four Araucaria species (Araucaria columnaris, A. heterophylla, A. angustifolia and A. bidwillii) and Agathis australis displayed no significant relationship in SI to [CO(2)] below current ambient levels (~380 ppm). However, representatives of the three extant genera within the Araucariaceae (A. bidwillii, A. australis and Wollemia nobilis) all exhibited significant reductions in SI when grown in atmospheres of elevated [CO(2)] (1,500 ppm). Stomatal conductance was reduced and WUE increased when grown under elevated [CO(2)]. Stomatal pore length did not increase alongside reduced stomatal density (SD) and SI in the three araucariacean conifers when grown at elevated [CO(2)]. These pronounced SD and SI reductions occur at higher [CO(2)] levels than in other species with more recent evolutionary origins, and may reflect an evolutionary legacy of the Araucariaceae in the high [CO(2)] world of the Mesozoic Era. Araucariacean conifers may therefore be suitable stomatal proxies of palaeo-[CO(2)] during periods of "greenhouse" climates and high [CO(2)] in the Earth's history.
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http://dx.doi.org/10.1007/s00442-011-1969-1DOI Listing
September 2011

Baseline intrinsic flammability of Earth's ecosystems estimated from paleoatmospheric oxygen over the past 350 million years.

Proc Natl Acad Sci U S A 2010 Dec 13;107(52):22448-53. Epub 2010 Dec 13.

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.

Atmospheric oxygen (O(2)) is estimated to have varied greatly throughout Earth's history and has been capable of influencing wildfire activity wherever fuel and ignition sources were present. Fires consume huge quantities of biomass in all ecosystems and play an important role in biogeochemical cycles. This means that understanding the influence of O(2) on past fire activity has far-reaching consequences for the evolution of life and Earth's biodiversity over geological timescales. We have used a strong electrical ignition source to ignite smoldering fires, and we measured their self-sustaining propagation in atmospheres of different oxygen concentrations. These data have been used to build a model that we use to estimate the baseline intrinsic flammability of Earth's ecosystems according to variations in O(2) over the past 350 million years (Ma). Our aim is to highlight times in Earth's history when fire has been capable of influencing the Earth system. We reveal that fire activity would be greatly suppressed below 18.5% O(2), entirely switched off below 16% O(2), and rapidly enhanced between 19-22% O(2). We show that fire activity and, therefore, its influence on the Earth system would have been high during the Carboniferous (350-300 Ma) and Cretaceous (145-65 Ma) periods; intermediate in the Permian (299-251 Ma), Late Triassic (285-201 Ma), and Jurassic (201-145 Ma) periods; and surprisingly low to lacking in the Early-Middle Triassic period between 250-240 Ma. These baseline variations in Earth's flammability must be factored into our understanding of past vegetation, biodiversity, evolution, and biogeochemical cycles.
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http://dx.doi.org/10.1073/pnas.1011974107DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3012516PMC
December 2010

Ancient atmospheres and the evolution of oxygen sensing via the hypoxia-inducible factor in metazoans.

Physiology (Bethesda) 2010 Oct;25(5):272-9

UCD Conway Institute, Systems Biology Ireland and School of Medicine and Medical Science, University College Dublin, Belfield, Dublin, Ireland.

Metazoan diversification occurred during a time when atmospheric oxygen levels fluctuated between 15 and 30%. The hypoxia-inducible factor (HIF) is a primary regulator of the adaptive transcriptional response to hypoxia. Although the HIF pathway is highly conserved, its complexity increased during periods when atmospheric oxygen concentrations were increasing. Thus atmospheric oxygen levels may have provided a selection force on the development of cellular oxygen-sensing pathways.
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http://dx.doi.org/10.1152/physiol.00029.2010DOI Listing
October 2010

An explanation for conflicting records of Triassic-Jurassic plant diversity.

Proc Natl Acad Sci U S A 2010 Aug 16;107(35):15351-6. Epub 2010 Aug 16.

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.

Macrofossils (mostly leaves) and sporomorphs (pollen and spores) preserve conflicting records of plant biodiversity during the end-Permian (P-Tr), Triassic-Jurassic (Tr-J), and end-Cretaceous (K-T) mass extinctions. Estimates of diversity loss based on macrofossils are typically much higher than estimates of diversity loss based on sporomorphs. Macrofossils from the Tr-J of East Greenland indicate that standing species richness declined by as much as 85% in the Late Triassic, whereas sporomorph records from the same region, and from elsewhere in Europe, reveal little evidence of such catastrophic diversity loss. To understand this major discrepancy, we have used a new high-resolution dataset of sporomorph assemblages from Astartekløft, East Greenland, to directly compare the macrofossil and sporomorph records of Tr-J plant biodiversity. Our results show that sporomorph assemblages from the Tr-J boundary interval are 10-12% less taxonomically diverse than sporomorph assemblages from the Late Triassic, and that vegetation composition changed rapidly in the boundary interval as a result of emigration and/or extirpation of taxa rather than immigration and/or origination of taxa. An analysis of the representation of different plant groups in the macrofossil and sporomorph records at Astartekløft reveals that reproductively specialized plants, including cycads, bennettites and the seed-fern Lepidopteris are almost absent from the sporomorph record. These results provide a means of reconciling the macrofossil and sporomorph records of Tr-J vegetation change, and may help to understand vegetation change during the P-Tr and K-T mass extinctions and around the Paleocene-Eocene Thermal Maximum.
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http://dx.doi.org/10.1073/pnas.1004207107DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2932585PMC
August 2010

Stomatal index responses of Agrostis canina to CO2 and sulphur dioxide: implications for palaeo-[CO2] using the stomatal proxy.

New Phytol 2010 Nov;188(3):845-55

School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.

• Stomatal index values of fossil plants are widely used in reconstructing palaeo-[CO(2)]. This depends upon the assumption that the stomatal index is determined by the atmospheric concentration of CO(2) ([CO(2)]). This study investigates whether fumigation with, and resistance to, sulphur dioxide (SO(2)) induces a reduction in the stomatal index that may affect stomatal reconstructions of palaeo-[CO(2)] coinciding with episodes of global-scale volcanism. • Agrostis canina from Mefite di Ansanto, Italy, grow in atmospheres of elevated-[CO(2)], SO(2) and hydrogen sulphide (H(2)S). Mefite A. canina were compared with a control population in a 'common-garden' experiment and a controlled-environment study under elevated-[CO(2)] and SO(2) fumigation. • In A. canina, resistance to toxic volcanic gases is not associated with reduced stomatal index, and fumigation with SO(2) does not cause a decrease in stomatal initiation. The two populations of A. canina analyzed in this study exhibit different stomatal index-[CO(2)] 'responses', with control plants showing a reduction in stomatal index and Mefite plants showing no response. • Stomatal reconstructions of palaeo-[CO(2)] during past episodes of global-scale volcanism probably reflect atmospheric [CO(2)] and not [SO(2)]. The lack of a reduction in the stomatal index in response to elevated [CO(2)] in the Mefite plants, suggests that resistance to toxic gases and/or long-term growth at high [CO(2)] reduces, or negates, sensitivity of the stomatal index-[CO(2)] relationship, or that stomatal index-[CO(2)] in the Mefite plants is attuned to [CO(2)] fluctuations at much higher concentrations.
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http://dx.doi.org/10.1111/j.1469-8137.2010.03403.xDOI Listing
November 2010

Differences in the response sensitivity of stomatal index to atmospheric CO2 among four genera of Cupressaceae conifers.

Ann Bot 2010 Mar 20;105(3):411-8. Epub 2010 Jan 20.

School of Biology & Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.

Background And Aims: The inverse relationship between stomatal density (SD: number of stomata per mm(2) leaf area) and atmospheric concentration of CO2 ([CO2]) permits the use of plants as proxies of palaeo-atmospheric CO2. Many stomatal reconstructions of palaeo-[CO2] are based upon multiple fossil species. However, it is unclear how plants respond to [CO2] across genus, family or ecotype in terms of SD or stomatal index (SI: ratio of stomata to epidermal cells). This study analysed the stomatal numbers of conifers from the ancient family Cupressaceae, in order to examine the nature of the SI-[CO2] relationship, and potential implications for stomatal reconstructions of palaeo-[CO2]. Methods Stomatal frequency measurements were taken from historical herbarium specimens of Athrotaxis cupressoides, Tetraclinis articulata and four Callitris species, and live A. cupressoides grown under CO2-enrichment (370, 470, 570 and 670 p.p.m. CO2).

Key Results: T. articulata, C. columnaris and C. rhomboidea displayed significant reductions in SI with rising [CO2]; by contrast, A. cupressoides, C. preissii and C. oblonga show no response in SI. However, A. cupressoides does reduce SI to increases in [CO2] above current ambient (approx. 380 p.p.m. CO2). This dataset suggests that a shared consistent SI-[CO2] relationship is not apparent across the genus Callitris. Conclusions The present findings suggest that it is not possible to generalize how conifer species respond to fluctuations in [CO2] based upon taxonomic relatedness or habitat. This apparent lack of a consistent response, in conjunction with high variability in SI, indicates that reconstructions of absolute palaeo-[CO2] based at the genus level, or upon multiple species for discrete intervals of time are not as reliable as those based on a single or multiple temporally overlapping species.
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http://dx.doi.org/10.1093/aob/mcp309DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2826259PMC
March 2010