Publications by authors named "John R Evans"

82 Publications

Wah Soon Chow, a teacher, a friend and a colleague.

Photosynth Res 2021 Aug;149(1-2):253-258

Climate Change Cluster (C3), University of Technology Sydney, Sydney, NSW, 2007, Australia.

To finish this special issue, some friends, colleagues and students of Prof. Chow (Emeritus Professor, the Research School of Biology, the Australian National University) have written small tributes to acknowledge not only his eminent career but to describe his wonderful personality.
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http://dx.doi.org/10.1007/s11120-021-00864-wDOI Listing
August 2021

A consensus on the Aquaporin Gene Family in the Allotetraploid Plant, .

Plant Direct 2021 May 7;5(5):e00321. Epub 2021 May 7.

ARC Centre of Excellence for Translational Photosynthesis Research School of Biology Australian National University Canberra ACT Australia.

Aquaporins (AQPs) are membrane-spanning channel proteins with exciting applications for plant engineering and industrial applications. Translational outcomes will be improved by better understanding the extensive diversity of plant AQPs. However, gene families are complex, making exhaustive identification difficult, especially in polyploid species. The allotetraploid species of (Nt; tobacco) plays a significant role in modern biological research and is closely related to several crops of economic interest, making it a valuable platform for AQP research. Recently, De Rosa et al., (2020) and Ahmed et al., (2020), concurrently reported on the AQP gene family in tobacco, establishing family sizes of 76 and 88 members, respectively. The discrepancy highlights the difficulties of characterizing large complex gene families. Here, we identify and resolve the differences between the two studies, clarify gene models, and yield a consolidated collection of 84 members that more accurately represents the complete NtAQP family. Importantly, this consensus NtAQP collection will reduce confusion and ambiguity that would inevitably arise from having two different descriptive studies and sets of NtAQP gene names. This report also serves as a case study, highlighting and discussing variables to be considered and refinements required to ensure comprehensive gene family characterizations, which become valuable resources for examining the evolution and biological functions of genes.
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http://dx.doi.org/10.1002/pld3.321DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8104905PMC
May 2021

Uncovering candidate genes involved in photosynthetic capacity using unexplored genetic variation in Spring Wheat.

Plant Biotechnol J 2021 08 27;19(8):1537-1552. Epub 2021 Feb 27.

The Earlham Institute, Norwich, UK.

To feed an ever-increasing population we must leverage advances in genomics and phenotyping to harness the variation in wheat breeding populations for traits like photosynthetic capacity which remains unoptimized. Here we survey a diverse set of wheat germplasm containing elite, introgression and synthetic derivative lines uncovering previously uncharacterized variation. We demonstrate how strategic integration of exotic material alleviates the D genome genetic bottleneck in wheat, increasing SNP rate by 62% largely due to Ae. tauschii synthetic wheat donors. Across the panel, 67% of the Ae. tauschii donor genome is represented as introgressions in elite backgrounds. We show how observed genetic variation together with hyperspectral reflectance data can be used to identify candidate genes for traits relating to photosynthetic capacity using association analysis. This demonstrates the value of genomic methods in uncovering hidden variation in wheat and how that variation can assist breeding efforts and increase our understanding of complex traits.
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http://dx.doi.org/10.1111/pbi.13568DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8384606PMC
August 2021

Stomatal, mesophyll conductance, and biochemical limitations to photosynthesis during induction.

Plant Physiol 2021 02;185(1):146-160

Division of Plant Science, Research School of Biology, The Australian National University, Canberra, Territory 2601, Australia.

The dynamics of leaf photosynthesis in fluctuating light affects carbon gain by plants. Mesophyll conductance (gm) limits CO2 assimilation rate (A) under the steady state, but the extent of this limitation under non-steady-state conditions is unknown. In the present study, we aimed to characterize the dynamics of gm and the limitations to A imposed by gas diffusional and biochemical processes under fluctuating light. The induction responses of A, stomatal conductance (gs), gm, and the maximum rate of RuBP carboxylation (Vcmax) or electron transport (J) were investigated in Arabidopsis (Arabidopsis thaliana (L.)) and tobacco (Nicotiana tabacum L.). We first characterized gm induction after a change from darkness to light. Each limitation to A imposed by gm, gs and Vcmax or J was significant during induction, indicating that gas diffusional and biochemical processes limit photosynthesis. Initially, gs imposed the greatest limitation to A, showing the slowest response under high light after long and short periods of darkness, assuming RuBP-carboxylation limitation. However, if RuBP-regeneration limitation was assumed, then J imposed the greatest limitation. gm did not vary much following short interruptions to light. The limitation to A imposed by gm was the smallest of all the limitations for most of the induction phase. This suggests that altering induction kinetics of mesophyll conductance would have little impact on A following a change in light. To enhance the carbon gain by plants under naturally dynamic light environments, attention should therefore be focused on faster stomatal opening or activation of electron transport.
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http://dx.doi.org/10.1093/plphys/kiaa011DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8133641PMC
February 2021

Facilitators and barriers to physical activity participation experienced by Aboriginal and Torres Strait Islander adults: a mixed methods systematic review protocol.

JBI Evid Synth 2020 Dec 31;Publish Ahead of Print. Epub 2020 Dec 31.

School of Population Health, The University of New South Wales, Sydney, NSW, Australia Faculty of Medicine and Health, Poche Centre for Indigenous Health, The University of Sydney, Sydney, NSW, Australia Wardliparingga Aboriginal Health Unit, South Australian Health and Medical Research Institute, Adelaide, SA, Australia Faculty of Health, University of Technology Sydney, Sydney, NSW, Australia Sydney School of Education and Social Work, The University of Sydney, Sydney, NSW, Australia.

Objective: To synthesis the existing research about physical activity and sport facilitators and barriers experienced by Aboriginal and Torres Strait Islander adults in Australia.

Introduction: Physical activity and sport have cultural importance for First Nations peoples. Achieving health and broader benefits from physical activity and sport is impacted by experiences of both facilitators and barriers to participation. Identifying how to facilitate participation and overcome barriers to physical activity and sport is important to develop strategies to increase physical activity levels and sport participation among Aboriginal and Torres Strait Islander adults. Several studies have examined physical activity and sport facilitators and barriers experienced by Aboriginal and Torres Strait Islander adults, and collective synthesis of these studies can provide a more comprehensive understanding of their findings.

Inclusion Criteria: This mixed methods systematic review will consider studies that include Aboriginal and Torres Strait Islander peoples aged 18 years and over from any setting or region of Australia. Studies will be considered if they report on facilitators and barriers to physical activity and/or sport participation.

Methods: Eleven databases will be searched, as well as gray literature sources, including a selection of websites containing resources relevant to physical activity participation for Aboriginal and Torres Strait Islander adults. Studies published in English will be included. No date limits will be set. After screening the titles and abstracts of identified citations, potentially relevant studies will be retrieved in full. Study selection, critical appraisal, data extraction, and data synthesis will be undertaken according to the convergent integrated approach to mixed methods reviews.

Systematic Review Registration Number: PROSPERO CRD42020162134.
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http://dx.doi.org/10.11124/JBIES-20-00339DOI Listing
December 2020

Effect of leaf temperature on the estimation of photosynthetic and other traits of wheat leaves from hyperspectral reflectance.

J Exp Bot 2021 02;72(4):1271-1281

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, The Australian National University, Canberra, ACT, Australia.

A growing number of leaf traits can be estimated from hyperspectral reflectance data. These include structural and compositional traits, such as leaf mass per area (LMA) and nitrogen and chlorophyll content, but also physiological traits such a Rubisco carboxylation activity, electron transport rate, and respiration rate. Since physiological traits vary with leaf temperature, how does this impact on predictions made from reflectance measurements? We investigated this with two wheat varieties, by repeatedly measuring each leaf through a sequence of temperatures imposed by varying the air temperature in a growth room. Leaf temperatures ranging from 20 °C to 35 °C did not alter the estimated Rubisco capacity normalized to 25 °C (Vcmax25), or chlorophyll or nitrogen contents per unit leaf area. Models estimating LMA and Vcmax25/N were both slightly influenced by leaf temperature: estimated LMA increased by 0.27% °C-1 and Vcmax25/N increased by 0.46% °C-1. A model estimating Rubisco activity closely followed variation associated with leaf temperature. Reflectance spectra change with leaf temperature and therefore contain a temperature signal.
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http://dx.doi.org/10.1093/jxb/eraa514DOI Listing
February 2021

Mesophyll conductance: walls, membranes and spatial complexity.

Authors:
John R Evans

New Phytol 2021 02 1;229(4):1864-1876. Epub 2020 Nov 1.

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, The Australian National University, Canberra, ACT, 2601, Australia.

A significant resistance to CO diffusion is imposed by mesophyll tissue inside leaves. Mesophyll resistance, r (or its reciprocal, mesophyll conductance, g ), reduces the rate at which Rubisco can fix CO , increasing the water and nitrogen costs of carbon acquisition. g varies in proportion to the surface area of chloroplasts exposed to intercellular airspace per unit leaf area. It also depends on the thickness and effective porosity of the cell wall and the CO permeabilities of membranes. As no measurements exist for the effective porosity of mesophyll cell walls, and CO permeability values are too low to account for observed rates of CO assimilation, conclusions from modelling must be treated with caution. There is great variation in the mesophyll resistance per unit chloroplast area for a given cell wall thickness, which may reflect differences in effective porosity. While apparent g can vary with CO and irradiance, the underlying conductance at the cellular level may remain unchanged. Dynamic changes in apparent g arise for spatial reasons and because chloroplasts differ in their photosynthetic composition and operate in different light environments. Measurements of the temperature sensitivity of membrane CO permeability are urgently needed to explain variation in temperature responses of g .
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http://dx.doi.org/10.1111/nph.16968DOI Listing
February 2021

Physical activity and sport participation characteristics of Indigenous children registered in the Active Kids voucher program in New South Wales.

J Sci Med Sport 2020 Dec 3;23(12):1178-1184. Epub 2020 Jul 3.

The University of Sydney, SPRINTER, Prevention Research Collaboration, The Charles Perkins Centre, Sydney School of Public Health, Australia. Electronic address:

Objectives: Investigate sociodemographic factors associated with physical activity and sport participation among Indigenous children registered in the New South Wales (NSW) government-funded Active Kids voucher program in 2018, including comparison with non-Indigenous children.

Design: Cross-sectional study.

Methods: The Active Kids voucher program aims to support the cost of children's sport and physical activities. All children aged 5-18 years in NSW are eligible for a voucher. To register, parent/carers report child sociodemographic characteristics, physical activity, sport participation and optional height and weight. Regression models were used to determine which sociodemographic characteristics were associated with meeting physical activity guidelines and sport participation for Indigenous and non-Indigenous children.

Results: Of the 671,375 children aged 5-18 years, 36,129 (5.4%) were Indigenous. More Indigenous children than non-Indigenous children met the physical activity guidelines before registering in the Active Kids program. Indigenous children had greater odds of meeting physical activity guidelines across all socio-economic quartiles. Among non-Indigenous children, odds reduced with social disadvantage. Indigenous children (38%) were less likely to participate in organised physical activity and sport sessions at least twice a week compared to non-Indigenous children (43%). Indigenous children living in major cities had higher sport participation levels compared with those living in outer regional and remote areas.

Conclusions: The Active Kids voucher program achieved population representative reach among Indigenous children, whose physical activity levels were higher than non-Indigenous children across all socioeconomic quartiles. The program has potential to supplement Indigenous children's physical activity levels using organised sessions and reduce sport drop-out among older children.
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http://dx.doi.org/10.1016/j.jsams.2020.06.016DOI Listing
December 2020

A Decrease in Mesophyll Conductance by Cell-Wall Thickening Contributes to Photosynthetic Downregulation.

Plant Physiol 2020 08 9;183(4):1600-1611. Epub 2020 Jun 9.

Australian Research Council Centre of Excellence for Translational Photosynthesis, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.

It has been argued that accumulation of nonstructural carbohydrates triggers a decrease in Rubisco content, which downregulates photosynthesis. However, a decrease in the sink-source ratio in several plant species leads to a decrease in photosynthesis and increases in both structural and nonstructural carbohydrate content. Here, we tested whether increases in cell-wall materials, rather than starch content, impact directly on photosynthesis by decreasing mesophyll conductance. We measured various morphological, anatomical, and physiological traits in primary leaves of soybean () and French bean () grown under high- or low-nitrogen conditions. We removed other leaves 2 weeks after sowing to decrease the sink-source ratio and conducted measurements 0, 1, and 2 weeks after defoliation.
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http://dx.doi.org/10.1104/pp.20.00328DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7401118PMC
August 2020

Genome-wide identification and characterisation of Aquaporins in Nicotiana tabacum and their relationships with other Solanaceae species.

BMC Plant Biol 2020 Jun 9;20(1):266. Epub 2020 Jun 9.

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, Australian National University, ACT, Canberra, 2601, Australia.

Background: Cellular membranes are dynamic structures, continuously adjusting their composition, allowing plants to respond to developmental signals, stresses, and changing environments. To facilitate transmembrane transport of substrates, plant membranes are embedded with both active and passive transporters. Aquaporins (AQPs) constitute a major family of membrane spanning channel proteins that selectively facilitate the passive bidirectional passage of substrates across biological membranes at an astonishing 10 molecules per second. AQPs are the most diversified in the plant kingdom, comprising of five major subfamilies that differ in temporal and spatial gene expression, subcellular protein localisation, substrate specificity, and post-translational regulatory mechanisms; collectively providing a dynamic transportation network spanning the entire plant. Plant AQPs can transport a range of solutes essential for numerous plant processes including, water relations, growth and development, stress responses, root nutrient uptake, and photosynthesis. The ability to manipulate AQPs towards improving plant productivity, is reliant on expanding our insight into the diversity and functional roles of AQPs.

Results: We characterised the AQP family from Nicotiana tabacum (NtAQPs; tobacco), a popular model system capable of scaling from the laboratory to the field. Tobacco is closely related to major economic crops (e.g. tomato, potato, eggplant and peppers) and itself has new commercial applications. Tobacco harbours 76 AQPs making it the second largest characterised AQP family. These fall into five distinct subfamilies, for which we characterised phylogenetic relationships, gene structures, protein sequences, selectivity filter compositions, sub-cellular localisation, and tissue-specific expression. We also identified the AQPs from tobacco's parental genomes (N. sylvestris and N. tomentosiformis), allowing us to characterise the evolutionary history of the NtAQP family. Assigning orthology to tomato and potato AQPs allowed for cross-species comparisons of conservation in protein structures, gene expression, and potential physiological roles.

Conclusions: This study provides a comprehensive characterisation of the tobacco AQP family, and strengthens the current knowledge of AQP biology. The refined gene/protein models, tissue-specific expression analysis, and cross-species comparisons, provide valuable insight into the evolutionary history and likely physiological roles of NtAQPs and their Solanaceae orthologs. Collectively, these results will support future functional studies and help transfer basic research to applied agriculture.
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http://dx.doi.org/10.1186/s12870-020-02412-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7285608PMC
June 2020

Deep Soil Water-Use Determines the Yield Benefit of Long-Cycle Wheat.

Front Plant Sci 2020 15;11:548. Epub 2020 May 15.

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) Agriculture and Food, Canberra, ACT, Australia.

Wheat production in southern Australia is reliant on autumn (April-May) rainfall to germinate seeds and allow timely establishment. Reliance on autumn rainfall can be removed by sowing earlier than currently practiced and using late summer and early autumn rainfall to establish crops, but this requires slower developing cultivars to match life-cycle to seasonal conditions. While slow-developing wheat cultivars sown early in the sowing window (long-cycle), have in some cases increased yield in comparison to the more commonly grown fast-developing cultivars sown later (short-cycle), the yield response is variable between environments. In irrigated wheat in the sub-tropics, the variable response has been linked to ability to withstand water stress, but the mechanism behind this is unknown. We compared short- vs. long-cycle cultivars × time of sowing combinations over four seasons (2011, 2012, 2015, and 2016) at Temora, NSW, Australia. Two seasons (2011 and 2012) had above average summer fallow (December-March) rain, and two seasons had below average summer fallow rain (2015 and 2016). Initial plant available water in each season was 104, 91, 28, and 27 mm, respectively. Rainfall in the 30 days prior to flowering (approximating the critical period for yield determination) in each year was 8, 6, 14, and 190 mm, respectively. We only observed a yield benefit in long-cycle treatments in 2011 and 2012 seasons where there was (i) soil water stored at depth (ii) little rain during the critical period. The higher yield of long-cycle treatments could be attributed to greater deep soil water extraction (<1.0 m), dry-matter production and grain number. In 2015, there was little rain during the critical period, no water stored at depth and no difference between treatments. In 2016, high in-crop rainfall filled the soil profile, but high rainfall during the critical period removed crop reliance on deep water, and yields were equivalent. A simulation study extended our findings to demonstrate a median yield benefit in long-cycle treatments when the volume of starting soil water was increased. This work reveals environmental conditions that can be used to quantify the frequency of circumstances where long-cycle wheat will provide a yield advantage over current practice.
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http://dx.doi.org/10.3389/fpls.2020.00548DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7242739PMC
May 2020

From green to gold: agricultural revolution for food security.

J Exp Bot 2020 04;71(7):2211-2215

School of Life Sciences, University of Essex, Wivenhoe Park, Colchester, UK.

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http://dx.doi.org/10.1093/jxb/eraa110DOI Listing
April 2020

Effect of N supply on the carbon economy of barley when accounting for plant size.

Funct Plant Biol 2020 03;47(4):368-381

Australian Research Council Centre of Excellence in Plant Energy Biology, Research School of Biology, Building 134, The Australian National University, Canberra, ACT 2601, Australia.

Nitrogen availability and ontogeny both affect the relative growth rate (RGR) of plants. In this study of barley (Hordeum vulgare L.) we determined which growth parameters are affected by nitrate (N) availability, and whether these were confounded by differences in plant size, reflecting differences in growth. Plants were hydroponically grown on six different nitrate (N) concentrations for 28 days, and nine harvests were performed to assess the effect of N on growth parameters. Most growth parameters showed similar patterns of responses to N supply whether compared at common time points or common plant sizes. N had a significant effect on the biomass allocation: increasing N increased leaf mass ratio (LMR) and decreased root mass ratio (RMR). Specific leaf area (SLA) was not significantly affected by N. RGR increased with increasing N supply up to 1 mM, associated with increases in both LMR and net assimilation rate (NAR). Increases in N supply above 1 mM did not increase RGR as increases in LMR were offset by decreases in NAR. The high RGR at suboptimal N supply suggest a higher nitrogen use efficiency (biomass/N supply). The reasons for the homeostasis of growth under suboptimal N levels are discussed.
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http://dx.doi.org/10.1071/FP19025DOI Listing
March 2020

Genetic variation for photosynthetic capacity and efficiency in spring wheat.

J Exp Bot 2020 04;71(7):2299-2311

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology. The Australian National University, Canberra, ACT, Australia.

One way to increase yield potential in wheat is screening for natural variation in photosynthesis. This study uses measured and modelled physiological parameters to explore genotypic diversity in photosynthetic capacity (Pc, Rubisco carboxylation capacity per unit leaf area at 25 °C) and efficiency (Peff, Pc per unit of leaf nitrogen) in wheat in relation to fertilizer, plant stage, and environment. Four experiments (Aus1, Aus2, Aus3, and Mex1) were carried out with diverse wheat collections to investigate genetic variation for Rubisco capacity (Vcmax25), electron transport rate (J), CO2 assimilation rate, stomatal conductance, and complementary plant functional traits: leaf nitrogen, leaf dry mass per unit area, and SPAD. Genotypes for Aus1 and Aus2 were grown in the glasshouse with two fertilizer levels. Genotypes for Aus3 and Mex1 experiments were grown in the field in Australia and Mexico, respectively. Results showed that Vcmax25 derived from gas exchange measurements is a robust parameter that does not depend on stomatal conductance and was positively correlated with Rubisco content measured in vitro. There was significant genotypic variation in most of the experiments for Pc and Peff. Heritability of Pc reached 0.7 and 0.9 for SPAD. Genotypic variation and heritability of traits show that there is scope for these traits to be used in pre-breeding programmes to improve photosynthesis with the ultimate objective of raising yield potential.
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http://dx.doi.org/10.1093/jxb/erz439DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7134913PMC
April 2020

Predicting dark respiration rates of wheat leaves from hyperspectral reflectance.

Plant Cell Environ 2019 07 28;42(7):2133-2150. Epub 2019 Mar 28.

ARC Centre of Excellence in Plant Energy Biology, Research School of Biology, Australian National University, Canberra, Australian Capital Territory, 2601, Australia.

Greater availability of leaf dark respiration (R ) data could facilitate breeding efforts to raise crop yield and improve global carbon cycle modelling. However, the availability of R data is limited because it is cumbersome, time consuming, or destructive to measure. We report a non-destructive and high-throughput method of estimating R from leaf hyperspectral reflectance data that was derived from leaf R measured by a destructive high-throughput oxygen consumption technique. We generated a large dataset of leaf R for wheat (1380 samples) from 90 genotypes, multiple growth stages, and growth conditions to generate models for R . Leaf R (per unit leaf area, fresh mass, dry mass or nitrogen, N) varied 7- to 15-fold among individual plants, whereas traits known to scale with R , leaf N, and leaf mass per area (LMA) only varied twofold to fivefold. Our models predicted leaf R , N, and LMA with r values of 0.50-0.63, 0.91, and 0.75, respectively, and relative bias of 17-18% for R and 7-12% for N and LMA. Our results suggest that hyperspectral model prediction of wheat leaf R is largely independent of leaf N and LMA. Potential drivers of hyperspectral signatures of R are discussed.
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http://dx.doi.org/10.1111/pce.13544DOI Listing
July 2019

Effects of mesophyll conductance on vegetation responses to elevated CO concentrations in a land surface model.

Glob Chang Biol 2019 05 23;25(5):1820-1838. Epub 2019 Mar 23.

Department of Ecosystem Physiology, University of Freiburg, Freiburg, Germany.

Mesophyll conductance (g ) is known to affect plant photosynthesis. However, g is rarely explicitly considered in land surface models (LSMs), with the consequence that its role in ecosystem and large-scale carbon and water fluxes is poorly understood. In particular, the different magnitudes of g across plant functional types (PFTs) are expected to cause spatially divergent vegetation responses to elevated CO concentrations. Here, an extensive literature compilation of g across major vegetation types is used to parameterize an empirical model of g in the LSM JSBACH and to adjust photosynthetic parameters based on simulated A  - C curves. We demonstrate that an explicit representation of g changes the response of photosynthesis to environmental factors, which cannot be entirely compensated by adjusting photosynthetic parameters. These altered responses lead to changes in the photosynthetic sensitivity to atmospheric CO concentrations which depend both on the magnitude of g and the climatic conditions, particularly temperature. We then conducted simulations under ambient and elevated (ambient + 200 μmol/mol) CO concentrations for contrasting ecosystems and for historical and anticipated future climate conditions (representative concentration pathways; RCPs) globally. The g -explicit simulations using the RCP8.5 scenario resulted in significantly higher increases in gross primary productivity (GPP) in high latitudes (+10% to + 25%), intermediate increases in temperate regions (+5% to + 15%), and slightly lower to moderately higher responses in tropical regions (-2% to +5%), which summed up to moderate GPP increases globally. Similar patterns were found for transpiration, but with a lower magnitude. Our results suggest that the effect of an explicit representation of g is most important for simulated carbon and water fluxes in the boreal zone, where a cold climate coincides with evergreen vegetation.
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http://dx.doi.org/10.1111/gcb.14604DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6487956PMC
May 2019

The nitrogen cost of photosynthesis.

J Exp Bot 2019 01;70(1):7-15

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, Australian National University, Canberra, ACT, Australia.

Global food security depends on three main cereal crops (wheat, rice and maize) achieving and maintaining high yields, as well as increasing their future yields. Fundamental to the production of this biomass is photosynthesis. The process of photosynthesis involves a large number of proteins that together account for the majority of the nitrogen in leaves. As large amounts of nitrogen are removed in the harvested grain, this needs to be replaced either from synthetic fertilizer or biological nitrogen fixation. Knowledge about photosynthetic properties of leaves in natural ecosystems is also important, particularly when we consider the potential impacts of climate change. While the relationship between nitrogen and photosynthetic capacity of a leaf differs between species, leaf nitrogen content provides a useful way to incorporate photosynthesis into models of ecosystems and the terrestrial biosphere. This review provides a generalized nitrogen budget for a C3 leaf cell and discusses the potential for improving photosynthesis from a nitrogen perspective.
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http://dx.doi.org/10.1093/jxb/ery366DOI Listing
January 2019

Embracing 3D Complexity in Leaf Carbon-Water Exchange.

Trends Plant Sci 2019 01 9;24(1):15-24. Epub 2018 Oct 9.

University of Sydney, Sydney, NSW 2006, Australia; www.sydney.edu.au/science/people/margaret.barbour. Electronic address:

Leaves are a nexus for the exchange of water, carbon, and energy between terrestrial plants and the atmosphere. Research in recent decades has highlighted the critical importance of the underlying biophysical and anatomical determinants of CO and HO transport, but a quantitative understanding of how detailed 3D leaf anatomy mediates within-leaf transport has been hindered by the lack of a consensus framework for analyzing or simulating transport and its spatial and temporal dynamics realistically, and by the difficulty of measuring within-leaf transport at the appropriate scales. We discuss how recent technological advancements now make a spatially explicit 3D leaf analysis possible, through new imaging and modeling tools that will allow us to address long-standing questions related to plant carbon-water exchange.
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http://dx.doi.org/10.1016/j.tplants.2018.09.005DOI Listing
January 2019

Phosphorus deficiency alters scaling relationships between leaf gas exchange and associated traits in a wide range of contrasting Eucalyptus species.

Funct Plant Biol 2018 Jul;45(8):813-826

Division of Plant Sciences, Research School of Biology, The Australian National University, Canberra, ACT 2601, Australia.

Phosphorus (P) limitation is known to have substantial impacts on leaf metabolism. However, uncertainty remains around whether P deficiency alters scaling functions linking leaf metabolism to associated traits. We investigated the effect of P deficiency on leaf gas exchange and related leaf traits in 17 contrasting Eucalyptus species that exhibit inherent differences in leaf traits. Saplings were grown under controlled-environment conditions in a glasshouse, where they were subjected to minus and plus P treatments for 15 weeks. P deficiency decreased P concentrations and increased leaf mass per area (LMA) of newly-developed leaves. Rates of photosynthesis (A) and respiration (R) were also reduced in P-deficient plants compared with P-fertilised plants. By contrast, P deficiency had little effect on the temperature sensitivity of R. Irrespective of P treatment, on a log-log basis A and R scaled positively with increasing leaf nitrogen concentration [N] and negatively with increasing LMA. Although P deficiency had limited impact on A-R-LMA relationships, rates of CO2 exchange per unit N were consistently lower in P-deficient plants. Our results highlight the importance of P supply for leaf carbon metabolism and show how P deficiencies (i.e. when excluding confounding genotypic and environmental effects) can have a direct effect on commonly used leaf trait scaling relationships.
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http://dx.doi.org/10.1071/FP17134DOI Listing
July 2018

Internal transport of CO from the root-zone to plant shoot is pH dependent.

Physiol Plant 2019 Mar 31;165(3):451-463. Epub 2018 Aug 31.

Division of Plant Sciences, Research School of Biology, The Australian National University, Acton ACT 2601, Australia.

We investigated the fate of carbon dioxide (CO ) absorbed by roots or internally produced by respiration using gas exchange and stable isotopic labeling. CO efflux from detached leaves supplied with bicarbonate/CO solutions was followed over six cycles. CO effluxes were detected when bicarbonate solution at high pH was used, corresponding to 71-85% of the expected efflux. No CO efflux was detected when CO solutions at low pH were used but CO efflux was subsequently detected as soon as bicarbonate solutions at high pH were supplied. By sealing the leaf and petiole in a plastic bag to reduce diffusion to the atmosphere, a small CO efflux signal (14-30% of the expected efflux) was detected suggesting that CO in the xylem stream can readily escape to the atmosphere before reaching the leaf. When the root-zones of intact plants were exposed to CO solutions, a significant efflux from leaf surface was observed (13% of the expected efflux). However, no signal was detected when roots were exposed to a high pH bicarbonate solution. Isotopic tracer experiments confirmed that CO supplied to the root-zone was transported through the plant and was readily lost to the atmosphere. However, little C moved to the shoot when roots were exposed to bicarbonate solutions at pH 8, suggesting that bicarbonate does not pass into the xylem.
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http://dx.doi.org/10.1111/ppl.12767DOI Listing
March 2019

Mesophyll conductance does not contribute to greater photosynthetic rate per unit nitrogen in temperate compared with tropical evergreen wet-forest tree leaves.

New Phytol 2018 04 13;218(2):492-505. Epub 2018 Feb 13.

ARC Centre of Excellence for Translational Photosynthesis, Division of Plant Sciences, Research School of Biology, Australian National University, Canberra, ACT, 2601, Australia.

Globally, trees originating from high-rainfall tropical regions typically exhibit lower rates of light-saturated net CO assimilation (A) compared with those from high-rainfall temperate environments, when measured at a common temperature. One factor that has been suggested to contribute towards lower rates of A is lower mesophyll conductance. Using a combination of leaf gas exchange and carbon isotope discrimination measurements, we estimated mesophyll conductance (g ) of several Australian tropical and temperate wet-forest trees, grown in a common environment. Maximum Rubisco carboxylation capacity, V , was obtained from CO response curves. g and the drawdown of CO across the mesophyll were both relatively constant. V estimated on the basis of intercellular CO partial pressure, C , was equivalent to that estimated using chloroplastic CO partial pressure, C , using 'apparent' and 'true' Rubisco Michaelis-Menten constants, respectively Having ruled out g as a possible factor in distorting variations in A between these tropical and temperate trees, attention now needs to be focused on obtaining more detailed information about Rubisco in these species.
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http://dx.doi.org/10.1111/nph.15031DOI Listing
April 2018

Hyperspectral reflectance as a tool to measure biochemical and physiological traits in wheat.

J Exp Bot 2018 01;69(3):483-496

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, The Australian National University, Canberra, ACT, Australia.

Improving photosynthesis to raise wheat yield potential has emerged as a major target for wheat physiologists. Photosynthesis-related traits, such as nitrogen per unit leaf area (Narea) and leaf dry mass per area (LMA), require laborious, destructive, laboratory-based methods, while physiological traits underpinning photosynthetic capacity, such as maximum Rubisco activity normalized to 25 °C (Vcmax25) and electron transport rate (J), require time-consuming gas exchange measurements. The aim of this study was to assess whether hyperspectral reflectance (350-2500 nm) can be used to rapidly estimate these traits on intact wheat leaves. Predictive models were constructed using gas exchange and hyperspectral reflectance data from 76 genotypes grown in glasshouses with different nitrogen levels and/or in the field under yield potential conditions. Models were developed using half of the observed data with the remainder used for validation, yielding correlation coefficients (R2 values) of 0.62 for Vcmax25, 0.7 for J, 0.81 for SPAD, 0.89 for LMA, and 0.93 for Narea, with bias <0.7%. The models were tested on elite lines and landraces that had not been used to create the models. The bias varied between -2.3% and -5.5% while relative error of prediction was similar for SPAD but slightly greater for LMA and Narea.
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http://dx.doi.org/10.1093/jxb/erx421DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5853784PMC
January 2018

Light Quality Affects Chloroplast Electron Transport Rates Estimated from Chl Fluorescence Measurements.

Plant Cell Physiol 2017 Oct;58(10):1652-1660

Australian Research Council Centre of Excellence for Translational Photosynthesis, Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT 2601, Australia.

Chl fluorescence has been used widely to calculate photosynthetic electron transport rates. Portable photosynthesis instruments allow for combined measurements of gas exchange and Chl fluorescence. We analyzed the influence of spectral quality of actinic light on Chl fluorescence and the calculated electron transport rate, and compared this with photosynthetic rates measured by gas exchange in the absence of photorespiration. In blue actinic light, the electron transport rate calculated from Chl fluorescence overestimated the true rate by nearly a factor of two, whereas there was closer agreement under red light. This was consistent with the prediction made with a multilayer leaf model using profiles of light absorption and photosynthetic capacity. Caution is needed when interpreting combined measurements of Chl fluorescence and gas exchange, such as the calculation of CO2 partial pressure in leaf chloroplasts.
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http://dx.doi.org/10.1093/pcp/pcx103DOI Listing
October 2017

Leaf water storage increases with salinity and aridity in the mangrove Avicennia marina: integration of leaf structure, osmotic adjustment and access to multiple water sources.

Plant Cell Environ 2017 Aug 20;40(8):1576-1591. Epub 2017 Jun 20.

Plant Science Division, Research School of Biology, The Australian National University, Acton, Australian Capital Territory, 2601, Australia.

Leaf structure and water relations were studied in a temperate population of Avicennia marina subsp. australasica along a natural salinity gradient [28 to 49 parts per thousand (ppt)] and compared with two subspecies grown naturally in similar soil salinities to those of subsp. australasica but under different climates: subsp. eucalyptifolia (salinity 30 ppt, wet tropics) and subsp. marina (salinity 46 ppt, arid tropics). Leaf thickness, leaf dry mass per area and water content increased with salinity and aridity. Turgor loss point declined with increase in soil salinity, driven mainly by differences in osmotic potential at full turgor. Nevertheless, a high modulus of elasticity (ε) contributed to maintenance of high cell hydration at turgor loss point. Despite similarity among leaves in leaf water storage capacitance, total leaf water storage increased with increasing salinity and aridity. The time that stored water alone could sustain an evaporation rate of 1 mmol m  s ranged from 77 to 126 min from subspecies eucalyptifolia to ssp. marina, respectively. Achieving full leaf hydration or turgor would require water from sources other than the roots, emphasizing the importance of multiple water sources to growth and survival of Avicennia marina across gradients in salinity and aridity.
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http://dx.doi.org/10.1111/pce.12962DOI Listing
August 2017

Biochemical model of C photosynthesis applied to wheat at different temperatures.

Plant Cell Environ 2017 Aug 2;40(8):1552-1564. Epub 2017 Jun 2.

Plant Science Division, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory, 2601, Australia.

We examined the effects of leaf temperature on the estimation of maximal Rubisco capacity (V ) from gas exchange measurements of wheat leaves using a C photosynthesis model. Cultivars of spring wheat (Triticum aestivum (L)) and triticale (X Triticosecale Wittmack) were grown in a greenhouse or in the field and measured at a range of temperatures under controlled conditions in a growth cabinet (2 and 21% O ) or in the field using natural diurnal variation in temperature, respectively. Published Rubisco kinetic constants for tobacco did not describe the observed CO response curves well as temperature varied. By assuming values for the Rubisco Michaelis-Menten constants for CO (K ) and O (K ) at 25 °C derived from tobacco and the activation energies of V from wheat and respiration in the light, R , from tobacco, we derived activation energies for K and K (93.7 and 33.6 kJ mol , respectively) that considerably improved the fit of the model to observed data. We confirmed that temperature dependence of dark respiration for wheat was well described by the activation energy for R from tobacco. The new parameters improved the estimation of V under field conditions, where temperatures increased through the day.
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http://dx.doi.org/10.1111/pce.12953DOI Listing
August 2017

Physiological and structural tradeoffs underlying the leaf economics spectrum.

New Phytol 2017 Jun 10;214(4):1447-1463. Epub 2017 Mar 10.

Department of Biological Sciences, Macquarie University, Sydney, NSW, 2109, Australia.

The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18-70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.
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http://dx.doi.org/10.1111/nph.14496DOI Listing
June 2017

Strong thermal acclimation of photosynthesis in tropical and temperate wet-forest tree species: the importance of altered Rubisco content.

Glob Chang Biol 2017 07 3;23(7):2783-2800. Epub 2017 Jan 3.

ARC Centre of Excellence in Plant Energy Biology, Research School of Biology, The Australian National University, Building 134, Canberra, ACT, 2601, Australia.

Understanding of the extent of acclimation of light-saturated net photosynthesis (A ) to temperature (T), and associated underlying mechanisms, remains limited. This is a key knowledge gap given the importance of thermal acclimation for plant functioning, both under current and future higher temperatures, limiting the accuracy and realism of Earth system model (ESM) predictions. Given this, we analysed and modelled T-dependent changes in photosynthetic capacity in 10 wet-forest tree species: six from temperate forests and four from tropical forests. Temperate and tropical species were each acclimated to three daytime growth temperatures (T ): temperate - 15, 20 and 25 °C; tropical - 25, 30 and 35 °C. CO response curves of A were used to model maximal rates of RuBP (ribulose-1,5-bisphosphate) carboxylation (V ) and electron transport (J ) at each treatment's respective T and at a common measurement T (25 °C). SDS-PAGE gels were used to determine abundance of the CO -fixing enzyme, Rubisco. Leaf chlorophyll, nitrogen (N) and mass per unit leaf area (LMA) were also determined. For all species and T , A at current atmospheric CO partial pressure was Rubisco-limited. Across all species, LMA decreased with increasing T . Similarly, area-based rates of V at a measurement T of 25 °C (V ) linearly declined with increasing T , linked to a concomitant decline in total leaf protein per unit leaf area and Rubisco as a percentage of leaf N. The decline in Rubisco constrained V and A for leaves developed at higher T and resulted in poor predictions of photosynthesis by currently widely used models that do not account for T -mediated changes in Rubisco abundance that underpin the thermal acclimation response of photosynthesis in wet-forest tree species. A new model is proposed that accounts for the effect of T -mediated declines in V on A , complementing current photosynthetic thermal acclimation models that do not account for T sensitivity of V .
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http://dx.doi.org/10.1111/gcb.13566DOI Listing
July 2017

Carbon dioxide and water transport through plant aquaporins.

Plant Cell Environ 2017 Jun 1;40(6):938-961. Epub 2016 Dec 1.

Australian Research Council Centre of Excellence for Translational Photosynthesis, Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT, 2601, Australia.

Aquaporins are channel proteins that function to increase the permeability of biological membranes. In plants, aquaporins are encoded by multigene families that have undergone substantial diversification in land plants. The plasma membrane intrinsic proteins (PIPs) subfamily of aquaporins is of particular interest given their potential to improve plant water relations and photosynthesis. Flowering plants have between 7 and 28 PIP genes. Their expression varies with tissue and cell type, through development and in response to a variety of factors, contributing to the dynamic and tissue specific control of permeability. There are a growing number of PIPs shown to act as water channels, but those altering membrane permeability to CO are more limited. The structural basis for selective substrate specificities has not yet been resolved, although a few key amino acid positions have been identified. Several regions important for dimerization, gating and trafficking are also known. PIP aquaporins assemble as tetramers and their properties depend on the monomeric composition. PIPs control water flux into and out of veins and stomatal guard cells and also increase membrane permeability to CO in mesophyll and stomatal guard cells. The latter increases the effectiveness of Rubisco and can potentially influence transpiration efficiency.
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http://dx.doi.org/10.1111/pce.12844DOI Listing
June 2017

Effects of reduced carbonic anhydrase activity on CO2 assimilation rates in Setaria viridis: a transgenic analysis.

J Exp Bot 2017 01 4;68(2):299-310. Epub 2016 Oct 4.

Australian Research Council Centre of Excellence for Translational Photosynthesis, Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT 2601, Australia.

In C species, the major β-carbonic anhydrase (β-CA) localized in the mesophyll cytosol catalyses the hydration of CO to HCO, which phosphoenolpyruvate carboxylase uses in the first step of C photosynthesis. To address the role of CA in C photosynthesis, we generated transgenic Setaria viridis depleted in β-CA. Independent lines were identified with as little as 13% of wild-type CA. No photosynthetic defect was observed in the transformed lines at ambient CO partial pressure (pCO). At low pCO, a strong correlation between CO assimilation rates and CA hydration rates was observed. COO isotope discrimination was used to estimate the mesophyll conductance to CO diffusion from the intercellular air space to the mesophyll cytosol (g) in control plants, which allowed us to calculate CA activities in the mesophyll cytosol (C). This revealed a strong relationship between the initial slope of the response of the CO assimilation rate to cytosolic pCO (AC) and cytosolic CA activity. However, the relationship between the initial slope of the response of CO assimilation to intercellular pCO (AC) and cytosolic CA activity was curvilinear. This indicated that in S. viridis, mesophyll conductance may be a contributing limiting factor alongside CA activity to CO assimilation rates at low pCO.
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http://dx.doi.org/10.1093/jxb/erw357DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5853810PMC
January 2017

Association between water and carbon dioxide transport in leaf plasma membranes: assessing the role of aquaporins.

Plant Cell Environ 2017 Jun 14;40(6):789-801. Epub 2016 Oct 14.

Australian Research Council Centre of Excellence in Plant Energy Biology, School of Agriculture, Food and Wine, Waite Research Institute, University of Adelaide, Glen Osmond, South Australia, 5064, Australia.

The role of some aquaporins as CO permeable channels has been controversial. Low CO permeability of plant membranes has been criticized because of unstirred layers and other limitations. Here we measured both water and CO permeability (P , P ) using stopped flow on plasma membrane vesicles (pmv) isolated from Pisum sativum (pea) and Arabidopsis thaliana leaves. We excluded the chemical limitation of carbonic anhydrase (CA) in the vesicle acidification technique for P using different temperatures and CA concentrations. Unstirred layers were excluded based on small vesicle size and the positive correlation between vesicle diameter and P . We observed high aquaporin activity (P 0.06 to 0.22 cm s ) for pea pmv based on all the criteria for their function using inhibitors and temperature dependence. Inhibitors of P did not alter P . P ranged from 0.001 to 0.012 cm s (mean 0.0079 + 0.0007 cm s ) with activation energy of 30.2 kJ mol . Intrinsic variation between pmv batches from normally grown or stressed plants revealed a weak (R  = 0.27) positive linear correlation between P and P . Despite the low P , aquaporins may facilitate CO transport across plasma membranes, but probably via a different pathway than for water.
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http://dx.doi.org/10.1111/pce.12830DOI Listing
June 2017
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