Plant responses to decadal scale increments in atmospheric CO2 concentration: comparing two stomatal conductance sampling methods

There are several lines of evidence suggesting that the vast majority of C3 plants respond to elevated atmospheric CO2 by decreasing their stomatal conductance (gs). However, in the majority of CO2 enrichment studies, the response to elevated CO2 are tested between plants grown under ambient (380–420 ppm) and high (538–680 ppm) CO2 concentrations and measured usually at single time points in a diurnal cycle. We investigated gs responses to simulated decadal increments in CO2 predicted over the next 4 decades and tested how measurements of gs may differ when two alternative sampling methods are employed (infrared gas analyzer [IRGA] vs. leaf porometer). We exposed Populus tremula, Popolus tremuloides and Sambucus racemosa to four different CO2 concentrations over 126 days in experimental growth chambers at 350, 420, 490 and 560 ppm CO2; representing the years 1987, 2025, 2051, and 2070, respectively (RCP4.5 scenario). Our study demonstrated that the species respond non-linearly to increases in CO2 concentration when exposed to decadal changes in CO2. Under natural conditions, maximum operational gs is often reached in the late morning to early afternoon, with a mid-day depression around noon. However, we showed that the daily maximum gs can, in some species, shift later into the day when plants are exposed to only small increases (70 ppm) in CO2. A non-linear decreases in gs and a shifting diurnal stomatal behavior under elevated CO2, could affect the long-term daily water and carbon budget of many plants in the future, and therefore alter soil–plant–atmospheric processes.Irish Research CouncilScience Foundation Irelan

Consistent Relationship between Field-Measured Stomatal Conductance and Theoretical Maximum Stomatal Conductance in C₃ Woody Angiosperms in Four Major Biomes

Premise of research. Understanding the relationship between field-measured operating stomatal conductance (gop) and theoretical maximum stomatal conductance (gmax), calculated from stomatal density and geometry, provides an important framework that can be used to infer leaf-level gas exchange of historical, herbarium, and fossil plants. To date, however, investigation of the nature of the relationship between gop and theoretical gmax remains limited to a small number of experiments on relatively few taxa and is virtually undefined for plants in natural ecosystems. Methodology. We used the gop measurements of 74 species and 35 families across four biomes from a published contemporary data set of field-measured leaf-level stomatal conductance in woody angiosperms and calculated the theoretical gmax from the same leaves to investigate the relationship between gop and gmax across multiple species and biomes and determine whether such relationships are widely conserved. Pivotal results. We observed significant relationships between gop and gmax, with consistency in the gop ∶ gmax ratio across biomes, growth habits (shrubs and trees), and habitats (open canopy and understory subcanopy). An overall mean gop ∶ gmax ratio of 0.26 ± 0.11 (mean ± SD) was observed. The consistently observed gop ∶ gmax ratio in this study strongly agrees with previous hypotheses that an ideal gop ∶ gmax ratio exists. Conclusions. These results build substantially on previous studies by presenting a new reference for a consistent gop ∶ gmax ratio across many levels and offer great potential to enhance paleoclimate proxies and vegetation-climate models alike

Expression of cyanobacterial genes enhanced CO2 assimilation and biomass production in transgenic Arabidopsis thaliana

Background - Photosynthesis is a key process in plants that is compromised by the oxygenase activity of Rubisco, which leads to the production of toxic compound phosphoglycolate that is catabolized by photorespiratory pathway. Transformation of plants with photorespiratory bypasses have been shown to reduce photorespiration and enhance plant biomass. Interestingly, engineering of a single gene from such photorespiratory bypasses has also improved photosynthesis and plant productivity. Although single gene transformations may not completely reduce photorespiration, increases in plant biomass accumulation have still been observed indicating an alternative role in regulating different metabolic processes. Therefore, the current study was aimed at evaluating the underlying mechanism (s) associated with the effects of introducing a single cyanobacterial glycolate decarboxylation pathway gene on photosynthesis and plant performance. Methods - Transgenic Arabidopsis thaliana plants (GD, HD, OX) expressing independently cyanobacterial decarboxylation pathway genes i.e., glycolate dehydrogenase, hydroxyacid dehydrogenase, and oxalate decarboxylase, respectively, were utilized. Photosynthetic, fluorescence related, and growth parameters were analyzed. Additionally, transcriptomic analysis of GD transgenic plants was also performed. Results - The GD plants exhibited a significant increase (16%) in net photosynthesis rate while both HD and OX plants showed a non-significant (11%) increase as compared to wild type plants (WT). The stomatal conductance was significantly higher (24%) in GD and HD plants than the WT plants. The quantum efficiencies of photosystem II, carbon dioxide assimilation and the chlorophyll fluorescence-based photosynthetic electron transport rate were also higher than WT plants. The OX plants displayed significant reductions in the rate of photorespiration relative to gross photosynthesis and increase in the ratio of the photosynthetic electron flow attributable to carboxylation reactions over that attributable to oxygenation reactions. GD, HD and OX plants accumulated significantly higher biomass and seed weight. Soluble sugars were significantly increased in GD and HD plants, while the starch levels were higher in all transgenic plants. The transcriptomic analysis of GD plants revealed 650 up-regulated genes mainly related to photosynthesis, photorespiratory pathway, sucrose metabolism, chlorophyll biosynthesis and glutathione metabolism. Conclusion - This study revealed the potential of introduced cyanobacterial pathway genes to enhance photosynthetic and growth-related parameters. The upregulation of genes related to different pathways provided evidence of the underlying mechanisms involved particularly in GD plants. However, transcriptomic profiling of HD and OX plants can further help to identify other potential mechanisms involved in improved plant productivity.Higher Education Commission of Pakista

Rising CO₂ drives divergence in water use efficiency of evergreen and deciduous plants

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 CO2 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 CO2 (~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

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

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

Green stemmed plants: a comparative anatomical and physiological study

Despite its significant contribution to the net carbon gain of plants and itsdistinct functional properties, stem photosynthesis has not yet received adequatescientific attention. For this reason, a combination of anatomical and physiologicalmethods was used to characterize the photosynthetic machinery of the green petiolesand pedicels of the monocotyledonous geophyte Zantedeschia aethiopica and thegreen stems of the dicotyledonous semi-woody species Dianthus caryophyllus, incomparison to the corresponding leaves. Both the green petioles/pedicels of Z.aethiopica and the green stems of D. caryophyllus possess all the anatomicalprerequisites of an actively photosynthesizing organ i.e. considerable number ofstomata with typical underlying substomatal chambers, chlorenchyma cells which aresimilar to the leaf palisade chlorenchyma cells and considerable amount of bothintercellular spaces and palisade free cell walls. Yet, the palisade cells of Z.aethiopica petioles/pedicels show a peculiar arrangement with their long axis parallelto the longitudinal organ axis. Furthermore, petiole/pedicel photosyntheticcharacteristics resemble those of leaves under adversity i.e. reduced Rubiscoactivity/content, high photorespiration rates and significant cyclic electron flowaround PSI. It is concluded that these are innate attributes of petiole/pedicelphotosynthesis serving particular functions like the increased nitrogen fixing activityof the species, the qualitative adjustment of the petiole/pedicel amino acid content, theactive decarboxylation of C4-organic acids and the rapid induction of nonphotochemicalquenching.Stem photosynthesis in D. caryophyllus was more efficient than leafphotosynthesis, as a result of the greater rates of stem C3 cycle and a possible organspecificvariation of the specificity factor of Rubisco. In general, D. caryophyllusstems display a photosynthetic pattern of optimal carbon assimilation in the expenseof photoprotection. It could be hypothesized that this kind of adaptation could be dueto the vertical orientation of stems, which results in lower incident light intensities invivo and may include the use of C4-organic acids coming up with the transpirationstream as an additional carbon source

Differences in the photosynthetic plasticity of ferns and Ginkgo grown in experimentally controlled low [O2]: [CO2] atmospheres may explain their contrasting ecological fate across the Triassic-Jurassic mass extinction boundary

Background and Aims: Fluctuations in [CO2] have been widely studied as a potential driver of plant evolution; however, the role of a fluctuating [O2]:[CO2] ratio is often overlooked. The present study aimed to investigate the inherent physiological plasticity of early diverging, extant species following acclimation to an atmosphere similar to that across the Triassic–Jurassic mass extinction interval (TJB, approx. 200 Mya), a time of major ecological change. Methods: Mature plants from two angiosperm (Drimys winteri and Chloranthus oldhamii), two monilophyte (Osmunda claytoniana and Cyathea australis) and one gymnosperm (Ginkgo biloba) species were grown for 2 months in replicated walk-in Conviron BDW40 chambers running at TJB treatment conditions of 16 % [O2]– 1900 ppm [CO2] and ambient conditions of 21 % [O2]–400 ppm [CO2], and their physiological plasticity was assessed using gas exchange and chlorophyll fluorescence methods. Key Results: TJB acclimation caused significant reductions in the maximum rate of carboxylation (VCmax) and the maximum electron flow supporting ribulose-1,5-bisphosphate regeneration (Jmax) in all species, yet this downregulation had little effect on their light-saturated photosynthetic rate (Asat). Ginkgo was found to photorespire heavily under ambient conditions, while growth in low [O2]:[CO2] resulted in increased heat dissipation per reaction centre (DIo/RC), severe photodamage, as revealed by the species' decreased maximum efficiency of primary photochemistry (Fv/Fm) and decreased in situ photosynthetic electron flow (Jsitu). Conclusions: It is argued that the observed photodamage reflects the inability of Ginkgo to divert excess photosynthetic electron flow to sinks other than the downregulated C3 and the diminished C2 cycles under low [O2]:[CO2]. This finding, coupled with the remarkable physiological plasticity of the ferns, provides insights into the underlying mechanism of Ginkgoales' near extinction and ferns' proliferation as atmospheric [CO2] increased to maximum levels across the TJB.European Research Counci

Co-ordination in morphological leaf traits of early diverging angiosperms is maintained following exposure to experimental palaeo-atmospheric conditions of sub-ambient O2 and elevated CO2

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 (Dv) 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 Dv and SD, five early-diverging plant species representing two different reproductive plant grades were grown for seven months in a palaeo-treatment comprising an O2:CO2 ratio that has occurred multiple times throughout plant evolutionary history. Results show a range of species-specific Dv and SD responses to the palaeo-treatment, however we show that the strong relationship between Dv 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 (Dm) to infer plant palaeo-physiology

Co-ordination in morphological leaf traits of early diverging angiosperms is maintained following exposure to experimental palaeoatmospheric conditions of sub-ambient O2 and elevated CO2

In order to be successful in a given environment a plant should invest in a vein and stomatal network that ensures balance between both water supply and demand. Vein density (Dv) 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 Dv and SD, five early-diverging plant species representing two different reproductive plant grades were grown for seven months in a palaeo-treatment comprising an O2:CO2 ratio that has occurred multiple times throughout plant evolutionary history. Results show a range of species-specific Dv and SD responses to the palaeo-treatment, however we show that the strong relationship between Dv and SD under modern ambient atmospheric composition is maintained following exposure to the palaeo-treatment. This suggests strong co-ordination between vein and stomatal traits even under relatively extreme environmental change. This co-ordination supports existing plant function proxies that use the distance between vein endings and stomata (Dm) to infer plant palaeo-physiology such as assimilation rate, and as a result, lends confidence to future application of palaeo-CO2 proxy models that require robust estimates of palaeo-assimilation rate as key initialisation parameters. European Research Counci

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

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 (δ13C) 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) δ13C, 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.European Research CouncilScience Foundation Irelan