The large Gunnera's (G. tinctoria and G. manicata) in Europe in relation to EU regulation 1143/2014.

Incorrect labelling of plants in the horticultural trade and misidentification is widespread. For the inspection services of the EU member states, correct identification of G. tinctoria has become important since the species was added to the List of Union concern in accordance with EU regulation 1143/2014 in August 2017. In the horticultural trade Gunnera plants are generally of modest dimensions and rarely flowering, so that the major distinguishing morphological characters for the identification of the two large species, G. tinctoria and G. manicata, are missing. As G. tinctoria is included in the EU regulation, its trade is prohibited, although the closely related species, G. manicata is not included on the list. Given that it is often difficult to distinguish between these two large herbaceous species using morphological attributes we used standard chloroplast DNA barcode markers, supplemented at a later stage by ITS markers. Plant material of putative G. tinctoria or G. manicata was obtained from the native and introduced range, both from "wild" sources, botanical gardens, and the horticultural trade. In western Europe plants circulating in the horticultural trade turned out to be predominantly G. tinctoria, with only one plant in cultivation identified as true G. manicata and the G. manicata found in botanical gardens was a hybrid recently described as G. x cryptica.

Variation in methane uptake by grassland soils in the context of climate change - A review of effects and mechanisms.

Grassland soils are climate-dependent ecosystems that have a significant greenhouse gas mitigating function through their ability to store large amounts of carbon (C). However, what is often not recognized is that they can also exhibit a high methane (CH4) uptake capacity that could be influenced by future increases in atmospheric carbon dioxide (CO2) concentration and variations in temperature and water availability. While there is a wealth of information on C sequestration in grasslands there is less consensus on how climate change impacts on CH4 uptake or the underlying mechanisms involved. To address this, we assessed existing knowledge on the impact of climate change components on CH4 uptake by grassland soils. Increases in precipitation associated with soils with a high background soil moisture content generally resulted in a reduction in CH4 uptake or even net emissions, while the effect was opposite in soils with a relatively low background moisture content. Initially wet grasslands subject to the combined effects of warming and water deficits may absorb more CH4, mainly due to increased gas diffusivity. However, in the longer-term heat and drought stress may reduce the activity of methanotrophs when the mean soil moisture content is below the optimum for their survival. Enhanced plant productivity and growth under elevated CO2, increased soil moisture and changed nutrient concentrations, can differentially affect methanotrophic activity, which is often reduced by increasing N deposition. Our estimations showed that CH4 uptake in grassland soils can change from -57.7 % to +6.1 % by increased precipitation, from -37.3 % to +85.3 % by elevated temperatures, from +0.87 % to +92.4 % by decreased precipitation, and from -66.7 % to +27.3 % by elevated CO2. In conclusion, the analysis suggests that grasslands under the influence of warming and drought may absorb even more CH4, mainly because of reduced soil water contents and increased gas diffusivity.

Post-Invasion Recovery of Plant Communities Colonised by Gunnera tinctoria after Mechanical Removal or Herbicide Application and Its Interaction with an Extreme Weather Event.

The interventions that are required for both the control and post-invasion restoration of native plant communities depends on several factors, including the efficacy of the measures that are used and how these interact with environmental factors. Here, we report on the results of an experiment on the effects of mechanical removal and herbicide application on the invasive plant Gunnera tinctoria and how an extreme weather event impacted on the invader and on the recovery of native coastal grassland communities. Both removal protocols were largely effective in eradicating mature plants, but the mechanical removal treatment resulted in a major increase in the number of G. tinctoria seedlings, which was exacerbated by the extreme event. Nine months after removal, the number of native species had recovered to c. 80% of that in uninvaded grasslands. In contrast to seedlings, mature plants of G. tinctoria showed a significant reduction in above-ground production after the extreme weather event, although these had largely recovered after six months. Overall, our results indicate that post-control restoration of the plant community may be possible without further significant management interventions. Nevertheless, since some invasive plants survived, further monitoring is required to ensure that recolonisation does not occur.

Contrasting Effects of Forest Type and Stand Age on Soil Microbial Activities: An Analysis of Local Scale Variability.

Understanding the functioning of different forest ecosystems is important due to their key role in strategies for climate change mitigation, especially through soil C sequestration. In controlled laboratory conditions, we conducted a preliminary study on six different forest soils (two coniferous, two deciduous, and two mixed sites comprising trees of different ages) collected from the same region. The aim was to explore any differences and assess seasonal changes in soil microbial parameters (basal respiration BR, microbial biomass Cmic, metabolic quotient qCO2, dehydrogenase activity DHA, and Cmic:Corg ratio). Indicator- and forest-specific seasonality was assessed. In addition to litter input, soil parameters (pH, nutrient content, texture and moisture) strongly regulated the analyzed microbial indicators. PCA analysis indicated similarity between mature mixed and deciduous forests. Among annual mean values, high Cmic and DHA with simultaneously low qCO2 suggest that the mature deciduous stand was the most sustainable in microbial activities among the investigated forest soils. Research on the interrelationship between soil parameters and forest types with different tree ages needs to be continued and extended to analyze a greater number of forest and soil types.

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.

Alien plant introductions and greenhouse gas emissions: Insights from Gunnera tinctoria invasions.

Plant invasions represent a major global change in land/vegetation cover with the potential to significantly modify greenhouse gas (GHG) emissions. To get a better understanding of the impacts of terrestrial invasive plants on soil GHG emissions we report, firstly, on experiments conducted on invasive populations of the N-fixing herbaceous species Gunnera tinctoria in Ireland, and secondly, compare our results with published information based on a systematic review of the literature. For G. tinctoria populations, there was a >50% reduction in soil CO2 emissions, mainly due to a reduction in autotrophic respiration, but with little impact on annual N2O or CH4 budgets. One year after the removal of G. tinctoria, soil GHG emissions returned to values comparable to uninvaded areas and this was associated with the reestablishment of the vegetation and an increased root biomass per unit area. If G. tinctoria covered 10% of abandoned agricultural land in Ireland, this could be associated with a reduction of approximately 8% (or 4.988 Mt CO2eq y-1) of the country's national CO2 emissions. Comparisons of these results with literature values were difficult because of the often low and limited sampling effort of previous investigations, a failure to assess all three major GHGs and because of marked seasonal variations. We found 46 studies that documented results for 16 species. From the studies that measured soil respiration, it was enhanced in only 45% of cases, questioning the assumption that invasive plants always increase soil CO2 emissions. In 25 cases that analysed methane, CH4 emissions increased in 76% of them, but all of these were conducted in wetlands. In only two cases were N-fixing species associated with enhanced N2O emissions. Our results argue for more detailed and comprehensive assessments of the effect of plant invasions on GHG emissions and their global impact.

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

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.

Dominant role of nitrogen stoichiometric flexibility in ecosystem carbon storage under elevated CO2.

Interactions between the carbon (C) and nitrogen (N) cycles can impact on the sensitivity of terrestrial C storage to elevated atmospheric carbon dioxide (CO2) concentrations (eCO2). However, the underlying mechanisms associated with CN interactions that influence terrestrial ecosystem C sequestration (Cseq) remains unclear. Here, we quantitatively analyzed published C and N responses to experimentally eCO2 using a meta-analysis approach. We determined the relative importance of three principal mechanisms (changes in the total ecosystem N amount, redistribution of N between plant and soil pools, and flexibility of the C:N ratio) that contribute to increases in ecosystem C storage in response to eCO2. Our results showed that eCO2 increased C and N accumulation, resulted in higher C:N ratios in plant, litter, and soil pools and induced a net shift of N from soils to vegetation. These three mechanisms largely explained the increment of ecosystem Cseq under eCO2, although the relative contributions differed across ecosystem types, with changes in the C:N ratio contributing 50% of the increment in forests Cseq, while the total N change contributed 60% of the increment in grassland Cseq. In terms of temporal variation in the relative importance of each of these three mechanisms to ecosystem Cseq: changes in the C:N ratio was the most important mechanism during the early years (~5 years) of eCO2 treatment, whilst the contribution to ecosystem Cseq by N redistribution remained rather small, and the contribution by total N change did not show a clear temporal pattern. This study highlights the differential contributions of the three mechanisms to Cseq, which may offer important implications for future predictions of the C cycle in terrestrial ecosystems subjected to global change.

Effect of soil microorganisms and labile C availability on soil respiration in response to litter inputs in forest ecosystems: A meta-analysis.

Litter inputs can influence soil respiration directly through labile C availability and, indirectly, through the activity of soil microorganisms and modifications in soil microclimate; however, their relative contributions and the magnitude of any effect remain poorly understood. We synthesized 66 recently published papers on forest ecosystems using a meta-analysis approach to investigate the effect of litter inputs on soil respiration and the underlying mechanisms involved. Our results showed that litter inputs had a strong positive impact on soil respiration, labile C availability, and the abundance of soil microorganisms, with less of an impact related to soil moisture and temperature. Overall, soil respiration was increased by 36% and 55%, respectively, in response to natural and doubled litter inputs. The increase in soil respiration induced by litter inputs showed a tendency for coniferous forests (50.7%)> broad-leaved forests (41.3%)> mixed forests (31.9%). This stimulation effect also depended on stand age with 30- to 100-year-old forests (53.3%) and ≥100-year-old forests (50.2%) both 1.5 times larger than ≤30-year-old forests (34.5%). Soil microbial biomass carbon and soil dissolved organic carbon increased by 21.0%-33.6% and 60.3%-87.7%, respectively, in response to natural and doubled litter inputs, while soil respiration increased linearly with corresponding increases in soil microbial biomass carbon and soil dissolved organic carbon. Natural and doubled litter inputs increased the total phospholipid fatty acid (PLFA) content by 6.6% and 19.7%, respectively, but decreased the fungal/bacterial PLFA ratio by 26.9% and 18.7%, respectively. Soil respiration also increased linearly with increases in total PLFA and decreased linearly with decreases in the fungal/bacterial PLFA ratio. The contribution of litter inputs to an increase in soil respiration showed a trend of total PLFA > fungal/bacterial PLFA ratio > soil dissolved organic carbon > soil microbial biomass carbon. Therefore, in addition to forest type and stand age, labile C availability and soil microorganisms are also important factors that influence soil respiration in response to litter inputs, with soil microorganisms being more important than labile C availability.

Author Correction: Nitrogen-rich organic soils under warm well-drained conditions are global nitrous oxide emission hotspots.

The original version of this Article contained an error in the first sentence of the Acknowledgements section, which incorrectly referred to the Estonian Research Council grant identifier as "PUTJD618". The correct version replaces the grant identifier with "PUTJD619". This has been corrected in both the PDF and HTML versions of the Article.

Nitrogen-rich organic soils under warm well-drained conditions are global nitrous oxide emission hotspots.

Nitrous oxide (N2O) is a powerful greenhouse gas and the main driver of stratospheric ozone depletion. Since soils are the largest source of N2O, predicting soil response to changes in climate or land use is central to understanding and managing N2O. Here we find that N2O flux can be predicted by models incorporating soil nitrate concentration (NO3-), water content and temperature using a global field survey of N2O emissions and potential driving factors across a wide range of organic soils. N2O emissions increase with NO3- and follow a bell-shaped distribution with water content. Combining the two functions explains 72% of N2O emission from all organic soils. Above 5 mg NO3--N kg-1, either draining wet soils or irrigating well-drained soils increases N2O emission by orders of magnitude. As soil temperature together with NO3- explains 69% of N2O emission, tropical wetlands should be a priority for N2O management.

A humidity shock leads to rapid, temperature dependent changes in coffee leaf physiology and gene expression.

Climate change is expected to increase the frequency of above-normal atmospheric water deficits contemporaneous with periods of high temperatures. Here we explore alterations in physiology and gene expression in leaves of Coffea canephora Pierre ex A. Froehner caused by a sharp drop in relative humidity (RH) at three different temperatures. Both stomatal conductance (gs) and CO2 assimilation (A) measurements showed that gs and A values fell quickly at all temperatures after the transfer to low RH.  However, leaf relative water content measurements indicated that leaves nonetheless experienced substantial water losses, implying that stomatal closure and/or resupply of water was not fast enough to stop excessive evaporative losses.  At 27 and 35 °C, upper leaves showed significant decreases in Fv/Fm compared with lower leaves, suggesting a stronger impact on photosystem II for upper leaves, while at 42 °C, both upper and lower leaves were equally affected. Quantitative gene expression analysis of transcription factors associated with conventional dehydration stress, and genes involved with abscisic acid signalling, such as CcNCED3, indicated temperature-dependent, transcriptional changes during the Humidity Shock ('HuS') treatments.  No expression was seen at 27 °C for the heat-shock gene CcHSP90-7, but it was strongly induced during the 42 °C 'HuS' treatment. Consistent with a proposal that important cellular damage occurred during the 42 °C 'HuS' treatment, two genes implicated in senescence were induced by this treatment. Overall, the data show that C. canephora plants subjected to a sharp drop in RH exhibit major, temperature-dependent alterations in leaf physiology and important changes in the expression of genes associated with abiotic stress and senescence. The results presented suggest that more detailed studies on the combined effects of low RH and high temperature are warranted.

Resource competition in plant invasions: emerging patterns and research needs.

Invasions by alien plants provide a unique opportunity to examine competitive interactions among plants. While resource competition has long been regarded as a major mechanism responsible for successful invasions, given a well-known capacity for many invaders to become dominant and reduce plant diversity in the invaded communities, few studies have measured resource competition directly or have assessed its importance relative to that of other mechanisms, at different stages of an invasion process. Here, we review evidence comparing the competitive ability of invasive species vs. that of co-occurring native plants, along a range of environmental gradients, showing that many invasive species have a superior competitive ability over native species, although invasive congeners are not necessarily competitively superior over native congeners, nor are alien dominants are better competitors than native dominants. We discuss how the outcomes of competition depend on a number of factors, such as the heterogeneous distribution of resources, the stage of the invasion process, as well as phenotypic plasticity and evolutionary adaptation, which may result in increased or decreased competitive ability in both invasive and native species. Competitive advantages of invasive species over natives are often transient and only important at the early stages of an invasion process. It remains unclear how important resource competition is relative to other mechanisms (competition avoidance via phenological differences, niche differentiation in space associated with phylogenetic distance, recruitment and dispersal limitation, indirect competition, and allelopathy). Finally, we identify the conceptual and methodological issues characterizing competition studies in plant invasions, and we discuss future research needs, including examination of resource competition dynamics and the impact of global environmental change on competitive interactions between invasive and native species.

Using soil seed banks to assess temporal patterns of genetic variation in invasive plant populations.

Most research on the genetics of invasive plant species has focused on analyzing spatial differences among existing populations. Using a long-established Gunnera tinctoria population from Ireland, we evaluated the potential of using plants derived from seeds associated with different soil layers to track genetic variation through time. This species and site were chosen because (1) G. tinctoria produces a large and persistent seed bank; (2) it has been present in this locality, Sraheens, for ∼90 years; (3) the soil is largely undisturbed; and (4) the soil's age can be reliably determined radiometrically at different depths. Amplified fragment length polymorphic markers (AFLPs) were used to assess differences in the genetic structure of 75 individuals sampled from both the standing population and from four soil layers, which spanned 18 cm (estimated at ∼90 years based on (210)Pb and (137)Cs dating). While there are difficulties in interpreting such data, including accounting for the effects of selection, seed loss, and seed migration, a clear pattern of lower total allele counts, percentage polymorphic loci, and genetic diversity was observed in deeper soils. The greatest percentage increase in the measured genetic variables occurred prior to the shift from the lag to the exponential range expansion phases and may be of adaptive significance. These findings highlight that seed banks in areas with long-established invasive populations can contain valuable genetic information relating to invasion processes and as such, should not be overlooked.

Simulating the effects of climate change on the distribution of an invasive plant, using a high resolution, local scale, mechanistic approach: challenges and insights.

The growing economic and ecological damage associated with biological invasions, which will likely be exacerbated by climate change, necessitates improved projections of invasive spread. Generally, potential changes in species distribution are investigated using climate envelope models; however, the reliability of such models has been questioned and they are not suitable for use at local scales. At this scale, mechanistic models are more appropriate. This paper discusses some key requirements for mechanistic models and utilises a newly developed model (PSS[gt]) that incorporates the influence of habitat type and related features (e.g., roads and rivers), as well as demographic processes and propagule dispersal dynamics, to model climate induced changes in the distribution of an invasive plant (Gunnera tinctoria) at a local scale. A new methodology is introduced, dynamic baseline benchmarking, which distinguishes climate-induced alterations in species distributions from other potential drivers of change. Using this approach, it was concluded that climate change, based on IPCC and C4i projections, has the potential to increase the spread-rate and intensity of G. tinctoria invasions. Increases in the number of individuals were primarily due to intensification of invasion in areas already invaded or in areas projected to be invaded in the dynamic baseline scenario. Temperature had the largest influence on changes in plant distributions. Water availability also had a large influence and introduced the most uncertainty in the projections. Additionally, due to the difficulties of parameterising models such as this, the process has been streamlined by utilising methods for estimating unknown variables and selecting only essential parameters.

Simulating the impacts of land use in northwest Europe on Net Ecosystem Exchange (NEE): the role of arable ecosystems, grasslands and forest plantations in climate change mitigation.

In this study, we compared measured and simulated Net Ecosystem Exchange (NEE) values from three wide spread ecosystems in the southeast of Ireland (forest, arable and grassland), and investigated the suitability of the DNDC (the DeNitrification-DeComposition) model to estimate present and future NEE. Although, the field-DNDC version overestimated NEE at temperatures >5 °C, forest-DNDC under-estimated NEE at temperatures >5 °C. The results suggest that the field/forest DNDC models can successfully estimate changes in seasonal and annual NEE from these ecosystems. Differences in NEE were found to be primarily land cover specific. The annual NEE was similar for the grassland and arable sites, but due to the contribution of exported carbon, the soil carbon increased at the grassland site and decreased at the arable site. The NEE of the forest site was an order of magnitude larger than that of the grassland or arable ecosystems, with large amounts of carbon stored in woody biomass and the soil. The average annual NEE, GPP and Reco values over the measurement period were -904, 2379 and 1475 g C m(-2) (forest plantations), -189, 906 and 715 g C m(-2) (arable systems) and -212, 1653 and 1444 g C m(-2) (grasslands), respectively. The average RMSE values were 3.8 g C m(-2) (forest plantations), 0.12 g C m(-2) (arable systems) and 0.21 g C m(-2) (grasslands). When these models were run with climate change scenarios to 2060, predictions show that all three ecosystems will continue to operate as carbon sinks. Further, climate change may decrease the carbon sink strength in the forest plantations by up to 50%. This study supports the use of the DNDC model as a valid tool to predict the consequences of climate change on NEE from different ecosystems.

Nitrogen deprivation stimulates symbiotic gland development in Gunnera manicata.

Gunnera is the only genus of angiosperms known to host cyanobacteria and the only group of land plants that hosts cyanobacteria intracellularly. Motile filaments of cyanobacteria, known as hormogonia, colonize Gunnera plants through cells in the plant's specialized stem glands. It is commonly held that Gunnera plants always possess functional glands for symbiosis. We found, however, that stem gland development did not occur when Gunnera manicata plants were grown on nitrogen (N)-replete medium but, rather, was initiated at predetermined positions when plants were deprived of combined N. While N status was the main determinant for gland development, an exogenous carbon source (sucrose) accelerated the process. Furthermore, a high level of sucrose stimulated the formation of callus-like tissue in place of the gland under N-replete conditions. Treatment of plants with the auxin transport inhibitor 1-naphthylphthalamic acid prevented gland development on N-limited medium, most likely by preventing resource reallocation from leaves to the stem. Optimized conditions were found for in vitro establishment of the Nostoc-Gunnera symbiosis by inoculating mature glands with hormogonia from Nostoc punctiforme, a cyanobacterium strain for which the full genome sequence is available. In contrast to uninoculated plants, G. manicata plants colonized by N. punctiforme were able to continue their growth on N-limited medium. Understanding the nature of the Gunnera plant's unusual adaptation to an N-limited environment may shed light on the evolution of plant-cyanobacterium symbioses and may suggest a route to establish productive associations between N-fixing cyanobacteria and crop plants.

Is acclimation required for success in high light environments? A case study using Mycelis muralis (L.) Dumort (Asteraceae).

In the field significant differences in maximum photosynthetic O2 -exchange rate (Pm ) were found between leaves of Mycelis muralis (L.) Dumort (Asteraceae) collected from woodland and exposed habitats, with the highest values in the exposed sites- However, there were no differences in the Pm of leaves collected from plants growing in grikes (fissures in the limestone pavement), of exposed limestone pavement, despite a greater than four-fold difference in the integrated daily irradiance. Leaves of plants from the open pavement had lower photon yields (ø1 ) and higher dark respiration rates and light compensation points, in comparison to shaded plants. Under controlled environmental conditions the highest Pm of leaves from plants subjected to variations in irradiance were found at the intermediate (8-6 mol photon m-2 d-1 growth light level used. At the highest growth irradiance 17.3 mol photon m-2 d-1 used in the laboratory both Pm and øl were reduced, although the latest plant biomass was found at this irradiance. No changes were found in the chlorophyll a:b ratio over the same range of irradiances. Examination of plant populations of M. muralis, collected from open or shaded habitats and exposed to growth irradiances that covered the range over which increases in photosynthesis were, observed in the laboratory (0.86-8.6 mol photon m-2 d-1 ), resulted in changes in leaf structure and pigment composition. The chlorophyll a:b ratio was low and largely independent of irradiance or the origin of the plant population. Differences in total chlorophyll content were small with the lowest values m the Durrow woodland populations at both irradiances. No variations were found in a number of chloroplast thylakoid structural features. In particular, the ratio of oppressed to non-appressed membranes was unchanged by growth at the two irradiances, consistent with an invariant chlorophyll a:b ratio. Based on peaks in the difference spectra the woodland populations had mi enhanced in vivo absorption at λlD= 650 and 706 nm when grown at low irradiance. These peaks were absent from the population collected from the open limestone pavement. The significance of the enhanced absorption at low irradiance and the possibility that these peaks represent long-wavelength forms of chlorophyll a (λlD = 706) and b (λlD = 650) is discussed. A particular feature of plants grown at high irradiance was an enhanced anthocyanin content in comparison to those grown at low irradiance. This was associated with an increase in absorptance. particularly in the green region (λlD = 550 nm) of the visible spectrum. Overall these results suggest that complete acclimation of photosynthesis and an ability to modulate light-harvesting is not a prerequisite, for success in a high light environment.

THE MINIMUM PHOTON REQUIREMENT FOR PHOTOSYNTHESIS: AN ANALYSIS OF THE DATA OF WARBURG & BURK (1950) AND YUAN, EVANS & DANIELS (1955).

Variations in the apparent photon requirement for photosynthesis (Φ-1 co 2 ) or (Φ-1 co 2 ) in the data of Warburg & Burk (1950) and Yuan, Evans & Daniels (1955) can be ascribed to changes in O2 uptake and energy-dependent processes which result in aberrant photon requirements in organisms subjected to non-optimal conditions. The increase in Φ-1 co 2 with increases in the gas exchange quotient (γ) in the observations of Yuan et al. (1955) is consistent with increases in photorespiratory production of glycollate, whilst changes in Φ-1 co 2 and Φ-1 co 2 in the results of Warburg & Burk (1950) can be explained by a variable Kok effect associated with nitrate assimilation at low light levels. When these O2 and energy-dependent processes are minimal, the lowest values should be observed. The minimum value obtained when Chlorella is photosynthesizing under optimal conditions is 6 mol photons mol-1 O2 . These results provide direct independent evidence for a photon requirement for photosynthesis of less than 8 mol photons mol-1 O2 . Such a value is not consistent with the Z scheme of photosynthesis.