De novo transcriptome assemblies of C3 and C4 non-model grass species reveal key differences in leaf development

BACKGROUND

C₄ photosynthesis is a mechanism that plants have evolved to reduce the rate of photorespiration during the carbon fixation process. The C₄ pathway allows plants to adapt to high temperatures and light while more efficiently using resources, such as water and nitrogen. Despite decades of studies, the evolution of the C₄ pathway from a C₃ ancestor remains a biological enigma. Interestingly, species with C₃-C₄ intermediates photosynthesis are usually found closely related to the C₄ lineages. Indeed, current models indicate that the assembly of C₄ photosynthesis was a gradual process that included the relocalization of photorespiratory enzymes, and the establishment of intermediate photosynthesis subtypes. More than a third of the C₄ origins occurred within the grass family (Poaceae). In particular, the Otachyriinae subtribe (Paspaleae tribe) includes 35 American species from C₃, C₄, and intermediates taxa making it an interesting lineage to answer questions about the evolution of photosynthesis.

RESULTS

To explore the molecular mechanisms that underpin the evolution of C₄ photosynthesis, the transcriptomic dynamics along four different leaf segments, that capture different stages of development, were compared among Otachyriinae non-model species. For this, leaf transcriptomes were sequenced, de novo assembled, and annotated. Gene expression patterns of key pathways along the leaf segments showed distinct differences between photosynthetic subtypes. In addition, genes associated with photorespiration and the C₄ cycle were differentially expressed between C₄ and C₃ species, but their expression patterns were well preserved throughout leaf development.

CONCLUSIONS

New, high-confidence, protein-coding leaf transcriptomes were generated using high-throughput short-read sequencing. These transcriptomes expand what is currently known about gene expression in leaves of non-model grass species. We found conserved expression patterns of C₄ cycle and photorespiratory genes among C₃, intermediate, and C₄ species, suggesting a prerequisite for the evolution of C₄ photosynthesis. This dataset represents a valuable contribution to the existing genomic resources and provides new tools for future investigation of photosynthesis evolution.

Meeting in the Middle: Lessons and Opportunities from Studying C3-C4 Intermediates

The discovery of C₃-C₄ intermediate species nearly 50 years ago opened up a new avenue for studying the evolution of photosynthetic pathways. Intermediate species exhibit anatomical, biochemical, and physiological traits that range from C₃ to C₄. A key feature of C₃-C₄ intermediates that utilize C₂ photosynthesis is the improvement in photosynthetic efficiency compared with C₃ species. Although the recruitment of some core enzymes is shared across lineages, there is significant variability in gene expression patterns, consistent with models that suggest numerous evolutionary paths from C₃ to C₄ photosynthesis. Despite the many evolutionary trajectories, the recruitment of glycine decarboxylase for C₂ photosynthesis is likely required. As technologies enable high-throughput genotyping and phenotyping, the discovery of new C₃-C₄ intermediates species will enrich comparisons between evolutionary lineages. The investigation of C₃-C₄ intermediate species will enhance our understanding of photosynthetic mechanisms and evolutionary processes and will potentially aid in crop improvement.

Lack of leaf carbonic anhydrase activity eliminates the C4 carbon-concentrating mechanism requiring direct diffusion of CO2 into bundle sheath cells

Carbonic anhydrase (CA) performs the first enzymatic step of C₄ photosynthesis by catalysing the reversible hydration of dissolved CO₂ that diffuses into mesophyll cells from intercellular airspaces. This CA-catalysed reaction provides the bicarbonate used by phosphoenolpyruvate carboxylase to generate products that flow into the C₄ carbon-concentrating mechanism (CCM). It was previously demonstrated that the Zea mays ca1ca2 double mutant lost 97% of leaf CA activity, but there was little difference in the growth phenotype under ambient CO₂ partial pressures (pCO₂ ). We hypothesise that since CAs are among the fastest enzymes, minimal activity from a third CA, CA8, can provide the inorganic carbon needed to drive C₄ photosynthesis. We observed that removing CA8 from the maize ca1ca2 background resulted in plants that had 0.2% of wild-type leaf CA activity. These ca1ca2ca8 plants had reduced photosynthetic parameters and could only survive at elevated pCO₂ . Photosynthetic and carbon isotope analysis combined with modelling of photosynthesis and carbon isotope discrimination was used to determine if ca1ca2ca8 plants had a functional C₄ cycle or were relying on direct CO₂ diffusion to ribulose 1,5-bisphosphate carboxylase/oxygenase within bundle sheath cells. The results suggest that leaf CA activity in ca1ca2ca8 plants was not sufficient to sustain the C₄ CCM.

Demonstration of local adaptation in maize landraces by reciprocal transplantation

Populations are locally adapted when they exhibit higher fitness than foreign populations in their native habitat. Maize landrace adaptations to highland and lowland conditions are of interest to researchers and breeders. To determine the prevalence and strength of local adaptation in maize landraces, we performed a reciprocal transplant experiment across an elevational gradient in Mexico. We grew 120 landraces, grouped into four populations (Mexican Highland, Mexican Lowland, South American Highland, South American Lowland), in Mexican highland and lowland common gardens and collected phenotypes relevant to fitness and known highland‐adaptive traits such as anthocyanin pigmentation and macrohair density. 67k DArTseq markers were generated from field specimens to allow comparisons between phenotypic patterns and population genetic structure. We found phenotypic patterns consistent with local adaptation, though these patterns differ between the Mexican and South American populations. Quantitative trait differentiation (Q_ST) was greater than neutral allele frequency differentiation (F_ST) for many traits, signaling directional selection between pairs of populations. All populations exhibited higher fitness metric values when grown at their native elevation, and Mexican landraces had higher fitness than South American landraces when grown in these Mexican sites. As environmental distance between landraces’ native collection sites and common garden sites increased, fitness values dropped, suggesting landraces are adapted to environmental conditions at their natal sites. Correlations between fitness and anthocyanin pigmentation and macrohair traits were stronger in the highland site than the lowland site, supporting their status as highland‐adaptive. These results give substance to the long‐held presumption of local adaptation of New World maize landraces to elevation and other environmental variables across North and South America.

The genetic architecture of leaf stable carbon isotope composition in Zea mays and the effect of transpiration efficiency on leaf elemental accumulation

With increased demand on freshwater resources for agriculture, it is imperative that more water-use efficient crops are developed. Leaf stable carbon isotope composition, δ13C, is a proxy for transpiration efficiency and a possible tool for breeders, but the underlying mechanisms effecting δ13C in C4 plants are not known. It has been suggested that differences in specific leaf area (SLA), which potentially reflects variation in internal CO2 diffusion, can impact leaf δ13C. Furthermore, although it is known that water movement is important for elemental uptake, it is not clear how manipulation of transpiration for increased water-use efficiency may impact nutrient accumulation. Here, we characterize the genetic architecture of leaf δ13C and test its relationship to SLA and the ionome in five populations of maize. Five significant QTL for leaf δ13C were identified, including novel QTL as well as some that were identified previously in maize kernels. One of the QTL regions contains an Erecta-like gene, the ortholog of which has been shown to regulate transpiration efficiency and leaf δ13C in Arabidopsis. QTL for δ13C were located in the same general chromosome region, but slightly shifted, when comparing data from two different years. Our data does not support a relationship between δ13C and SLA, and of the 19 elements analyzed, only a weak correlation between molybdenum and δ13C was detected. Together these data add to the genetic understanding of leaf δ13C in maize and suggest that improvements to plant water use may be possible without significantly influencing elemental homeostasis.

Genomic Prediction of Yield Traits in Single-Cross Hybrid Rice (Oryza sativa L.)

Hybrid rice varieties can outyield the best inbred varieties by 15 – 30% with appropriate management. However, hybrid rice requires more inputs and management than inbred rice to realize a yield advantage in high-yielding environments. The development of stress-tolerant hybrid rice with lowered input requirements could increase hybrid rice yield relative to production costs. We used genomic prediction to evaluate the combining abilities of 564 stress-tolerant lines used to develop Green Super Rice with 13 male sterile lines of the International Rice Research Institute for yield-related traits. We also evaluated the performance of their F₁ hybrids. We identified male sterile lines with good combining ability as well as F₁ hybrids with potential further use in product development. For yield per plant, accuracies of genomic predictions of hybrid genetic values ranged from 0.490 to 0.822 in cross-validation if neither parent or up to both parents were included in the training set, and both general and specific combining abilities were modeled. The accuracy of phenotypic selection for hybrid yield per plant was 0.682. The accuracy of genomic predictions of male GCA for yield per plant was 0.241, while the accuracy of phenotypic selection was 0.562. At the observed accuracies, genomic prediction of hybrid genetic value could allow improved identification of high-performing single crosses. In a reciprocal recurrent genomic selection program with an accelerated breeding cycle, observed male GCA genomic prediction accuracies would lead to similar rates of genetic gain as phenotypic selection. It is likely that prediction accuracies of male GCA could be improved further by targeted expansion of the training set. Additionally, we tested the correlation of parental genetic distance with mid-parent heterosis in the phenotyped hybrids. We found the average mid-parent heterosis for yield per plant to be consistent with existing literature values at 32.0%. In the overall population of study, parental genetic distance was significantly negatively correlated with mid-parent heterosis for yield per plant (r = −0.131) and potential yield (r = −0.092), but within female families the correlations were non-significant and near zero. As such, positive parental genetic distance was not reliably associated with positive mid-parent heterosis.

Heterosis and Hybrid Crop Breeding: A Multidisciplinary Review

Although hybrid crop varieties are among the most popular agricultural innovations, the rationale for hybrid crop breeding is sometimes misunderstood. Hybrid breeding is slower and more resource-intensive than inbred breeding, but it allows systematic improvement of a population by recurrent selection and exploitation of heterosis simultaneously. Inbred parental lines can identically reproduce both themselves and their F1 progeny indefinitely, whereas outbred lines cannot, so uniform outbred lines must be bred indirectly through their inbred parents to harness heterosis. Heterosis is an expected consequence of whole-genome non-additive effects at the population level over evolutionary time. Understanding heterosis from the perspective of molecular genetic mechanisms alone may be elusive, because heterosis is likely an emergent property of populations. Hybrid breeding is a process of recurrent population improvement to maximize hybrid performance. Hybrid breeding is not maximization of heterosis per se, nor testing random combinations of individuals to find an exceptional hybrid, nor using heterosis in place of population improvement. Though there are methods to harness heterosis other than hybrid breeding, such as use of open-pollinated varieties or clonal propagation, they are not currently suitable for all crops or production environments. The use of genomic selection can decrease cycle time and costs in hybrid breeding, particularly by rapidly establishing heterotic pools, reducing testcrossing, and limiting the loss of genetic variance. Open questions in optimal use of genomic selection in hybrid crop breeding programs remain, such as how to choose founders of heterotic pools, the importance of dominance effects in genomic prediction, the necessary frequency of updating the training set with phenotypic information, and how to maintain genetic variance and prevent fixation of deleterious alleles.

Leaf Angle eXtractor: A high-throughput image processing framework for leaf angle measurements in maize and sorghum

PREMISE

Maize yields have significantly increased over the past half-century owing to advances in breeding and agronomic practices. Plants have been grown in increasingly higher densities due to changes in plant architecture resulting in plants with more upright leaves, which allows more efficient light interception for photosynthesis. Natural variation for leaf angle has been identified in maize and sorghum using multiple mapping populations. However, conventional phenotyping techniques for leaf angle are low throughput and labor intensive, and therefore hinder a mechanistic understanding of how the leaf angle of individual leaves changes over time in response to the environment.

METHODS

High-throughput time series image data from water-deprived maize (Zea mays subsp. mays) and sorghum (Sorghum bicolor) were obtained using battery-powered time-lapse cameras. A MATLAB-based image processing framework, Leaf Angle eXtractor (LAX), was developed to extract and quantify leaf angles from images of maize and sorghum plants under drought conditions.

RESULTS

Leaf angle measurements showed differences in leaf responses to drought in maize and sorghum. Tracking leaf angle changes at intervals as short as one minute enabled distinguishing leaves that showed signs of wilting under water deprivation from other leaves on the same plant that did not show wilting during the same time period.

DISCUSSION

Automating leaf angle measurements using LAX makes it feasible to perform large-scale experiments to evaluate, understand, and exploit the spatial and temporal variations in plant response to water limitations.

Insights from transcriptome profiling on the non-photosynthetic and stomatal signaling response of maize carbonic anhydrase mutants to low CO2

BACKGROUND

Carbonic anhydrase (CA) catalyzes the hydration of CO2 in the first biochemical step of C4 photosynthesis, and has been considered a potentially rate-limiting step when CO2 availability within a leaf is low. Previous work in Zea mays (maize) with a double knockout of the two highest-expressed β-CA genes, CA1 and CA2, reduced total leaf CA activity to less than 3% of wild-type. Surprisingly, this did not limit photosynthesis in maize at ambient or higher CO2concentrations. However, the ca1ca2 mutants exhibited reduced rates of photosynthesis at sub-ambient CO2, and accumulated less biomass when grown under sub-ambient CO2 (9.2 Pa). To further clarify the importance of CA for C4 photosynthesis, we assessed gene expression changes in wild-type, ca1 and ca1ca2 mutants in response to changes in pCO2 from 920 to 9.2 Pa.

RESULTS

Leaf samples from each genotype were collected for RNA-seq analysis at high CO2 and at two time points after the low CO2 transition, in order to identify early and longer-term responses to CO2 deprivation. Despite the existence of multiple isoforms of CA, no other CA genes were upregulated in CA mutants. Although photosynthetic genes were downregulated in response to low CO2, differential expression was not observed between genotypes. However, multiple indicators of carbon starvation were present in the mutants, including amino acid synthesis, carbohydrate metabolism, and sugar signaling. In particular, multiple genes previously implicated in low carbon stress such as asparagine synthetase, amino acid transporters, trehalose-6-phosphate synthase, as well as many transcription factors, were strongly upregulated. Furthermore, genes in the CO2 stomatal signaling pathway were differentially expressed in the CA mutants under low CO2.

CONCLUSIONS

Using a transcriptomic approach, we showed that carbonic anhydrase mutants do not compensate for the lack of CA activity by upregulating other CA or photosynthetic genes, but rather experienced extreme carbon stress when grown under low CO2. Our results also support a role for CA in the CO2 stomatal signaling pathway. This study provides insight into the importance of CA for C4 photosynthesis and its role in stomatal signaling.

Leaf stable carbon isotope composition reflects transpiration efficiency in Zea mays

The increasing demand for food production and predicted climate change scenarios highlight the need for improvements in crop sustainability. The efficient use of water will become increasingly important for rain-fed agricultural crops even in fertile regions that have historically received ample precipitation. Improvements in water-use efficiency in Zea mays have been limited, and warrant a renewed effort aided by molecular breeding approaches. Progress has been constrained by the difficulty of measuring water-use in a field environment. The stable carbon isotope composition (δ¹³ C) of the leaf has been proposed as an integrated signature of carbon fixation with a link to stomatal conductance. However, additional factors affecting leaf δ¹³ C exist, and a limited number of studies have explored this trait in Z. mays. Here we present an extensive characterization of leaf δ¹³ C in Z. mays. Significant variation in leaf δ¹³ C exists across diverse lines of Z. mays, which we show to be heritable across several environments. Furthermore, we examine temporal and spatial variation in leaf δ¹³ C to determine the optimum sampling time to maximize the use of leaf δ¹³ C as a trait. Finally, our results demonstrate the relationship between transpiration and leaf δ¹³ C in the field and the greenhouse. Decreasing transpiration and soil moisture are associated with decreasing leaf δ¹³ C. Taken together these results outline a strategy for using leaf δ¹³ C and reveal its usefulness as a measure of transpiration efficiency under well-watered conditions rather than a predictor of performance under drought.

Carbonic Anhydrase Mutants in Zea mays Have Altered Stomatal Responses to Environmental Signals

Stomata regulate transpirational water loss and CO2 uptake for photosynthesis in response to changing environmental conditions. Research investigating stomatal movement has mostly been conducted in C3 eudicot species, which have very different CO2 requirements for photosynthesis relative to C4 grasses. Carbonic anhydrase (CA) catalyzes the hydration of CO2, and its activity has been linked to stomatal aperture regulation in eudicots. The number of Ca genes and their evolutionary history differ between monocots and dicots, and many questions remain unanswered about potential neofunctionalization and subfunctionalization of grass Ca paralogs and their roles in photosynthesis and stomatal conductance. To investigate the roles of different Ca genes in maize (Zea mays), we examined stomatal responses in ca1 and ca2 single mutants as well as a ca1ca2 double mutant. The ca1 and ca2 single mutants had 10% and 87% of the CA activity exhibited by the wild type, respectively, while ca1ca2 had less than 5% of wild-type CA activity. The ca mutants had higher stomatal conductance than the wild type and slower stomatal closure in response to increases in CO2 partial pressure. Contrary to previous reports in eudicots, ca mutants showed slowed stomatal closure in response to the light-dark transition and did not show differences in stomatal density compared with the wild type. These results implicate CA-mediated signaling in the control of stomatal movement but not stomatal development. Drought experiments with ca1ca2 mutant plants suggest a role for CA in water-use efficiency and reveal that Z. mays is not optimized for water-use efficiency under well-watered conditions.

Biochemical and transcriptomic analysis of maize diversity to elucidate drivers of leaf carbon isotope composition

Carbon isotope discrimination is used to study CO2 diffusion, substrate availability for photosynthesis, and leaf biochemistry, but the intraspecific drivers of leaf carbon isotope composition (δ13C) in C4 species are not well understood. In this study, the role of photosynthetic enzymes and post-photosynthetic fractionation on δ13C (‰) was explored across diverse maize inbred lines. A significant 1.3‰ difference in δ13C was observed between lines but δ13C did not correlate with in vitro leaf carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), or ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity. RNA-sequencing was used to identify potential differences in post-photosynthetic metabolism that would influence δ13C; however, no correlations were identified that would indicate significant differences in post-photosynthetic fractionation between lines. Variation in δ13C has been observed between C4 subtypes, but differential expression of NADP-ME and PEP-CK pathways within these lines did not correlate with δ13C. However, co-expression network analysis provided novel evidence for isoforms of C4 enzymes and putative transporters. Together, these data indicate that diversity in maize δ13C cannot be fully explained by variation in CA, PEPC, or Rubisco activity or gene expression. The findings further emphasise the need for future work exploring the influence of stomatal sensitivity and mesophyll conductance on δ13C in maize.

Cross species selection scans identify components of C4 photosynthesis in the grasses

C4 photosynthesis is perhaps one of the best examples of convergent adaptive evolution with over 25 independent origins in the grasses (Poaceae) alone. The availability of high quality grass genome sequences presents new opportunities to explore the mechanisms underlying this complex trait using evolutionary biology-based approaches. In this study, we performed genome-wide cross-species selection scans in C4 lineages to facilitate discovery of C4 genes. The study was enabled by the well conserved collinearity of grass genomes and the recently sequenced genome of a C3 panicoid grass, Dichanthelium oligosanthes This method, in contrast to previous studies, does not rely on any a priori knowledge of the genes that contribute to biochemical or anatomical innovations associated with C4 photosynthesis. We identified a list of 88 candidate genes that include both known and potentially novel components of the C4 pathway. This set includes the carbon shuttle enzymes pyruvate, phosphate dikinase, phosphoenolpyruvate carboxylase and NADP malic enzyme as well as several predicted transporter proteins that likely play an essential role in promoting the flux of metabolites between the bundle sheath and mesophyll cells. Importantly, this approach demonstrates the application of fundamental molecular evolution principles to dissect the genetic basis of a complex photosynthetic adaptation in plants. Furthermore, we demonstrate how the output of the selection scans can be combined with expression data to provide additional power to prioritize candidate gene lists and suggest novel opportunities for pathway engineering.

A Gene for Genetic Background in Zea mays: Fine-Mapping enhancer of teosinte branched1.2 to a YABBY Class Transcription Factor

The effects of an allelic substitution at a gene often depend critically on genetic background, i.e., the genotypes at other genes in the genome. During the domestication of maize from its wild ancestor (teosinte), an allelic substitution at teosinte branched (tb1) caused changes in both plant and ear architecture. The effects of tb1 on phenotype were shown to depend on multiple background loci, including one called enhancer of tb1.2 (etb1.2). We mapped etb1.2 to a YABBY class transcription factor (ZmYAB2.1) and showed that the maize alleles of ZmYAB2.1 are either expressed at a lower level than teosinte alleles or disrupted by insertions in the sequences. tb1 and etb1.2 interact epistatically to control the length of internodes within the maize ear, which affects how densely the kernels are packed on the ear. The interaction effect is also observed at the level of gene expression, with tb1 acting as a repressor of ZmYAB2.1 expression. Curiously, ZmYAB2.1 was previously identified as a candidate gene for another domestication trait in maize, nonshattering ears. Consistent with this proposed role, ZmYAB2.1 is expressed in a narrow band of cells in immature ears that appears to represent a vestigial abscission (shattering) zone. Expression in this band of cells may also underlie the effect on internode elongation. The identification of ZmYAB2.1 as a background factor interacting with tb1 is a first step toward a gene-level understanding of how tb1 and the background within which it works evolved in concert during maize domestication.

The draft genome of the C3 panicoid grass species Dichanthelium oligosanthes

BACKGROUND

Comparisons between C3 and C4 grasses often utilize C3 species from the subfamilies Ehrhartoideae or Pooideae and C4 species from the subfamily Panicoideae, two clades that diverged over 50 million years ago. The divergence of the C3 panicoid grass Dichanthelium oligosanthes from the independent C4 lineages represented by Setaria viridis and Sorghum bicolor occurred approximately 15 million years ago, which is significantly more recent than members of the Bambusoideae, Ehrhartoideae, and Pooideae subfamilies. D. oligosanthes is ideally placed within the panicoid clade for comparative studies of C3 and C4 grasses.

RESULTS

We report the assembly of the nuclear and chloroplast genomes of D. oligosanthes, from high-throughput short read sequencing data and a comparative transcriptomics analysis of the developing leaf of D. oligosanthes, S. viridis, and S. bicolor. Physiological and anatomical characterizations verified that D. oligosanthes utilizes the C3 pathway for carbon fixation and lacks Kranz anatomy. Expression profiles of transcription factors along developing leaves of D. oligosanthes and S. viridis were compared with previously published data from S. bicolor, Zea mays, and Oryza sativa to identify a small suite of transcription factors that likely acquired functions specifically related to C4 photosynthesis.

CONCLUSIONS

The phylogenetic location of D. oligosanthes makes it an ideal C3 plant for comparative analysis of C4 evolution in the panicoid grasses. This genome will not only provide a better C3 species for comparisons with C4 panicoid grasses, but also highlights the power of using high-throughput sequencing to address questions in evolutionary biology.

Complex Field-Induced States in Linarite PbCuSO4(OH)2 with a Variety of High-Order Exotic Spin-Density Wave States

Low-temperature neutron diffraction and NMR studies of field-induced phases in linarite are presented for magnetic fields H∥b axis. A two-step spin-flop transition is observed, as well as a transition transforming a helical magnetic ground state into an unusual magnetic phase with sine-wave-modulated moments ∥H. An effective J[over ˜]_{1}-J[over ˜]_{2} single-chain model with a magnetization-dependent frustration ratio α_{eff}=-J[over ˜]_{2}/J[over ˜]_{1} is proposed. The latter is governed by skew interchain couplings and shifted to the vicinity of the ferromagnetic critical point. It explains qualitatively the observation of a rich variety of exotic longitudinal collinear spin-density wave, SDW_{p}, states (9≥p≥2).

A Limited Role for Carbonic Anhydrase in C4 Photosynthesis as Revealed by a ca1ca2 Double Mutant in Maize

Carbonic anhydrase (CA) catalyzes the first biochemical step of the carbon-concentrating mechanism of C₄ plants, and in C₄ monocots it has been suggested that CA activity is near limiting for photosynthesis. Here, we test this hypothesis through the characterization of transposon-induced mutant alleles of Ca1 and Ca2 in maize (Zea mays). These two isoforms account for more than 85% of the CA transcript pool. A significant change in isotopic discrimination is observed in mutant plants, which have as little as 3% of wild-type CA activity, but surprisingly, photosynthesis is not reduced under current or elevated CO₂ partial pressure (pCO₂). However, growth and rates of photosynthesis under subambient pCO₂ are significantly impaired in the mutants. These findings suggest that, while CA is not limiting for C₄ photosynthesis in maize at current pCO₂, it likely maintains high rates of photosynthesis when CO₂ availability is reduced. Current atmospheric CO₂ levels now exceed 400 ppm (approximately 40.53 Pa) and contrast with the low-pCO₂ conditions under which C₄ plants expanded their range approximately 10 million years ago, when the global atmospheric CO₂ was below 300 ppm (approximately 30.4 Pa). Thus, as CO₂ levels continue to rise, selective pressures for high levels of CA may be limited to arid climates where stomatal closure reduces CO₂ availability to the leaf.

Driving magnetostructural transitions in layered intermetallic compounds

We report the dramatic effect of applied pressure and magnetic field on the layered intermetallic compound Pr(0.5)Y(0.5)Mn(2)Ge(2). In the absence of pressure or magnetic field this compound displays interplanar ferromagnetism at room temperature and undergoes an isostructural first order magnetic transition (FOMT) to an antiferromagnetic state below 158 K, followed by another FOMT at 50 K due to the reemergence of ferromagnetism as praseodymium orders (T(C)(Pr)). The application of a magnetic field drives these two transitions towards each other, whereas the application of pressure drives them apart. Pressure also produces a giant magnetocaloric effect such that a threefold increase of the entropy change associated with the lower FOMT (at T(C)(Pr)) is seen under a pressure of 7.5 kbar. First principles calculations, using density functional theory, show that this remarkable magnetic behavior derives from the strong magnetoelastic coupling of the manganese layers in this compound.

The magnetocaloric effect and critical behaviour of the Mn(0.94)Ti(0.06)CoGe alloy

Structural, magnetic and magnetocaloric properties of the Mn(0.94)Ti(0.06)CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected, at T(str) = 235 K and T(C) = 270 K. A giant magnetocaloric effect has been obtained at around T(str) associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni(2)In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, at T(str), the magnetic entropy change, -ΔS(M) reached a maximum value of 14.8 J kg(-1) K(-1) under a magnetic field of 5 T, which is much higher than that previously reported for the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around T(C) = 270 K from the ferromagnetic to the paramagnetic state, we performed a detailed critical exponent study. The critical components γ, β and δ determined using the Kouvel-Fisher method, the modified Arrott plot and the critical isotherm analysis agree well. The values deduced for the critical exponents are close to the theoretical prediction from the mean-field model, indicating that the magnetic interactions are long range. On the basis of these critical exponents, the magnetization, field and temperature data around T(C) collapse onto two curves obeying the single scaling equation M(H,ε) = ε(β)f ± (H/ε(β+γ)).

The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8)

The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) were made up of stacked perovskite-like layers of FeO(6) octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca(2)Fe(2)O(5)) structure type. We have gone further and determined a likely space group for this Grenier phase and determined the magnetic structure of the compounds at room temperature. The space group is found to be P 2(1)ma (b axis as the long axis), and the crystal structure has been refined, subject to the stacking faulting along the long axis that is apparent in electron diffraction patterns. The magnetic structure of LaCa(2)Fe(3)O(8) is shown to consist of antiferromagnetically ordered Fe(3+) ions on a collinear G-type antiferromagnetic structure, with the magnetic moments most likely (anti)parallel with the c axis, and of magnitude 3.4 ± 0.2μ(B) (3.6 ± 0.2μ(B) for NdCa(2)Fe(3)O(8)). The result is reasonable given the magnetic structures of the end members of the La(1-x)Ca(x)FeO(3) series, LaFeO(3) (x = 0) and Ca(2)Fe(2)O(5) (x = 1).

Magnetic phase transitions in Pr(1-x)Lu(x)Mn(2)Ge(2) compounds

The effects of replacing Pr by Lu on the magnetic behaviour and structures of Pr(1-x)Lu(x)Mn(2)Ge(2) (x = 0.2,x = 0.4) have been investigated using x-ray diffraction, Mössbauer spectroscopy, magnetization and neutron diffraction measurements. The substitution of Lu for Pr leads to a decrease in the lattice constants a, c and the unit cell volume V at room temperature with this contraction of the unit cell resulting in modifications of the Pr(1-x)Lu(x)Mn(2)Ge(2) magnetic structures. Four and five magnetic phase transitions-linked primarily with temperature driven changes in the intralayer Mn-Mn separation distances-have been detected within the temperature range 4.5-550 K for Pr(0.8)Lu(0.2)Mn(2)Ge(2) and Pr(0.6)Lu(0.4)Mn(2)Ge(2), respectively, with re-entrant ferromagnetism being detected around T(C)(Pr)∼31 K for Pr(0.6)Lu(0.4)Mn(2)Ge(2). It was found that T(C)(inter) and T(C)(Pr) increase with increasing applied field while T(N)(inter) decreases for Pr(0.6)Lu(0.4)Mn(2)Ge(2), indicating that the canted antiferromagnetic AFmc region contracts with increasing field. The Debye temperatures for Pr(1-x)Lu(x)Mn(2)Ge(2) with x = 0.2 and 0.4 were evaluated as θ(D) = 320 ± 40 K and θ(D) = 400 ± 20 K respectively from the temperature dependence of the average isomer shift. The magnetic structures of both compounds have been determined by means of neutron diffraction measurements over the temperature range 3-300 K with formation of the Fmi magnetic state below T(c/c) = 192 K for Pr(0.8)Lu(0.2)Mn(2)Ge(2) and the occurrence of re-entrant ferromagnetism below T(C)(Pr) = 31 K for Pr(0.6)Lu(0.4)Mn(2)Ge(2) being confirmed.

Light emission from a titanium vacuum arc using Fizeau interferometry with parallel detection

The lineshape of light emission from a titanium vacuum arc was studied using a Fizeau interferometer coupled with an optical multichannel analyzer (OMA). A viewing geometry normal to the cathode surface was employed. Temperatures of ~3 x 10(5) K and ~3.5 x 10(4) K were obtained for titanium ions and titanium atoms present in the cathode spot, respectively. In light of results from previous work, a case is made for the latter temperature being the actual heavy species temperature in the cathode spot.