Tipburn resilience in lettuce (Lactuca spp.) - the importance of germplasm resources and production system-specific assays.

BACKGROUND: Tipburn is a physiological disorder of lettuce (Lactuca spp.). It causes discoloration and collapse of leaf margins, leading to unsaleable crops in both protected (glasshouse, hydroponic) and outdoor production systems. The occurrence of tipburn is hard to predict and is sensitive to environmental conditions. Phenotyping for tipburn resilience requires diverse germplasm resources and, to date, limited material has been investigated for this condition. RESULTS: Using a Lactuca diversity fixed foundation set (DFFS) under glasshouse conditions, we identified a significant (P < 0.001) genotypic effect on tipburn resilience across both the entire population and across lines belonging to the cultivated species L. sativa alone. Latuca sativa lines exhibited significantly (P < 0.05) higher average tipburn severity than those belonging to the wild species L. saligna, L. serriola, and L. virosa but we were able to identify both cultivated and wild tipburn-resilient lines. Leaf morphology factors, which included pigmentation, width, and serration, also significantly (P < 0.05) influenced tipburn resilience. Using a recombinant inbred line (RIL) mapping population derived from two DFFS lines, different small-effect quantitative trait loci (QTLs) accounting for 12.3% and 25.2% of total tipburn variation were identified in glasshouse and field conditions, respectively. CONCLUSIONS: These results reflect the advantages of phenotyping under production-system-specific conditions for the examination of environmentally sensitive traits and highlight genetic markers and germplasm resources for the development of tipburn resilient lines for use in both protected and outdoor lettuce production. © 2023 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The Evolutionary History of Wild, Domesticated, and Feral Brassica oleracea (Brassicaceae).

Understanding the evolutionary history of crops, including identifying wild relatives, helps to provide insight for conservation and crop breeding efforts. Cultivated Brassica oleracea has intrigued researchers for centuries due to its wide diversity in forms, which include cabbage, broccoli, cauliflower, kale, kohlrabi, and Brussels sprouts. Yet, the evolutionary history of this species remains understudied. With such different vegetables produced from a single species, B. oleracea is a model organism for understanding the power of artificial selection. Persistent challenges in the study of B. oleracea include conflicting hypotheses regarding domestication and the identity of the closest living wild relative. Using newly generated RNA-seq data for a diversity panel of 224 accessions, which represents 14 different B. oleracea crop types and nine potential wild progenitor species, we integrate phylogenetic and population genetic techniques with ecological niche modeling, archaeological, and literary evidence to examine relationships among cultivars and wild relatives to clarify the origin of this horticulturally important species. Our analyses point to the Aegean endemic B. cretica as the closest living relative of cultivated B. oleracea, supporting an origin of cultivation in the Eastern Mediterranean region. Additionally, we identify several feral lineages, suggesting that cultivated plants of this species can revert to a wild-like state with relative ease. By expanding our understanding of the evolutionary history in B. oleracea, these results contribute to a growing body of knowledge on crop domestication that will facilitate continued breeding efforts including adaptation to changing environmental conditions.

Identification of genetic loci in lettuce mediating quantitative resistance to fungal pathogens.

KEY MESSAGE: We demonstrate genetic variation for quantitative resistance against important fungal pathogens in lettuce and its wild relatives, map loci conferring resistance and predict key molecular mechanisms using transcriptome profiling. Lactuca sativa L. (lettuce) is an important leafy vegetable crop grown and consumed globally. Chemicals are routinely used to control major pathogens, including the causal agents of grey mould (Botrytis cinerea) and lettuce drop (Sclerotinia sclerotiorum). With increasing prevalence of pathogen resistance to fungicides and environmental concerns, there is an urgent need to identify sources of genetic resistance to B. cinerea and S. sclerotiorum in lettuce. We demonstrated genetic variation for quantitative resistance to B. cinerea and S. sclerotiorum in a set of 97 diverse lettuce and wild relative accessions, and between the parents of lettuce mapping populations. Transcriptome profiling across multiple lettuce accessions enabled us to identify genes with expression correlated with resistance, predicting the importance of post-transcriptional gene regulation in the lettuce defence response. We identified five genetic loci influencing quantitative resistance in a F6 mapping population derived from a Lactuca serriola (wild relative) × lettuce cross, which each explained 5-10% of the variation. Differential gene expression analysis between the parent lines, and integration of data on correlation of gene expression and resistance in the diversity set, highlighted potential causal genes underlying the quantitative trait loci.

Crystal Memory near Discontinuous Triacylglycerol Phase Transitions: Models, Metastable Regimes, and Critical Points.

It is proposed that "crystal memory", observed in a discontinuous solid-liquid phase transition of saturated triacylglycerol (TAG) molecules, is due to the coexistence of solid TAG crystalline phases and a liquid TAG phase, in a superheated metastable regime. Such a coexistence has been detected. Solid crystals can act as heterogeneous nuclei onto which molecules can condense as the temperature is lowered. We outlined a mathematical model, with a single phase transition, that shows how the time-temperature observations can be explained, makes predictions, and relates them to recent experimental data. A modified Vogel-Fulcher-Tammann (VFT) equation is used to predict time-temperature relations for the observation of "crystal memory" and to show boundaries beyond which "crystal memory" is not observed. A plot of the lifetime of a metastable state versus temperature, using the modified VFT equation, agrees with recent time-temperature data. The model can be falsified through its predictions: the model possesses a critical point and we outline a procedure describing how it could be observed by changing the hydrocarbon chain length. We make predictions about how thermodynamic functions will change as the critical point is reached and as the system enters a crossover regime. The model predicts that the phenomenon of "crystal memory" will not be observed unless the system is cooled from a superheated metastable regime associated with a discontinuous phase transition.

Assembly and characterisation of a unique onion diversity set identifies resistance to Fusarium basal rot and improved seedling vigour.

KEY MESSAGE: A unique, global onion diversity set was assembled, genotyped and phenotyped for beneficial traits. Accessions with strong basal rot resistance and increased seedling vigour were identified along with associated markers. Conserving biodiversity is critical for safeguarding future crop production. Onion (Allium cepa L.) is a globally important crop with a very large (16 Gb per 1C) genome which has not been sequenced. While onions are self-fertile, they suffer from severe inbreeding depression and as such are highly heterozygous as a result of out-crossing. Bulb formation is driven by daylength, and accessions are adapted to the local photoperiod. Onion seed is often directly sown in the field, and hence seedling establishment is a critical trait for production. Furthermore, onion yield losses regularly occur worldwide due to Fusarium basal rot caused by Fusarium oxysporum f. sp. cepae. A globally relevant onion diversity set, consisting of 10 half-sib families for each of 95 accessions, was assembled and genotyping carried out using 892 SNP markers. A moderate level of heterozygosity (30-35%) was observed, reflecting the outbreeding nature of the crop. Using inferred phylogenies, population structure and principal component analyses, most accessions grouped according to local daylength. A high level of intra-accession diversity was observed, but this was less than inter-accession diversity. Accessions with strong basal rot resistance and increased seedling vigour were identified along with associated markers, confirming the utility of the diversity set for discovering beneficial traits. The onion diversity set and associated trait data therefore provide a valuable resource for future germplasm selection and onion breeding.

Analysis of Brassica oleracea early stage abiotic stress responses reveals tolerance in multiple crop types and for multiple sources of stress.

BACKGROUND: Brassica oleracea includes a number of important crop types such as cabbage, cauliflower, broccoli and kale. Current climate conditions and weather patterns are causing significant losses in these crops, meaning that new cultivars with improved tolerance of one or more abiotic stress types must be sought. In this study, genetically fixed B. oleracea lines belonging to a Diversity Fixed Foundation Set (DFFS) were assayed for their response to seedling stage-imposed drought, flood, salinity, heat and cold stress. RESULTS: Significant (P ≤ 0.05) variation in stress tolerance response was found for each stress, for each of four measured variables (relative fresh weight, relative dry weight, relative leaf number and relative plant height). Lines tolerant to multiple stresses were found to belong to several different crop types. There was no overall correlation between the responses to the different stresses. CONCLUSION: Abiotic stress tolerance was identified in multiple B. oleracea crop types, with some lines exhibiting resistance to multiple stresses. For each stress, no one crop type appeared significantly more or less tolerant than others. The results are promising for the development of more environmentally robust lines of different B. oleracea crops by identifying tolerant material and highlighting the relationship between responses to different stresses. © 2017 Society of Chemical Industry.

Intramembrane congestion effects on lysenin channel voltage-induced gating.

All cell membranes are packed with proteins. The ability to investigate the regulatory mechanisms of protein channels in experimental conditions mimicking their congested native environment is crucial for understanding the environmental physicochemical cues that may fundamentally contribute to their functionality in natural membranes. Here we report on investigations of the voltage-induced gating of lysenin channels in congested conditions experimentally achieved by increasing the number of channels inserted into planar lipid membranes. Typical electrophysiology measurements reveal congestion-induced changes to the voltage-induced gating, manifested as a significant reduction of the response to external voltage stimuli. Furthermore, we demonstrate a similar diminished voltage sensitivity for smaller populations of channels by reducing the amount of sphingomyelin in the membrane. Given lysenin's preference for targeting lipid rafts, this result indicates the potential role of the heterogeneous organization of the membrane in modulating channel functionality. Our work indicates that local congestion within membranes may alter the energy landscape and the kinetics of conformational changes of lysenin channels in response to voltage stimuli. This level of understanding may be extended to better characterize the role of the specific membrane environment in modulating the biological functionality of protein channels in health and disease.

Modeling the surface of Campylobacter fetus: protein surface layer stability and resistance to cationic antimicrobial peptides.

Campylobacter fetus is a Gram negative bacterium recognized for its virulence in animals and humans. This bacterium possesses a paracrystalline array of high molecular weight proteins known as surface-layer proteins covering its cell surface. A mathematical model has been made of the outer membrane of this bacterium, both with its surface-layer proteins (S+) and without (S-). Monte Carlo computer simulation was used to understand the stability of the surface-layer protein structure as a function of ionic concentration. The interactions of an electrically-charged antimicrobial agent, the cationic antimicrobial peptide protamine, with surface-layer proteins and with the lipopolysaccharides of the outer membrane were modeled and analyzed. We found that (1) divalent ions stabilize the surface-layer protein array by reducing the fluctuations perpendicular and parallel to the membrane plane thereby promoting adhesion to the LPS region. This was achieved via (2) divalent ions bridging the negatively-charged LPS Core. The effect of this bridging is to bring individual Core regions closer together so that the O-antigens can (3) increase their attractive van der Waals interactions and "collapse" to form a surface with reduced perpendicular fluctuations. These findings provide support for the proposal of Yang et al. [1]. (4) No evidence for a significant increase in Ca(2+) concentration in the region of the surface-layer protein subunits was observed in S+ simulations compared to S- simulations. (5) We predicted the trends of protamine MIC tests performed on C. fetus and these were in good agreement with our experimental results.

Interaction of protamine with gram-negative bacteria membranes: possible alternative mechanisms of internalization in Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa.

This study was concerned with the interaction between the cationic antimicrobial peptide, protamine (Ptm) and the cytoplasmic membranes of the gram-negative bacteria Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa. The objective of the study was to explain the observed paradox of internalization without permanent disruption of the cell envelope. We carried out Monte Carlo computer simulation of Ptm in an aqueous environment in the presence of ~100 mM NaCl and model membranes consisting of either (65:35) or (75:25) PE:PG molar ratios. The (75:25) model, representative of the gram-negative cytoplasmic membrane, showed that the Ptm center of mass remained at least 7 nm from the membrane surface leading to the prediction that Ptm would not internalize via disruption of the inner membrane. By using immunoelectron microscopy of Ptm-treated cells, we showed that Ptm internalization to the cytoplasm took place rapidly in the presence or absence of the outer envelope. Ultrastructural examination revealed no obvious morphological changes to cells that were treated with subinhibitory or bactericidal levels of Ptm. Reconstituted phospholipid bilayers were constructed and were unperturbed by Ptm treatment over a wide range of concentrations and applied transmembrane voltages. We conclude that in the cases of the cell envelopes of E. coli, S. typhimurium and P. aeruginosa, Ptm internalized by means independent of the phospholipid bilayer, most likely mediated by one or more membrane proteins such as cation-selective barrel-like proteins. Work is currently underway to test this hypothesis.

An intra-specific linkage map of lettuce (Lactuca sativa) and genetic analysis of postharvest discolouration traits.

Minimally processed salad packs often suffer from discolouration on cut leaf edges within a few days after harvest. This limits shelf life of the product and results in high wastage. Recombinant inbred lines (RILs) derived from a cross between lettuce cvs. Saladin and Iceberg were shown to be suitable for genetic analysis of postharvest discolouration traits in lettuce. An intra-specific linkage map based on this population was generated to enable genetic analysis. A total of 424 markers were assigned to 18 linkage groups covering all nine chromosomes. The linkage map has a total length of 1,040 cM with an average marker distance of 2.4 cM within the linkage groups and was anchored to the ultra-dense, transcript-based consensus map. Significant genetic variation in the postharvest traits 'pinking', 'browning' and 'overall discolouration' was detected among the RILs. Seven significant quantitative trait loci (QTL) were identified for postharvest discolouration traits providing markers linked to the QTL that can be used for marker-assisted selection. Phenotypic stability was confirmed for extreme lines possessing the corresponding QTL parental alleles and which had shown transgressive segregation. This study indicates that a desired phenotype with reduced levels of postharvest discolouration can be achieved by breeding using natural variation.

Quantitative determination of ion distributions in bacterial lipopolysaccharide membranes by grazing-incidence X-ray fluorescence.

A model of the outer membrane of Gram-negative bacteria was created by the deposition of a monolayer of purified rough mutant lipopolysaccharides at an air/water interface. The density profiles of monovalent (K(+)) and divalent (Ca(2+)) cations normal to the lipopolysaccharides (LPS) monolayers were investigated using grazing-incidence X-ray fluorescence. In the absence of Ca(2+), a K(+) concentration peak was found in the negatively charged LPS headgroup region. With the addition of CaCl(2), Ca(2+) ions almost completely displaced K(+) ions from the headgroup region. By integrating the experimentally reconstructed excess ion density profiles, we obtained an accurate measurement of the effective charge density of LPS monolayers. The experimental findings were compared to the results of Monte Carlo simulations based on a coarse-grained minimal model of LPS molecules and showed excellent agreement.

A new broccoli × broccoli immortal mapping population and framework genetic map: tools for breeders and complex trait analysis.

A unique broccoli × broccoli doubled haploid (DH) population has been created from the F(1) of a cross between two DH broccoli lines derived from cultivars Green Duke and Marathon. We genotyped 154 individuals from this population with simple sequence repeat and amplified fragment length polymorphism markers to create a B. oleracea L. var. italica 'intra-crop' specific framework linkage map. The map is composed of nine linkage groups with a total length of 946.7 cM. Previous published B. oleracea maps have been constructed using diverse crosses between morphotypes of B. oleracea; this map therefore represents a useful breeding resource for the dissection of broccoli specific traits. Phenotype data have been collected from the population over five growing seasons; the framework linkage map has been used to locate quantitative trait loci for agronomically important broccoli traits including head weight (saleable yield), head diameter, stalk diameter, weight loss and relative weight loss during storage, as well as traits for broccoli leaf architecture. This population and associated linkage map will aid breeders to directly map agronomically important traits for the improvement of elite broccoli cultivars.

Genotype and environment effects on nitrate accumulation in a diversity set of lettuce accessions at commercial maturity: the influence of nitrate uptake and assimilation, osmotic interactions and shoot weight and development.

BACKGROUND: The causes of the natural variation in nitrate accumulation and associated traits are studied using a diverse population of 48 mature lettuce accessions grown hydroponically in winter and summer seasons. Information on the effects of genotype, environment and their interactions will inform future selection strategies for the production of low-nitrate varieties more suited to meeting EU requirements for harvested produce. RESULTS: The effects of genotype (G), environment (E) and G × E interactions were all significant, with nitrate concentrations lower but covering a wider range in summer. Concentrations of nitrate-N were positively correlated with those of water and total-N and negatively with assimilated-C in the shoot in both seasons, with all relationships partitioned according to morphotype and/or seasonal type. Corresponding relationships between nitrate-N and assimilated-N or with shoot fresh or dry weight were generally weak or inconsistent. Nitrate concentrations at an early growth stage were strongly related to those at maturity in winter, but not in summer when light levels were less variable. CONCLUSION: The effects of genotype and environment on nitrate accumulation in lettuce are strongly influenced by morphotype, with most G × E interactions between accessions within the same morphotype predominantly of the non-crossover type. All low-nitrate-accumulating genotypes have increased concentrations of organic solutes (concentration regulation) and reduced water (volume regulation) to help stabilise osmotic potential within the shoots. Variability in nitrate accumulation arises more from differences in uptake than in efficiency of its chemical reduction. Genotypic differences in nitrate accumulation can be masked by changes in head morphology during maturation, provided that they are not confounded by substantial changes in intercepted light. Recent selection strategies do not appear to have produced lower-nitrate-accumulating cultivars.

Screening for genotype and environment effects on nitrate accumulation in 24 species of young lettuce.

BACKGROUND: Nitrate accumulates in plants in response to N supply, aerial environment (predominantly light), and genotype. This paper characterises the effects of genotype, environment, and their interactions on nitrate accumulation by 24 cultivated and wild lettuce accessions grown hydroponically in winter and summer. The results will inform future strategies for selecting for low-nitrate varieties. RESULTS: A preliminary study in which two accessions were sampled for nitrate over time showed largest differences between cultivars in the early-middle period of growth. Sampling the whole population of lettuce at this stage revealed significant effects of genotype, environment (with nitrate concentrations generally higher in winter), and genotype × environment interactions (largely due to a wider range of concentrations in summer). Changes in the ranking of accessions for nitrate accumulation between the two growing seasons were generally small for cultivated morphotypes. Shoot nitrate concentrations and water contents were positively associated, particularly in summer when separate relationships for different cultivated morphotypes (butterhead, cos/Romaine, crisp, leaf, and stem lettuce) were detected. Expressing nitrate concentration on either a shoot fresh or dry matter basis had relatively little effect on the ranking of most cultivated accessions, but not for the wild types. CONCLUSION: There is a well-defined sampling window when differences in nitrate accumulation between lettuce genotypes are at a maximum. Delaying sampling may allow morphological changes in head form to mask earlier genotypic differences. Genotype × environment interactions are predominantly of the non-crossover type and have only a small effect on changes in the ranking of accessions between seasons, allowing selections to be made at any time of year. At least part of the genotypic variation in nitrate accumulation is associated with differences in shoot water content.

Using USAXS to predict the under-tempered chocolate microstructure.

Chocolate is a manufactured product enjoyed worldwide. Over the years, manufacturers have learned how to appeal to humans using this rich-fat food that arouses all the senses. Good quality chocolate is recognized by its smoothness, a slow melt in the mouth, and a snap when bitten, and described as well-tempered. This work compares dark chocolate samples manufactured to obtain under- and well-tempered chocolate, where under-tempered does not show all the physical properties desired by consumers. The microstructure was studied using the ultra small angle X-ray scattering (USAXS) technique, complemented by small and wide angle X-ray scattering to identify the polymorphs. It was observed that under- and well-tempered chocolates exhibited differences in the q-region ~ 2 × 10-5 Å-1 < q < ~1.5 × 10-4 Å-1, which correspond to spatial length scales from 32 µm to 3.2 µm. The differences are manifested in the value of the mass fractal dimension, D, obtained when the USAXS data were fitted using the Unified Fit model (Irena software). The characteristic length scale at which these differences were observed falls in length scales detected by humans in the oral cavity. This work proposes that a D = 2.1 characterizes an under-tempered 70% dark chocolate while a D = 2.3 characterizes a well-tempered 70% dark chocolate. This work also presents a simple model that describes the disintegration of those aggregates formed by the basic scatter units for under- and well-tempered chocolate. The model proposes that aggregates formed in under-tempered chocolate persist after the bulk chocolate has melted, which can be perceived as grittiness. However, the model proposes that the aggregates for well-tempered chocolate melt at the same or lower temperatures than the bulk chocolate melting temperature; hence no grittiness is perceived. The model is supported by the observation that the heat of transition for the under-tempered chocolate is smaller than that of the well-tempered case.

Fresh fruit and vegetables as vehicles for the transmission of human pathogens.

Much research into food-borne human pathogens has focused on transmission from foods of animal origin. However, recent investigations have identified fruits and vegetables are the source of many disease outbreaks. Now believed to be a much larger contributor to produce-associated outbreaks than previously reported, norovirus outbreaks are commonly caused by contamination of foods from hands of infected workers. Although infections with Shiga toxin-producing E. coli O157 have been linked to beef more often than to any other food product, severe outbreaks have been traced to consumption of contaminated radish sprouts and pre-packaged spinach. Similarly, while infections with Salmonella have mainly been linked to consumption of foods of animal origin, many outbreaks have been traced to contaminated fresh produce. E. coli O157 binds to lettuce leaves by alternative mechanisms involving the filamentous type III secretions system, flagella and the pilus curli. Association of Salmonella with fresh produce appears to be serovar-specific involving flagella, curli, cellulose, and O antigen capsule. A better understanding of plant, microbiological, environmental, processing and food handling factors that facilitate contamination will allow development of evidence-based policies, procedures and technologies aimed at reducing the risk of contamination of fresh produce.

Both leaf properties and microbe-microbe interactions influence within-species variation in bacterial population diversity and structure in the lettuce (Lactuca Species) phyllosphere.

Morphological and chemical differences between plant genera influence phyllosphere microbial populations, but the factors driving within-species variation in phyllosphere populations are poorly understood. Twenty-six lettuce accessions were used to investigate factors controlling within-species variation in phyllosphere bacterial populations. Morphological and physiochemical characteristics of the plants were compared, and bacterial community structure and diversity were investigated using terminal restriction fragment length polymorphism (T-RFLP) profiling and 16S rRNA gene clone libraries. Plant morphology and levels of soluble carbohydrates, calcium, and phenolic compounds (which have long been associated with plant responses to biotic stress) were found to significantly influence bacterial community structure. Clone libraries from three representative accessions were found to be significantly different in terms of both sequence differences and the bacterial genera represented. All three libraries were dominated by Pseudomonas species and the Enterobacteriaceae family. Significant differences in the relative proportions of genera in the Enterobacteriaceae were detected between lettuce accessions. Two such genera (Erwinia and Enterobacter) showed significant variation between the accessions and revealed microbe-microbe interactions. We conclude that both leaf surface properties and microbial interactions are important in determining the structure and diversity of the phyllosphere bacterial community.

Electrostatic interactions affect nanoparticle-mediated toxicity to gram-negative bacterium Pseudomonas aeruginosa PAO1.

Nanoscale materials can have cytotoxic effects. Here we present the first combined empirical and theoretical investigation of the influence of electrostatic attraction on nanoparticle cytotoxicity. Modeling electrostatic interactions between cells and 13 nm spheres of zinc oxide nanoparticles provided insight into empirically determined variations of the minimum inhibitory concentrations between four differently charged isogenic strains of Pseudomonas aeruginosa PAO1. We conclude that controlling the electrostatic attraction between nanoparticles and their cellular targets may permit the modulation of nanoparticle cytotoxicity.

Protein specificity of charged sequences in polyanions and heparins.

Long-range electrostatic interactions are generally assigned a subordinate role in the high-affinity binding of proteins by glycosaminoglycans, the most highly charged biopolyelectrolytes. The discovery of high and low sulfation domains in heparan sulfates, however, suggests selectivity via complementarity of their linear sulfation patterns with protein charge patterns. We examined how charge sequences in anionic/nonionic copolymers affect their binding to a protein with prominent charge anisotropy. Experiments and united-atom Monte Carlo simulations, together with Delphi electrostatic modeling for the protein, confirm strongest binding when polyanion sequences allow for optimization of repulsive and attractive electrostatics. Simulations also importantly identified retention of considerable polyion conformational freedom, even for strong binding. The selective affinity for heparins of high and low charge density found for this protein is consistent with nonspecific binding to distinctly different protein charge domains. These findings suggest a more nuanced view of specificity than previously proposed for heparinoid-binding proteins.

Modeling the solid-liquid phase transition in saturated triglycerides.

We investigated theoretically two competing published scenarios for the melting transition of the triglyceride trilaurin (TL): those of (1) Corkery et al. [Langmuir 23, 7241 (2007)], in which the average state of each TL molecule in the liquid phase is a discotic "Y" conformer whose three chains are dynamically twisted, with an average angle of approximately 120 degrees between them, and those of (2) Cebula et al. [J. Am. Oil Chem. Soc. 69, 130 (1992)], in which the liquid-state conformation of the TL molecule in the liquid phase is a nematic h*-conformer whose three chains are in a modified "chair" conformation. We developed two competing models for the two scenarios, in which TL molecules are in a nematic compact-chair (or "h") conformation, with extended, possibly all-trans, chains at low-temperatures, and in either a Y conformation or an h* conformation in the liquid state at temperatures higher than the phase-transition temperature, T*=319 K. We defined an h-Y model as a realization of the proposal of Corkery et al. [Langmuir 23, 7241 (2007)], and explored its predictions by mapping it onto an Ising model in a temperature-dependent field, performing a mean-field approximation, and calculating the transition enthalpy DeltaH. We found that the most plausible realization of the h-Y model, as applied to the solid-liquid phase transition in TL, and likely to all saturated triglycerides, gave a value of DeltaH in reasonable agreement with the experiment. We then defined an alternative h-h* model as a realization of the proposal of Cebula et al. [J. Am. Oil Chem. Soc. 69, 130 (1992)], in which the liquid phase exhibits an average symmetry breaking similar to an h conformation, but with twisted chains, to see whether it could describe the TL phase transition. The h-h* model gave a value of DeltaH that was too small by a factor of approximately 3-4. We also predicted the temperature dependence of the 1132 cm(-1) Raman band for both models, and performed measurements of the ratios of three TL Raman bands in the temperature range of -20 degrees C < or = T < or = 90 degrees C. The experimental results were in accord with the predictions of the h-Y model and support the proposal of Corkery et al. [Langmuir 23, 7241 (2007)] that the liquid state is made up of molecules that are each, on average, in a Y conformation. Finally, we carried out computer simulations of minimal-model TLs in the liquid phase, and concluded that although the individual TL molecules are, on average, Y conformers, long-range discotic order is unlikely to exist.