José Graça

Glycerol and glyceryl esters of ω-hydroxyacids in cutins

Cutins from the leaves and fruits of seven plant species were depolymerized by NaOCH(3)-methanolysis. The monomers that were released mostly included C16 and C18 omega-hydroxyacids with mid-chain oxygenated substitutions, namely epoxy and hydroxyl groups. Glycerol was also solubilized as a monomer in quantities that ranged from 1 to 14% of the methanolysates. Partial depolymerization of three cutins by CaO-methanolysis released the same monomers as had been obtained in the previous reaction, as well as small quantities of 1- and 2-monoacylglyceryl esters of omega-hydroxyacids. Molar proportions of glycerol permit the esterification of a significant part of the aliphatic omega-hydroxyacids, thereby possibly playing a major role in the polyester structure of cutin. Glycerol had not previously been known to form part of the cutin polymer.

Extractive composition and summative chemical analysis of thermally treated eucalypt wood.

Abstract Eucalypt wood (Eucalyptus globulus) was heated in an oven for 2–24 h at 170–200°C and in an autoclave with superheated and saturated steam for 2–12 h at 190–210°C. The chemical composition of untreated wood and thermally treated wood with different mass losses in the range of 1.1–11.9% was studied by summative analysis, and the composition of dichloromethane, ethanol and water extracts was determined by gas chromatography mass spectometry (GC-MS). The hemicelluloses degraded first, mainly regarding the arabinose and xylose moieties. Lignin degraded at a slower rate and cellulose was only slightly affected under severe treatment conditions. The extractive content increased first with heat treatment and decreased later on. Almost all of the original extractives disappeared and new compounds were formed, such as anhydrosugars, mannosan, galactosan, levoglucosan and two C5 anhydrosugars. The most prominent lignin derived compounds were syringaldehyde, syringic acid and sinapaldehyde. The main difference between autoclave and oven treated samples was the appearance of more oxidized extractives for the oven treatment.

Methanolysis of bark suberins: analysis of glycerol and acid monomers

The depolymerization and subsequent analysis of cork suberins from the outer barks of Pseudotsuga menziesii and Quercus suber was performed using a simplified methanolysis procedure. The amount of sodium methoxide catalyst was maintained at 20–30 mM and the methanolysis mixture was submitted to trimethylsilyl derivatisation and used directly for gas chromatographic analysis, allowing simultaneous quantification of glycerol and long-chain monomers. Response factors for glycerol, ferulic acid and one saturated homologue representing each of the suberinic families (i.e. the 1-alkanols, 1-alkanoic acids, ω-hydroxyacids and α,ω-diacids) were determined. Effective depolymerization of suberin was checked using the infrared specta of the residues after methanolysis. Glycerol is a major constituent of the suberins from P. menziesii (26% of total) and from Q. suber (14%). In both suberins, α,ω-diacids are dominant, i.e. 54% of the long-chain monomers in P. menziesii (mostly saturated C16-C22 homologues and the C18 unsaturated diacid), and 53% in Q. suber (mostly the C18 unsaturated diacid and mid-chain oxygenated (epoxide and vic-diol) derivatives). In P. menziesii epoxyacids are absent. The importance of glycerol and α,ω-diacids as suberin monomers supports a polymeric structure based on their successive esterification. Copyright

Cork Suberin: A Glyceryl Based Polyester

Total depolymerization of cork from the cork oak (Quercus suber L.) by using a sodium methoxide catalysed methanolysis solubilized 53.2 % of the material, including 5.2 % of glycerol, 48,0 % of suberinic fatty acids and alcohols and minor amounts of ferulic acid. A very mild depolymerization using calcium oxide treated methanol, which solubilized only 2.0% of the cork material was studied by GC-MS. In the solubilized material, the total amount of aliphatic acids was 43.8 % (including alkanoic acids 4.0%. ω-hydroxyacids 13.2% and α,ω-diacids 26.6%), of 1-alkanols 2.1 % and of monoacylglycenols 32.1%. It was possible to identify 1-monoacylglycerols and 2-monoacylglycerols of alkanoic acids (1,2%), ω-hydroxyacids (3.7 %) and α,ω-diacids (22.8 %). It is proposed that suberin is a glyceryl based polymer and that its insoluble character is given, at least in part, by the cross-linking of dicarboxylic fatty acids with glycerol. The term suberin should be used for this aliphatic polyester component of the cell wall.

Influence of tree eccentric growth on syringyl/guaiacyl ratio in Eucalyptus globulus wood lignin assessed by analytical pyrolysis

Abstract Wood disks from 9-year-old Eucalyptus globulus trees that showed eccentric growth were analysed by analytical pyrolysis to determine lignin syringyl/guaiacyl (s/g) ratio, lignin content and polysaccharide composition (hexosans/hexosans+pentosans). Three cross-sectional fractions were analysed: the long radius, the short radius and the intermediate sections. Between-tree variation was observed for the s/g ratio from 2.0 to 2.8, the lignin content from 26.5 to 28.0% and the hesosans/hexosans+pentosans ratio from 0.76 to 0.83. The reliability of the pyrolysis analysis assessed by the pooled standard deviation was very high for the three determinations (2–3% coefficients of variation). Eccentric tree growth in E. globulus has no impact in the s/g ratio and does not necessarily imply differences in lignin content or in polysaccharide composition. When differences occur, lower lignin and higher cellulose are found in the cross section part with the highest radial growth. Sampling for wood chemical analysis in eccentric cross sections should avoid the stem parts with high radial differences between opposite sides. Analytical pyrolysis can be used to prospect for the presence of tension wood.

The evaluation of the quality of cork planks by image analysis

The quality evaluation of cork planks in relation to porosity may use image analysis techniques. Results are presented on the number, dimensions, form and orientation of the lenticular channels in tangential, transverse and radial sections of cork planks of different thickness and qualities. The porosity coefficient (% pore area in relation to total area) varied between 1.1%-18.9% and 2,1%-16.4%, respectively in tangential and transverse/radial sections and is determined mostly by the number of pores larger than 0.81 mm 2 and by their average area. The porosity may be used to define cork quality classes. Due to the heterogeneous porosity distribution in cork planks, an assessment of quality made on small cork samples will give a result with a large error. For an error below 15%, the observation in the tangential section should be made on a sample with a minimum area of 225 cm 2 (e.g. 15 cm x 15 cm) and in the transverse and radial sections on a sample with a minimum length of 15 cm.

THE PERIDERM DEVELOPMENT IN QUERCUS SUBER

In the cork oak (Quercus suber L.), the phellogen differentiates during the first year of growth in the cell layer immediately under the epidermis and divides to form 3–6 suberized phellem cells. Division of the phellogen only occurs after suberization of the previous divided cell. During the first four years of growth, the phellem cells have tannin-filled lumens and it is only in the 5th to 7th years that they acquire the characteristics of ʻadultʼ cork cells with empty lumens and thin suberized walls. The lenticels are formed by the lenticular phellogen, which differentiates under the stomata and has a high meristematic activity. In this region, the cells are unsuberized, with a loose arrangement and intercellular voids, constituting the filling or complementary tissue. After three years, the lenticels appear as small protuberances that soon become conspicuous. Inclusions of sclerenchymatous nodules and isolated sclereids occur occasionally mostly in the vicinity of, or in, the lenticels.

Suberin: the biopolyester at the frontier of plants.

Suberin is a lipophilic macromolecule found in specialized plant cell walls, wherever insulation or protection toward the surroundings is needed. Suberized cells form the periderm, the tissue that envelops secondary stems as part of the bark, and develop as the sealing tissue after wounding or leaf abscission. Suberin is a complex polyester built from poly-functional long-chain fatty acids (suberin acids) and glycerol. The suberin acids composition of a number of plant tissues and species is now established, but how the polyester macromolecule is assembled within the suberized cell walls is not known. In the last years contributions from several areas have however significantly enriched our understanding of suberin. The primary structure of the polyester, i.e., how the suberin acids and glycerol are sequentially linked was revealed, together with the stereochemistry of the mid-chain functional groups some suberin acids have; solid-state NMR studies showed the presence of methylene chains spatially separated and with different molecular mobility; biophysical studies showed the membrane behavior of suberin acids derivatives, allowing new insights on structure-properties relationships; and a number of candidate genes were conclusively related to suberin biosynthesis. The comprehension of suberin as a macromolecule will be essential to understand its vital protective roles in plants and how they will deal with eventual environmental changes. Suberin is also expected to be a source for high-performing bio-based chemicals, taking advantage of the structural uniqueness of their constituent suberin acids.

Cuticle Structure in Relation to Chemical Composition: Re-assessing the Prevailing Model.

The surface of most aerial plant organs is covered with a cuticle that provides protection against multiple stress factors including dehydration. Interest on the nature of this external layer dates back to the beginning of the 19th century and since then, several studies facilitated a better understanding of cuticular chemical composition and structure. The prevailing undertanding of the cuticle as a lipidic, hydrophobic layer which is independent from the epidermal cell wall underneath stems from the concept developed by Brongniart and von Mohl during the first half of the 19th century. Such early investigations on plant cuticles attempted to link chemical composition and structure with the existing technologies, and have not been directly challenged for decades. Beginning with a historical overview about the development of cuticular studies, this review is aimed at critically assessing the information available on cuticle chemical composition and structure, considering studies performed with cuticles and isolated cuticular chemical components. The concept of the cuticle as a lipid layer independent from the cell wall is subsequently challenged, based on the existing literature, and on new findings pointing toward the cell wall nature of this layer, also providing examples of different leaf cuticle structures. Finally, the need for a re-assessment of the chemical and structural nature of the plant cuticle is highlighted, considering its cell wall nature and variability among organs, species, developmental stages, and biotic and abiotic factors during plant growth.

Glyceryl-acyl and aryl-acyl dimers in Pseudotsuga menziesii bark suberin

Summary The cork from the outer bark of Douglas-fir (Pseudotsuga menziesii) was exhaustively extracted and submitted to a mild CaO-catalyzed methanolic depolymerization. The solubilized material (2.5 % yield) was derivatized and analysed with GC-MS. Glycerol was the main component (41.2%), followed by aliphatic acid monomers (30.6%) and glyceryl and feruloyl dimers (14.5%). Monoacylglyceryl esters of α,ω-diacids were dominant (68% of glyceryl dimers), followed by monoacylglyceryl esters of alkanoic acids and of ω-hydroxyacids (respectively 20% and 9%). For all cases 1- and 2-monoacyl-glyceryl esters were present. Dimers of ω-hydroxyacids esterified to ferulic acid through their primary hydroxyl were also found. The feruloyl ester of docosanol could also be identified. The relative abundance of the dimers followed the proportion of acid monomers. The direct evidence found here for P. menziesii, and previously for Q. suber, that glycerol is esterified to all suberinic acids and that ω-hydroxyacids are esterified to ferulic acid supported the discussion of a suberin structure developing on glyceryl-diacyl-glyceryl and glyceryl-hydroxyacyl-feruloyl backbones.