Discovering the corky bark of Eucalyptus suberea from anatomical and chemical perspectives

This paper presents the first scientific investigation of the corky bark of Eucalyptus suberea (cork mallee). The study involved outer bark samples from two trees with diameters of 23 cm (A) and 47 cm (B), collected from a plantation in Portugal. Anatomical analysis was performed using light and scanning electron microscopy (SEM), enabling detailed observation of bark to reveal the cork cells presence. The chemical characterization included summative chemical analysis, FTIR, and GC-MS/FID for suberin composition and Py-GC-MS for lignin composition. The extractive-free bark mass of E. suberea from trees A and B contained 29% and 25% suberin, 30% and 37% lignin, and 39% and 38% polysaccharides, respectively. The bark A exhibited a higher extractives content (49%) than just 14% for bark B. Conversely, dichloromethane extract from bark A was notably rich in alkanol ferulates. FTIR spectra revealed a characteristic cork tissue pattern, showing a low suberin content but high levels of polysaccharides and free carboxylic acids. The main components of E. suberea A and B suberins were glycerol, with 15% and 11%, {omega}-hydroxyacids, with 45% and 50%, and ,{omega}-diacids, with 30% and 17%, representing a total of ca. 90% and 78% of E. suberea A and B suberins mass, respectively. Both suberins showed low contents of epoxides and vic-diols; also, both presented the particularity of having high contents of free acid groups, especially the suberin of bark B, suggesting a structure with short aliphatic chains. E. suberea corky bark presents a lignin composition that differentiates from the cork G-lignins analyzed until now with a monomer composition exhibiting a GS character, with more G- (53-56%) than S-units (34-37%) with an S/G ratio between 0.64 to 0.66, for A and B lignins, respectively.