Quercus rotundifolia Bark as a Source of Polar Extracts: Structural and Chemical Characterization

Abstract

Quercus rotundifolia bark was studied regarding anatomical, chemical, and antioxidant properties from trees in two sites in southern Portugal and are here reported for the first time. The general structure and anatomy of Q. rotundifolia bark showed a rhytidome with sequential undulated and anastomosed periderms with a small proportion of cork, while the phloem included broad rays with strong cell sclerification, groups of sclereids with embed large prismatic crystals, and abundant druses in parenchyma cells. The mean chemical composition was 15.5% ash, 1.6% dichloromethane extractives, 6.4% ethanol and 9.3% water extractives, 3.0% suberin, 30.5% total lignin, and 33.8% carbohydrates. Carbohydrates included mainly glucose (50.7% of total monomers) and xylose (23.8%), with uronic (3.0%) and acetic acids (1.0%). Suberin was mainly composed of ω-hydroxyacids (48.0% of all compounds) and α,ω-diacids (19.5%). The main compounds found in the lipophilic extracts were triterpenes (43.6%–56.2% of all compounds) and alkanoic acids (32.7%–41.7%). Phenolic content was high especially in the ethanol extracts, ranging from 219.5–572.9 mg GAE/g extract and comprising 162.5–247.5 CE/g extract of flavonoids and 41.2–294.1 CE/g extract of condensed tannins. The extracts revealed very good antioxidant properties with IC50 values of 4.4 µg ethanol extract/mL and 4.7 µg water extract/mL. Similar anatomical, chemical, and antioxidant characteristics were found in the bark from both sites. The high phenolic content and excellent antioxidant characteristics of polar extracts showed holm oak barks to be a promising natural source of antioxidants with possible use in industry and pharmaceutical/medical areas.

Qualitative and quantitative analysis of the constitutive bark of Q. ilex x Q. suber hybrids

“…On their side, a larger proportion of non-suberized inactive phloem (almost 70% of their outer bark) characterizes Q. ilex samples, with a significant proportion of lignified rays and schlerenchymatic nodules (9-14%). Phellem tissue accounts for approximately 20% of the bark transverse area, and is comprised of several thin (∼10 cells thick), anastomosed periderms, which is consistent with previous description provided, for instance, by Sousa et al . (2021), and similar to other Quercus species, as, for instance Q. petraea (Gričar et al .…”

“…Macroscopically, holm oak outer bark is finely crackled in small tiles, which eventually fall from the tree. In a transverse section, periderms can be distinguished as dark, thin lines; inactive phloem presents a brown-reddish color, with whitish multiseriate lignified rays and schlerenchymatic nodules, consistently with the observations by Sousa et al (2021). As seen under the microscope, periderms are few cells thick, and become more abundant in the outer part of the bark, as mechanical tensions induce the differentiation of new phellogens in the inactive phloem comprised between periderms.…”

“…Samples were also collected from one cork oak ( Q. suber ) and two holm oak ( Q. ilex ssp. rotundifolia ) trees as a reference, since previous works have reported almost no variability among individuals within these species (Sousa et al 2021). Outer bark samples (5×12 cm) were harvested from the south-facing part of the trees at DBH (1.30 m) using a hammer and a chisel.…”

“…This type of outer bark has been described in detail for Q. robur (Trockenbrodt 1991), Q. petraea (Gricar et al 2015), Q. faginea (Quilhó et al 2013), Q. cerris (Sen et al 2011) and Q. ilex ssp. rotundifolia (Sousa et al 2021).…”

Wood Density and Ring Width in Quercus rotundifolia Trees in Southern Portugal

“…[64]. A recent study on Q. rotundifolia barks structure and chemical composition revealed high phenolic content and excellent antioxidant characteristics of polar extracts similar at both sites [83]. Regarding wood quality, two regional types of Q. rotundifolia quality were assigned in Portugal [12], one for more qualified uses (site 1) and the other for current uses (site 2), which could not be confirmed with these results.…”