Earticle

Cork is a part of the periderm in the outer bark of cork oak that covers the stem and branches. Worldwide, cork is commonly known as a sealant in wine bottles. Cork has outstanding properties, such as low density, low permeability to liquids and gases, low conductivity, chemical stability and durability, and high compressibility. There are three types of cork in the cork industry: virgin cork, second cork, and reproduction cork. Virgin and second cork have deep fractures and a distorted structure and are commonly used as agglomerate products, whereas reproduction cork has an even surface and is the most essential material for making solid products such as wine stoppers. Quercus suber reproduction cork is the main resource of cork in the world because it has a large quantity of cork in its periderm. In Korea, Quercus variabilis is abundant and a common commercial wood species. Quercus variabilis is another species that has large quantity of cork in its periderm. However, there is still a lack of information regarding the quality of Quercus variabilis cork grown in Korea. Therefore, to give further valuable insight for effective utilization of cork resource, the present study investigated the qualitative anatomical characteristics such as growth ring characteristics, lenticular channel, and cellular cork structure of Quercus variabilis virgin cork grown in Korea and compared with those of Quercus suber reproduction cork grown in Portugal. The qualitative anatomical characteristics of both species were observed by optical and scanning electron microscopy, and the results of this study will be presented at the International Symposium.

Cork is an impermeable material, the phellem layer of bark tissue that is harvested for commercial use primarily from Quercus suber (the cork oak), which is native to southwest Europe and northwest Africa. Cork is mainly composed of suberin, a hydrophobic substance. Because of its impermeable, buoyant, elastic, and fire retardant properties, it is used in a variety of products such as wine stoppers, panels, boards and paving materials. Cork production is generally considered eco-friendly and sustainable because the cork tree is not cut down to obtain cork; only the bark is stripped to harvest the cork. The tree continues to live and grow. The sustainability of production and the easy recycling of cork products and by-products are two of its most distinctive aspects. In our previous studies, corks in Quercus variablis (in South Korea) and Quercus suber (in Portucal) were analyzed to compare the chemical compositions and its natural durability. Both species had similar composition of chemical components. In particular, the contents of suberin in the species were shown as 47.6% and 50.2%, respectively. As a result of fungal decay resistance test, there was no big difference in weight losses and morphological changes of both species. Based on the results, it was thought that the cork of Q. variabilis could be fully utilized as an alternative resource for that of Q. suber. As a main substance in corks, suberin is a complex polymer composed of two domains, polyaliphatic and polyphenolic, and thus it has great potential to be used for various purposes. The monomers, di-acids and phenolic acids have several applications in food, polymer and pharmaceutical industries. For examples, phenolic acid monomer of suberin is a secondary metabolite. Extraction of antioxidants from natural sources can replace synthetic antioxidants that do not have toxic and carcinogenic property. In polymer industries, suberin monomers are used as macromolecular materials either as additives or inactive starting monomeric form. Recent studies have explored the polyester nature of the suberin monomer to synthesize various polyester polymeric materials. Therefore, the isolation of suberin monomers or low-molecular weight compounds from renewable sources is the topic of focus nowadays. Hence, waste from corks rich in suberin components could be a promising source for these renewable chemicals extraction.

Cork granules are by-products after the production of various cork products, such as wine stoppers, and are considered as a valuable resource because of their good elasticity and slip resistance. In this study, the applicability of cork granules as paving materials for walkways and children's playgrounds was investigated. Cork granules with various particle sizes from 1 mm to 8 mm were obtained from Portuguese cork oak (Quercus suber). As a result of manufacturing cork paving materials with a thickness of 15 mm, their tensile strength was found to be 0.5 MPa or more. However, the tensile properties of the paving material made only with cork granules with a particle size smaller than 2 mm were less than 0.2 MPa. Furthermore, other physical properties of cork paving materials such as shock absorption, sliding resistance, and vertical deformation were all suitable for human activity. As a result of surface temperature measurement by a thermal imaging camera in summer, it was found that the cork paving material was more effective in alleviating the heat island phenomenon compare to the rubber one.

The present work aims to study the induction of somatic embryogenesis in cork oak (Quercus suber L.) from immature zygotic embryos and young apical leaves obtained from 2-month-old seedlings through acorn germination on sterilized peat. The immature zygotic embryos were grown for 1 month on the mineral solution of MS in the presence of 4.52 µM 2,4-D and 30 g/L sucrose. They were then transferred to the same mineral solution with no added growth regulators. In the third subculture, yellow somatic embryos, characterized by two voluminous cotyledons, were differentiated from the radicle of the immature zygotic embryos. The induction of somatic embryogenesis in young leaves required a series of transfers on different culture media containing 30 g/L sucrose and 100 mg/L myo-inositol. Secondary or recurrent somatic embryogenesis occurred within the immature somatic embryo radicles after 1 month of culture on growth regulator-free medium containing WPM macronutrients, MS micronutrients, and vitamins.