Cork Development: What Lies Within

Abstract

The cork layer present in all dicotyledonous plant species with radial growth is the result of the phellogen activity, a secondary meristem that produces phellem (cork) to the outside and phelloderm inwards. These three different tissues form the periderm, an efficient protective tissue working as a barrier against external factors such as environmental aggressions and pathogen attacks. The protective function offered by cork cells is mainly due to the abundance of suberin in their cell walls. Chemically, suberin is a complex aliphatic network of long chain fatty acids and alcohols with glycerol together with aromatic units. In most woody species growing in temperate climates, the first periderm is replaced by a new functional periderm upon a few years after being formed. One exception to this bark development can be found in cork oak (Quercus suber) which display a single periderm that grows continuously. Quercus suber stands by its thick cork layer development with continuous seasonal growth. Cork raw material has been exploited by man for centuries, especially in Portugal and Spain. Nowadays, its applications have widened vastly, from the most known product, stoppers, to purses or insulating materials used in so many industries, such as construction and car production. Research on how cork develops, and the effect environmental factors on cork oak trees is extremely important to maintain production of good-quality cork, and, by maintaining cork oak stands wealthy, we are preserving a very important ecosystem both by its biodiversity and its vital social and economic role in areas already showing a population declination.

Keywords: cork; exploitation; phellem; phellogen; stress; suberin.

Figure 1

The cork layer of Quercus suber. (a) Virgin cork. This cork oak has never been debarked is still displaying its first cork layer. (b,c) Different quality types of amadia cork. (b) Cork plank thick enough and with very few discontinuities allowing stopper punching (red dots circle). It is possible to distinguish the growth rings (black arrow). (c) Example of a reduced-quality cork plank, thin in thickness and intersected by numerous lenticular channels (black arrowhead). White arrows indicate the growth direction from the surface in contact with the trunk and where the phellogen is (also known as cork belly) towards the surface in contact with the environment. In—cork plant inside; out—outermost cork plank surface.

Figure 2

Cork develops as early as one-year stems in Quercus suber. (a–i) Cross-sectional stereomicroscopic images of one-, two- and three-year-old stems. (Scale bars: (a,b) 100 µm, c 500 µm). (d–f) Details of the xylem with empty tracheids (*). Moving outwardly, one finds the phloem (dark layer) surrounded by the periderm comprising the phelloderm (orange arrow -phd), a thin layer of phellogenic cells (white arrowhead) and several rows of suber cells. The most external layer is the remaining epidermis. (Scale bars: (d,e) 100 µm, f 500 µm). (g–i) Cork cells exhibit autofluorescence when excited with UV light [18]. The stems observed in d-f were excited with UV using a stereoscope filter for “Lumar01” (BP 365/12; LP397) after which, the suberin layer is clearly distinguishable (white arrow). In older stems, as xylem cells mature, autofluorescence can be observed since lignin also possesses some autofluorescence properties under UV light. (j,k) Light microscopy of one-year stem cross sections stained with toluidine blue. (Scale bars: (g,h) 100 µm, i 500 µm). (j) Some remaining epidermis (ep) is still present and right below, six to seven cell cork layers with cells displaying a hyaline cell wall appearance and filled with electrodense material (phenolic compounds) (orange line) can be observed. Inwardly and adjacent to suberin cells, is the phellogen composed of only one to two cells (red line). Cells are filled with cytoplasm and cell walls do not present a secondary thickening. Right below is the phelloderm with the characteristic round-shaped parenchymatous cells. (Scale bar: 10 µm). (k) Cross section showing a lenticel forming in a one-year stem (black arrow). The disorganized division of the meristematic cells inside the lenticel structure is starting to push the suberin layer outwards leading to a complete rupture of this layer to form an aperture that allows gas exchanges. (Scale bar: 40 µm). (l) Transmission electron microscopy image of amadia cork cell wall clearly showing a plasmodesmata (black arrow) crossing both suberized cell walls. It is possible to observe cytoplasmic deposition in the inner part of the cell, with thickening appearing on both sides of the plasmodesmata channel (amp. 20,000×). ep—epidermis; ph—phloem; phd—phelloderm; phl—phellogen; s—suberin; xy—xylem.