Survival strategies of plants during secondary growth: barrier properties of phellems and lenticels towards water, oxygen, and carbon dioxide

Ever since plants began to conquer the terrestrial environment, a simple but effective evolutionary strategy has been employed to cope with the combined necessities of preventing an excessive loss of water via the aerial surface while also supporting the vital exchange of CO(2) and O(2) for photosynthesis and respiration. Large areas of the primary above-ground surface of plants are covered by a hydrophobic, non-cellular cuticle which effectively minimizes evaporation and very strongly reduces exchange of CO(2) and O(2). Hence, gas exchange is controlled by regulating stomatal apertures. Upon wounding or entering into secondary growth, however, the epidermis, cuticle, and stomata are replaced by a phellem (cork), which is produced by a lateral cambium, the phellogen. Former stomata are replaced by lenticels, which are multicellular structures and functionally analogous to stomata. In the secondary plant body, phellems effectively prevent the loss of water from the cortex of the stem while lenticels support the exchange of vital gases such as CO(2), O(2), and water vapour. The permeance of these gases via the lenticels reaches a maximum during July and is minimal during autumn and winter. In contrast to stomatal control, gas exchange through phellems is regulated by long-term structural changes. The permeances of cuticles, phellems, and lenticels are compared and discussed.