Holocrine Secretion and Kino Flow in Angiosperms: Their Role and Physiological Advantages in Plant Defence Mechanisms

Kinos are plant exudates, rich in polyphenols, produced by several angiosperms in reaction to damage. They flow out of kino veins, schizolysigenous ducts composing an anatomically distinct continuous system of tangentially anastomosing lacunae produced by the vascular cambium, which encircle the plant. Kino is secreted holocrinously into the vein lumen by a cambiform epithelium lined by suberized cells that separate kino veins from the surrounding axial parenchyma. A model describing kino flow in eucalypts is presented to investigate how vein distribution and structure, as well as kino holocrine loading, crystallization, and viscosity affect flow. Considering viscosity, vein anatomy, and a time-dependent holocrine loading of kino, the unsteady Stokes equation was applied. Qualitatively, kino flow is similar to resin flow. There is an increase in flow towards the vein open end, and both pressure and flow depend on vein dimensions, kino properties and holocrine loading. However, kino veins present a much smaller specific resistance to flow compared to resin ducts. Also, unlike resin loading in conifers, holocrine kino loading is not pressure-driven. The pressure and pressure gradient required to drive an equally fast flow are smaller than what is observed on the resin ducts of conifers. These results agree with previous observations on some angiosperms and suggest that holocrinous gum flow may have lower metabolic energy costs; thus presenting physiological advantages and possibly constituting an evolutionary step of angiosperms in using internal secretory systems in plant defence mechanisms compared to resin flow in conifers. Understanding of how these physiological and morphological parameters affect kino flow might be useful for selecting species and developing more sustainable and economically viable methods of tapping gum and gum resin in angiosperms.