IMMUNOCYTOCHEMICAL EVIDENCE THAT SECRETION OF PECTIN OCCURS DURING GEL (GUM) AND TYLOSIS FORMATION IN TREES

During gel (gum) formation in angiosperm trees, fibrillar material acc umulated in protective layers of xylem parenchyma cells before being s ecreted across half-bordered pit membranes into vessel elements. Immun ogold labeling demonstrated that this fibrillar material was mainly co mposed of partially esterified pectic polysaccharides. The primary wal l of expanding tyloses, an extension of the parenchyma protective laye r, secreted similar pectic substances to completely block vessel eleme nts. In most studies, these occluding structures were reported to be f ormed in response to causative factors such as aging processes, injuri es, or infections. Current observations support the view that partial to complete embolism, which almost always accompanies these factors, m ight be the main cause triggering the formation of vessel occlusions. Whereas pectin seems to be the basic component of gels (gums) and of t he external layer of tyloses, other substances, such as phenols, were also detected either as a part of these plugs or as accumulations besi de them in vessels. Finally, it is proposed that the term 'gel' instea d of 'gum' be used in future studies to describe the occluding materia l secreted by ray and paratracheal parenchyma cells.