Histo- and cytopathology of trunk phloem necrosis, a form of rubber tree (Hevea brasiliensis Müll. Arg.) tapping panel dryness

Abstract

Trunk phloem necrosis (TPN) is a physiological disease of rubber tree (Hevea brasiliensis Müll. Arg.) discovered in the 1980s. It has been distinguished from rubber tree tapping panel dryness (TPD) by its macroscopic symptoms and presumed origin. But little attention has been paid to its microscopic features, and there is now some evidence that both syndromes could be linked to an impaired cyanide metabolism. In order to characterise TPN and compare it with TPD microscopically, the inner phloem of tapping panels was investigated by light and transmission electron microscopy in healthy trees and TPN-affected trees. TPN-affected phloem presented numerous and varied structural and ultrastructural features. There were signs of cellular deterioration in a great number of specialised cells, i.e. laticifers and sieve tubes, and not very specialised cells, i.e. parenchyma cells and companion cells. There were also signs of cellular dedifferentiation in other parenchymatous cells, e.g. in tylosoids and hyperplasic cells. These cells were derived from parenchyma cells that ensheath laticifers in which the latex coagulated. Numerous structural features of TPN are common to TPD, notably tylosoids associated with in situ coagulated latex, which are also known to be early structural markers of TPD and cyanide-induced. It is therefore concluded that TPN is identical to or a variant of TPD, and is a degenerative disease of rubber tree trunk phloem resembling plant stress response, programmed cell death and plant tumourigenesis in some aspects.