The diaphragm is a ‘sclerenchymatous’ tissue located in vine shoots and two- or three-year-old wood.
Figures (a, b, c, d) show an example of a diaphragm in a one-year-old vine shoot.
This tissue, located at a node, separates/isolates the continuity of the pith between two internodes (Figures c, d).
A lugol stain reveals the presence of starch in the parenchymal rays of the wood (secondary xylem); the starch is coloured blue-purple (Figures e, f, g).
When pruning a vine shoot, it is recommended to leave one or two cm (or more) above the bud that is to be preserved and to protect it from possible necrosis of the conducting tissue due to the pruning wound.
In general, the pith above the node/bud left after pruning, dries out and creates an entry point for pathogens: the diaphragm will therefore protect the pith area below the node/bud (it should be noted that in some shoots the diaphragms are necrotic and therefore no longer play their role as a barrier).
The conducting tissues are not isolated from the diaphragm, as shown in the figures (e, f, g).
In fact, lugol stain shows a continuity of wood conduction (xylem II) in the area of the diaphragm. As these tissues are alive, they are generally able to defend themselves.
Figure (g) shows a longitudinal section of a shoot at a node, cut in both planes: the plane of the phyllotaxis and the plane orthogonal to the plane of the phyllotaxis. Lugol’s staining confirms the continuous vascular connection at the node, between the two internodes.
The following issues are not addressed in this post:
I) the issue of so-called ‘flush’ pruning wounds
II) the role of the spare wood that is left at pruning, which respects the crown buds (basal buds) and limits the progression of necrosis of the conducting tissue underneath the pruning wound (example with figure (h))
III) the (real or presumed) role of the diaphragm in cases I) and II).
Original article (in French): https://www.linkedin.com/feed/update/urn:li:activity:7032748675218948097/