Anatomical Structure of the Matured Grapevine Shoot

Before the onset of the winter rest period, grapevine shoots manage to mature and lignify along almost their entire length, except for the topmost herbaceous tip, which usually perishes from the first autumn frosts. A matured annual shoot features characteristic swollen, short sections called nodes, which are separated by internodes of varying lengths. Formed wintering buds (eyes) enclosed in brown leathery scales are located on the nodes, and below them, semi-lunar scars—petiole traces from fallen leaves—are clearly visible.

A cross-section passing through an internode clearly reveals that the grapevine shoot has an irregular oval shape rather than a round one (Fig. 23). In the anatomical structure of the cane, four sides are strictly distinguished:

  • Dorsal side: In a freely growing shoot, it is always directed upward toward the sun and features a more intensive coloration. The tissues of the xylem (wood) and secondary phloem (bast) are most strongly developed here. By the end of the season, 3–4 dense layers of hard bast are formed on the dorsal side.
  • Ventral side: It faces downward toward the ground and is more weakly colored. The development of mechanical and conductive tissues on this side is also very powerful (the thickness of the xylem and the presence of 3–4 layers of hard bast are analogous to the dorsal side). It is worth noting that the leaves and laterals on the vine are always directed toward the dorsal side, while the wintering buds face the ventral side.
  • Flat side: This is a lateral (side) side. Tissues are more weakly developed here than on the dorsal and ventral sides; by the end of the vegetation period, usually only 2 layers of hard bast form here.
  • Grooved side: It is opposite to the flat lateral side and always corresponds to the wintering bud. This is the anatomically weakest section of the cane. The wood is least developed here, and only one layer of hard bast forms (in some cases, hard bast fibers may be completely absent on the grooved side). On each subsequent internode, the lateral sides (flat and grooved) systematically alternate places.

Significance of the Cane Structure for Grafting

Differences in the thickness of conductive and mechanical tissues on different sides of the shoot must be taken into account when producing bench winter grafting of cuttings. For successful and rapid fusion of the scion with the rootstock, you need to achieve ideal alignment of their cambial layers.

If, when cutting oblique sections or tongues on a grafting machine, one does not account for the asymmetry of the cane and aligns, for example, the grooved (anatomically thin) side of the scion with the dorsal (thick) side of the rootstock, the cambial rings will not match. Because of this, the tissues will not be able to fuse normally, excessive formation of tyloses and gum exudation will begin in the cut planes, and the graft itself will eventually fail or yield a weak, non-viable sapling.

Structure and Functions of the Diaphragm

A longitudinal section of a matured shoot shows that the hollow pith in each node is completely interrupted by a dense transverse partition—the diaphragm. It consists of live parenchymal cells with thick, lignified walls. By the end of the vegetative period, the tissues of the diaphragm become overfilled with starch grains, especially in the immediate vicinity of the wintering eyes. The cells of the diaphragm are also rich in tannins.

The diaphragm performs a critical function of protection and isolation: the live cells of the partition are separated from the easily disintegrating dead tissue of the pith on both sides by several layers of suberized cells. It serves as a unique thermal and hydraulic buffer for the node and the bud.

It is important for the viticulturist to know that the live tissues of the diaphragm are biologically more sensitive to extremely low temperatures than the wintering buds themselves. During excavations or inspection of canes after severe winters, latent damage to the nodes is frequently recorded: the tissues of the diaphragm turn heavily brown due to frost injury, even though the buds themselves remain green and suffer no damage at all.