Wintering buds are situated on the nodes of shoots, resting upon small, flattened cushions whose tissues contain an abundant reserve of organic nutrients. In autumn, prior to entering the dormancy period, these buds acquire their definitive structure, representing shortened shoots equipped with the primordia of leaves, tendrils, and inflorescences, protected from environmental influences by brown, coriaceous scales. In practical viticulture, these wintering buds are referred to as compound eyes.
Internal Anatomy and Structure of the Compound Eye
Anatomical analysis of longitudinal sections reveals that compound eyes generally consist of entire bud complexes enclosed by shared protective scales. Most frequently, a larger primary bud occupies the center of the eye, flanked by two smaller secondary (accessory) buds. The inner surface of the protective scales, as well as all components of these three individual buds, are densely covered with long, brownish trichomes (hairs) that fill all vacant spaces.
The primary bud possesses a conically widened, shortened central axis on which future nodes and internodes can be distinguished. This axis is covered by scale-like embryonic leaves, the number of which can reach up to 24; opposite these leaves lie the primordia of inflorescences and tendrils. Notably, the plane of phyllotaxis in the primary bud runs almost parallel to the plane of phyllotaxis of the parent shoot, whereas in the secondary buds, it invariably intersects with the latter at an approximate right angle.
Biological Role of Secondary Buds
Secondary buds exhibit the same shape and structural organization as the primary bud, though all their constituent organs are significantly less differentiated. Under typical conditions at the onset of the subsequent vegetative period, only the primary bud resumes growth. If it suffers frost damage during winter, one or both of the secondary buds will initiate growth; the same phenomenon is observed following severe injury or manual removal of the young shoot that developed from the primary bud. Not infrequently, both the primary and some of the secondary buds break dormancy simultaneously in spring, resulting in the emergence of two or even three shoots from a single compound eye.
Initiation of Inflorescence Primordia and Yield Pre-determination
In the lower buds of a shoot, inflorescence primordia appear relatively early, from mid-June to early July. In higher-positioned buds, this process occurs later; however, by the end of the vegetative period (approximately in October), the primordia of all inflorescences destined to appear in the following year are already present in all fully formed buds. No neoformation of inflorescences takes place after this period.
Research indicates that within fully matured buds, the inflorescence primordia are morphologically differentiated to such a degree that their definitive structural shape for the following year can be predicted (i.e., whether they will develop into large, ramified clusters or small, cylindrical ones). Consequently, viticultural researchers conclude that the grapevine yield is largely predetermined by the environmental conditions under which bud development progressed during the preceding vegetative period.
