Grape Seed Morphology

Seed Formation and Stages of Maturity

The grape seed acquires its final external characteristics, anatomical structure, and maximum dimensions precisely at the onset of berry ripening, known in viticulture as "veraison" (the stage when berries change color from green to purple in red varieties or golden in white ones). This stage strictly coincides with the moment of full physiological maturity of the seed itself. After passing the veraison stage, only a process of deeper lignification occurs within the seed tissues, strengthening its internal structure.

Subsequent biochemical processes occurring within the fruit, such as the intensive accumulation of sugars in the juice and the progressive reduction in free acid concentration, determine the technical (industrial) maturity of the berry. However, these processes no longer exert any biological influence on the internal constitution and morphology of the seed. If the ripe clusters are left on the vine, berry turgor drops due to the progressive desiccation of the pulp. The wrinkled skin tightly adheres to the seeds, which are subsequently released naturally as the residual cellular structures of the pulp and skin break down, dry out, or decay.

Significance of Morphological Seed Characteristics

Seeds completely freed from the surrounding pericarp and their translucent outer integument possess stable and clearly defined characteristics. In scientific literature, these features are recognized as crucial criteria for the precise differentiation of species within the genus Vitis. The fundamental importance of these morphological characteristics was first demonstrated in the works of scientists-viticulturists Dr. G. Engelmann and A. Millardet. Subsequently, their findings formed the basis of the systematics proposed by leading researchers, including J.-E. Planchon, G. Foëx, and P. Viala.

Later, viticulturists-researchers H. Goethe, V. Vannuccini, and A. Bonnet demonstrated that the described anatomical features of seeds can be utilized not only to identify and differentiate wild species, but also to identify specific cultivated grape varieties. Although differences in seed structure are much less pronounced among varieties derived from the same botanical species, their unique external markers still serve as a reliable tool for precise varietal identification. Unfortunately, in practical research, this method is still not always given sufficient attention.

Seed Shape and Quantity

The geometric shape of a grape seed is directly dependent on the total number of seeds within the berry. When more than two seeds develop in a fruit (which is most typical for the majority of wild species and cultivated varieties), mutual pressure causes them to acquire a characteristic three-sided (trigonal) shape (Fig. 310). In cases where a seed develops in isolation or as a pair strictly within a single locule of the ovary, its walls become more rounded and swollen, and the central ventral ridge (keel) is significantly less prominent.

A reduction in the normal number of seeds, up to their complete absence, can be either accidental or genetically fixed. Accidental seedlessness, known as millerandage (shot berries), occurs under the influence of adverse weather conditions or cultural disruptions. In this case, vines that normally produce well-formed seeds develop clusters with berries of variable size, the smallest of which either completely lack seeds or contain only their underdeveloped rudiments.

Conversely, in certain varieties of the species Vitis vinifera (such as Corinth or Sultanina), seedlessness is a stable, hereditarily fixed trait. Their fruits develop and successfully reach maturity without seed formation due to congenital malformations in the structure of the ovules that render fertilization impossible. Such plants can only be propagated vegetatively (via cuttings).

An inverse phenomenon also occurs—an increase in the number of seeds beyond the norm. Individual berries may contain 5 or 6 normally developed seeds. In certain American hybrids, researchers have recorded up to 9 fully developed seeds, and up to 15 in the seeds of Croton, though in the latter case, most remained immature and differed in size. At the same time, multi-seeded grape berries do not differ externally from ordinary ones, and the exact biological causes of such multiplication have not yet been determined by science.

Morphological Structure of the Grape Seed (Vitis)

The grape seed is small and possesses a distinct individual shape depending on the subgenus:

• In representatives of the subgenus Euvitis (True Grapes), the seeds have characteristic outlines that taper toward the base (Figs. 308–310).
• In representatives of the subgenus Muscadinia, the seeds are flattened and boat-shaped (Figs. 311–314).

The elongated tip, or beak of the seed (Fig. 308, b), which corresponds to the micropyle and indicates the position of the embryonic radicle, is morphologically the true apex of the seed, despite being oriented downward inside the berry. Due to this lower position, the beak is conventionally referred to as the base of the seed in ampelographic descriptions. The opposite, most expanded upper part is considered the apex. The ventral side of the seed always faces the geometric center of the ovarian cavity (Fig. 308, 1), while the opposite dorsal side bears the chalaza (Fig. 309, 2).

The parameters of the dorsal side of the seed, and particularly the relief of the chalazal zone, possess the highest stability and systematic value for species identification. The dorsal surface is typically convex and smoothly rounds at the apex (Fig. 309). Its maximum width almost always coincides with the location of the chalaza, which is situated approximately at the center of the seed (with the exception of Vitis vinifera). Closer to the base, the seed narrows significantly, transitioning into the beak.

A longitudinal groove runs along the entire dorsal surface (Fig. 309, s). It is most clearly defined at the apex of the seed and practically disappears or becomes highly attenuated toward the beak (in some species, it is entirely absent in this zone, as shown in Fig. 315). In the central portion of the seed, this groove expands, forming a rounded depression in which the chalaza is embedded (Fig. 309, c).

Radiating grooves or striae may extend from the central chalazal depression toward the margins (Fig. 309, st). In most species, they are barely perceptible or absent, leaving the periphery of the seed smooth. However, in Vitis rotundifolia, these radial grooves are so deep that they distinctly cut the margins of the seed (Fig. 311), resembling the structure of ruminate seeds characteristic of the genus Ampelocissus. A similar feature is clearly traceable in the closely related species Vitis Munsoniana (Fig. 313), and is also well-marked in Vitis rubra (Fig. 343) and Vitis Balansaeana (Fig. 359).

The longitudinal groove of the dorsal side of the seed is filled by the chalaza in its central portion, and by the seed seam, or raphe, in the upper zone (Fig. 309, r). The raphe curves around the apex of the seed and passes onto its ventral side, extending down to the beak itself. Anatomically, the chalaza and raphe consist of thickened fibrovascular tissues of the seed, the degree of prominence and shape of which serve as crucial species markers:

• Vitis vinifera: The raphe on the dorsal side is frequently completely absent or barely perceptible. The chalaza itself is shifted significantly upward (into the upper third of the seed) and is morphologically expressed only as a rounded tubercle at the bottom of the chalazal depression (Fig. 365). The beak in this species is the longest in the genus Vitis and averages at least one-third of the total length of the seed (Figs. 363–374).
• Vitis labrusca: The chalaza and raphe are entirely absent as relief structures. In their place, only a large central rounded depression is found, from which flat connecting grooves extend upward and downward (Fig. 347). The beak in this species is short, angular, as if abruptly truncated at the end, and wider at its apex than at its point of attachment to the body of the seed.
• Vitis æstivalis and Vitis cinerea: The chalaza and raphe, by contrast, project very sharply above the surface. The chalaza has a distinct rounded shape, and the raphe appears as a narrow, highly elevated, and abruptly detached cord (Figs. 335, 339).
• Vitis cordifolia and Vitis Blancoii: The chalaza has a rounded base near the beak and elongates sharply toward the apex, acquiring the characteristic shape of a triangular teardrop or drop (Figs. 319, 337).
• Vitis riparia: The chalaza also has a teardrop shape with an ascending point; however, the raphe cord in this species is shortened and does not reach the connecting groove at the very apex of the seed (Fig. 345).
• Vitis rotundifolia: The raphe cord is completely absent. The chalaza is embedded within a massive central depression, smoothly narrowing toward both the beak and the apex of the seed (Fig. 311).

The beak itself (Fig. 308, b), though present in all species without exception, remains short in most of them (excluding Vitis vinifera) and sometimes appears as a barely detached, weakly defined point. Examples of this structure include the seeds of Vitis Romaneti (Fig. 357) and Vitis Blancoii (Fig. 319). The surface of the beak is most frequently completely smooth, but in certain forms, it can be slightly wrinkled or rugose (Figs. 367, 368).

The ventral surface of the seed (Fig. 308, 1) is anatomically formed by two sloping faces that meet at the center, creating a longitudinal ridge—the ventral keel. Along this keel runs a connecting groove inside which the raphe cord lies (if it is developed). It is worth noting that the vascular strand of the raphe practically never reaches the very tip of the micropylar beak, and the line of the keel itself is rarely perfectly straight—more often it follows a slightly wavy (sinuous) trajectory. On a cross-section, the seed has a distinct triangular or three-sided shape due to this structure of the faces, where the dorsal side serves as the base and the ventral keel as the apex (Fig. 310, 3).

On either side of the central keel on the ventral faces are two longitudinal depressions called fossae, or ventral pits (Fig. 308, 1, f). Their depth, width, and length also serve as important diagnostic features:

• In Vitis rotundifolia and Vitis Munsoniana, the ventral side is practically flat, and the pits are very long, reaching the very apex of the seed (Figs. 311, 313). The cross-section of such seeds is discoid rather than triangular due to the pronounced flattening of both faces.
• In Vitis candicans, the ventral pits are extremely narrow and slit-like (Fig. 329).
• In Vitis æstivalis and Vitis Lincecumii, the pits, on the contrary, are maximally expanded and occupy almost the entire width of the two faces of the ventral side (Figs. 331, 339).

The surface of the ventral faces and the pits themselves may be covered with fine transverse striae or grooves. In the subgenus Muscadinia, these grooves are very deeply pronounced (Figs. 311, 313); in Vitis labrusca, they are vaguely indicated around the keel (Fig. 347); and in Vitis caribæa, they are localized strictly along the edges of the pits (Fig. 315).

Although the morphological characteristics of the ventral side of the seed are inferior in systematic importance to those of the dorsal side, they possess high genetic stability. As demonstrated in the research of viticulturist A. Bonnet, all the described structural elements of the ventral and dorsal sides are transmitted in detail to hybrid grape forms (especially binary hybrids), allowing for the precise determination of their parental species.