Depending on the character of the specific breeding task and the available parent material, the grape breeding process can be directed along several pathways. The first is the introduction of varieties, which is applicable when transferring ready-made forms. The second and most active pathway is artificial morphogenesis and selection among the resulting new forms, associated with the sowing of seeds obtained from hybridization, inbreeding (inbreeding/inbred lines), or simply natural pollination. The third pathway is clonal selection, aimed at improving existing varieties by selecting the most productive and high-quality genotypes from a population (a mixture of clones). This method allows for an increase in the quantity and an improvement in the quality of the yield without creating new varieties.
Introduction of Varieties
In certain cases, when varieties with the required economically valuable properties already exist in other regions, the breeding task can be resolved through their introduction. This method is advisable in cases where grape culture is being introduced to a region for the first time (for example, in the Far East) or when grape culture in a given locality has been little tested so far (in northern regions, on the Don sands, etc.). By this pathway, high-quality table varieties can be selected from the existing global assortment for Eastern and Western Georgia, as well as wine varieties for regions of Central Asia where the local assortment does not fully satisfy production requirements.
However, in most cases, this simple method is incapable of resolving the set tasks, since varieties combining all necessary properties (for example, phylloxera resistance with high fruit quality) do not exist in a ready-made form within the global assortment. Furthermore, introduction must be approached with sufficient caution. If a certain variety demonstrates high results in one region, this provides no grounds for asserting that similar indicators will be obtained when it is transferred to other conditions. Grapes react extremely sharply to changes in the external environment, under the influence of which fruit quality, chemical composition, yield indicators, and the general resistance of the vine are significantly modified. Thus, the Riesling variety yields famous high-quality wines only on specific plots of the Mosel and in Anapa, whereas in other ecological conditions it provides only a mediocre product. The Korinka variety yields high crops and a conditional product in Greece, Turkey, and Australia, while in Central Asia it frequently drops its blossoms and fails to yield a crop entirely.
Obtaining New Varieties via Artificial Morphogenesis
To create new grape varieties that combine traits absent in the parent forms, the following methods of artificial morphogenesis are applied: 1) sowing seeds from natural pollination, 2) inbreeding (self-pollination), 3) intraspecific hybridization, 4) interspecific hybridization.
Sowing Seeds from Natural Pollination
Sowing seeds from the natural pollination of various varieties is historically the main method through which the bulk of existing cultivated grape varieties was obtained. In the 18th and 19th centuries, a whole group of originators (Jacques Vibert, Malingre, Courtiller, Bouchet, Salomon, Foster, Pirovano, Van Mons, etc.) obtained hundreds of varieties by this pathway. However, in modern scientific breeding, this method possesses significantly less value compared to others, since it excludes the possibility of deliberate and directed planning of breeding work. With this method, only the maternal plants from which the seeds were collected are known to the researcher, while the paternal varieties whose pollen effected fertilization remain unknown. For this reason, sowing seeds from natural pollination is practically not applied in grape breeding today, except in cases of technical difficulty in conducting artificial hybridization or when organizing mass seed sowings.
Inbreeding (Self-Pollination)
Inbreeding for breeding purposes in grapes was previously practically unused. At present, the method of self-pollination is used to obtain the second generation of hybrids, as well as for the in-depth study of the inheritance and genetic structure of existing varieties. Growing seedlings obtained by sowing seeds from the inbreeding of various varieties made it possible to establish experimentally that most grape varieties are heterozygous. The segregation of traits in the offspring of different varieties proceeds unequally, revealing the following regularities:
- The first group of varieties: upon inbreeding, it yields extremely weak shoots and segregates a significant number of albinos, dwarfs, and various monstrous forms, which indicates severe inbreeding depression.
- The second group of varieties: it yields significantly fewer of the indicated anomalies, but nevertheless reveals a certain degree of depression.
- The third group of varieties: it yields normal, viable shoots with plants of good development, which in some cases prove to be completely identical to the original maternal form.
Such behavior of the offspring indicates that the method of inbreeding can have practical significance in breeding only on the condition of a preliminary selection of varieties that yield normal offspring unsuppressed by depression. Subject to this condition, inbreeding can be applied to obtain new varieties close to the maternal type. When obtaining an inbred second generation of hybrids, it is also necessary to select such hybrid combinations that do not segregate undesirable recessive traits. For example, the Riparia × Gamay hybrid segregates albinos and variegated recessive forms in F₂, which were introduced into the genotype of the hybrid by the Gamay variety. At the same time, the Mourvèdre × Rupestris hybrid does not reveal such lethal and semi-lethal recessive forms in F₂.
Intraspecific Hybridization
Obtaining new forms by crossbreeding varieties belonging to the cultivated European grape V. vinifera has traditionally been performed to combine the valuable properties of two parental organisms. For instance, to obtain highly productive varieties with an intense black berry color in the south of France, Bouchet crossed Aramon and Teinturier (black), breeding the variety Grand Noir de la Calmette. In Germany, to obtain early forms, Rasche successfully crossed Madeleine Angevine with Riesling. This method is of decisive significance for breeding.
Crossbreeding two varieties yields the first generation of seedlings (F₁). Since grape varieties are heterozygous, strong segregation of properties by morphological and biological traits is observed in F₁, making it practically impossible to find two absolutely identical plants in the first generation. All grape traits are divided into two groups:
- Strongly modifying under the influence of the environment: resistance, sugar content, acidity of berries, and yield. Their inheritance is studied taking into account ecological conditions, age, and the developmental stage of the plant.
- Weakly modifying: coloration and aroma of berries, seed shape, leaf pubescence, and flower sex. The inheritance of these traits is of a more definite character.
Regularities of Trait Inheritance in Intraspecific Crosses:
- Leaf traits: by leaf size, F₁ hybrids occupy an intermediate position with a tendency toward reduction. When crossing an entire-leaf variety with a dissected-leaf variety, dissected forms predominate in F₁ and F₂ (however, the extreme dissection of Chasselas persillé is recessive). Pubescence (tomentose or bristly) dominates over glabrous leaves. Red autumn leaf coloration dominates over yellow in F₁, and in F₂, heterozygotes give a 3:1 segregation. Autumn leaf coloration in European varieties strictly correlates with berry color: yellow coloration corresponds to white berries, while red corresponds to black and red berries (this correlation is absent in American species and their hybrids).
- Inheritance of sex: sex is rigidly linked to flower form. Functionally female flowers (self-sterile, with reflexed stamens and sterile triangular pollen without pores) when crossed with male forms (erect stamens, underdeveloped ovary) yield a 1:1 segregation in the offspring, which has been proven in the species V. amurensis, V. riparia, V. labrusca, and V. rotundifolia. When crossing female forms with hermaphroditic ones (erect stamens, barrel-shaped fertile pollen), either a 1:1 segregation is observed in F₁, or all seedlings turn out to be hermaphroditic. Crossbreeding or self-pollination of hermaphrodites yields either completely hermaphroditic offspring or a segregation of 3 hermaphrodites : 1 female vine. Genetically (Negrul, 1936), female forms carry ff sex chromosomes, male forms are heterogametic (Fm f), and hermaphroditic forms are Fh f or Fh Fh. To obtain economically valuable bisexual varieties, it is recommended to use parental pairs with the Fh Fh formula.
- Seedlessness (obligate parthenocarpy): only 10–15 seedless varieties are known in the global assortment (Korinka, Kishmishes, Esqueri). When crossing a seeded variety with a seedless one, all seedlings in F₁ are obtained with seeds. However, if a variety with a high percentage of partial parthenocarpy (e.g., Muscat of Alexandria) is crossed with a seedless one (White Kishmish), 3 to 10% of seedless seedlings can be obtained already in F₁.
- Berry quality: by berry size, a tendency toward reduction is fixed in F₁, so to obtain large-fruited forms, it is necessary to cross exclusively large-berried varieties. Black berry color dominates over white (e.g., Nimrang × Saperavi yields only black berries in F₁), but heterozygous black varieties can give segregation. Muscat and labrusca aromas dominate over simple aroma. Muscat of Alexandria is heterozygous and, when crossed, yields a segregation of 50% muscat and 50% non-muscat seedlings in F₁. When crossing fleshy varieties with each other, fleshy-juicy and fleshy forms are obtained in F₁; crossbreeding table varieties with wine varieties does not yield good results in terms of quality. Sugar content and acidity under southern conditions occupy an intermediate position. Segregation of wild, low-quality forms during intraspecific crosses of V. vinifera does not occur.
- Time of ripening: a complex trait. When crossing late varieties with early ones, seedlings with an intermediate ripening period are obtained in F₁. To breed early forms, it is necessary to cross early parental pairs, although in rare cases, early-maturing forms can also segregate when crossing late varieties.
Interspecific Hybridization
Since the cultivated grape V. vinifera is susceptible to frost, phylloxera, and fungal diseases, and varieties with relative resistance do not fully solve the problem, breeders use American species (resistant to phylloxera and mildew) and the Asian species V. amurensis (which withstands forty-degree frosts). All these species cross easily with each other and with V. vinifera, having the same chromosome number (2n = 38). The F₁ and F₂ hybrids develop without abnormalities during the formation of germ cells and are characterized by heterosis (enhanced growth).
However, the fruit quality in F₁ interspecific hybrids remains close to American wild forms (small black berry, coarse taste, specific flavor), and resistance is of an intermediate character. Because of this, F₁ hybrids could not yield high-quality direct producers, but became valuable rootstocks (e.g., Riparia × Rupestris 3309, 101-14). In obtaining lime-resistant rootstocks, Riparia × Berlandieri crosses proved successful, where the lime resistance from Berlandieri was preserved, but the poor rooting capacity of its cuttings disappeared.
The complex "direct producers" currently cultivated (in Ukraine and Moldavia) are still poorly resistant and yield low-quality fruit. To solve this problem, the Ukrainian Institute of Viticulture (since 1928) and the Sochi point of VIR (since 1934) switched to the mass production of the second generation (F₂) of interspecific hybrids. In F₂, a diverse segregation of complex traits of resistance and quality occurs. The required combinations segregate extremely rarely — one out of several thousand or tens of thousands of seedlings, which requires growing huge F₂ populations. It has been established that sex, berry quality, and resistance inherit and combine freely. For example, the Mourvèdre × Rupestris hybrid yields more seedlings resistant to leaf and root phylloxera in F₂ than Riparia × Gamay and Chasselas rose × Rupestris, while the Riparia × Gamay hybrid segregates more mildew-resistant forms. Interspecific crosses with V. amurensis to obtain cold-hardy varieties for the northern zone were first performed by I. V. Michurin (as well as Khudyakov's hybrid in the Far Eastern Krai); however, the fruit quality of these hybrids remains low. Interspecific hybridization is a complex but currently the only method for obtaining varieties with complex resistance and high fruit quality.