Grape breeding for early ripening and seedless berries

How early and seedless grape cultivars are developed

To obtain grapes with an early ripening period and large seedless berries, breeders perform inter-cultivar crosses within the Vitis vinifera species. An early ripening period is extremely important for commercial vineyards of any intended use. To pass this trait on to hybrids, very early and early cultivars are used as parent forms, crossing them with high-yield and resistant forms.

Modern practice has refuted the old opinion that it is impossible to combine extra-early ripening, high yield, and excellent taste in one plant. New cultivars successfully combine these qualities with high productivity and valuable biological properties. Among such cultivars, the following have proven themselves well in vineyards:

• Ranniy Magaracha;
• Muscat Tairovskiy;
• Gissarskiy Ranniy;
• Karakoz;
• Sverkhranniy Bessemyanniy Magaracha;
• Ranniy Shredera, and others.

Seedlessness is another trait of high market value for grapes, especially for table cultivars and sultanas. Old seedless cultivars usually have small berries, which reduces their overall marketability and yield. Modern hybridization is aimed at breeding large seedless berries with an early ripening period.

• Total number of cultivars in the register — 263
• Zoned seedless cultivars — 10
• Yield of seedless seedlings during selection — up to 40%

Seedlessness is passed on to offspring as a recessive trait, which significantly complicates breeding work. Physiologically, seedless cultivars can only act as the paternal form in crosses. However, with targeted pairing, the proportion of seedless seedlings in the offspring reaches 40%. The successful transmission of this trait is genetically linked to the maternal cultivar's tendency toward stenospermocarpy, its origin from the eastern ecological-geographical group, or the participation of seedless cultivars in the first and second generations (F1 and F2) of hybrids.

As a result of targeted breeding, new valuable table and sultana cultivars have been obtained that combine large berry size, yield, and excellent taste:

• Kishmish Khishrau;
• Kishmish Zarafshan;
• Kishmish VIRA;
• Kishmish Magaracha;
• Mechta;
• Kishmish Moldavskiy;
• Parvana, etc.

Clonal selection: using mutations to improve the vineyard

Clonal selection helps to increase the yield of existing grape cultivars, improve their commercial qualities, and clean nursery plant material from dangerous viruses and other pathogens. The method is based on the natural ability of grapes to mutate — changes in the genotype that are stably transmitted during vegetative propagation. Such changes affect the appearance of the vine, its physiology, and biochemical processes.

It is important for an agronomist to distinguish true mutations from temporary modifications. Modifications represent only an adaptive reaction of the vine to specific weather conditions or changes in the level of agricultural technology. Unlike mutations, modification changes are not fixed in the offspring during propagation by cuttings.

Genetic changes in grapes are divided into three groups depending on the nature of the damage to the hereditary apparatus. Genomic mutations change the number of chromosomes in cells (polyploidy, haploidy, aneuploidy), with polyploidy being the most important for practice. Chromosomal mutations rearrange the structure of chromosomes through their inversion, translocation, duplication, or fragmentation. Gene mutations change the chemical structure of individual genes, which often disrupts the metabolism of the vine.

By selecting valuable bud mutations that affected individual genes or segments of chromosomes, breeders have developed entire groups of popular cultivars:

• muscat cultivars (Muscat Belyy, Muscat Rozovyy, Muscat Fioletovyy);
• seedless (Kishmish Belyy Kruglyy, Kishmish Belyy Oval'nyy, Kishmish Rozovyy, Kishmish Chernyy, Kishmish Mramornyy);
• cultivar groups Khusayne, Pinot, Chasselas, Feteasca, etc.

A clone in viticulture should be understood as vegetative offspring obtained as a result of a mutation and differing from the original cultivar by one or more traits that are preserved during vegetative propagation. The best results in clonal selection can be expected from working with cultivars of ancient origin, which have accumulated stable intra-cultivar deviations over a long period of cultivation.

Clonal selection is no less important for the breeding refinement of new cultivars. When specifically choosing original cultivars for clonal selection, the general value of the base cultivar, its prevalence, and its inclusion in the cultivar zoning are taken into account. The task of clonal selection includes eliminating those shortcomings that lower the overall assessment of the cultivar (low yield, small clusters, berry shriveling).

The most complex and critical stage of clonal selection is the determination of types of variation, which boils down to attempting to answer the question of whether the discovered change is of a mutational (hereditary) or modificational nature. Mutational changes serve as the theoretical and practical basis for conducting clonal selection of grapes.

Unfortunately, clonal selection does not currently possess sufficiently reliable rapid methods for determining the nature of these changes. There are many cases where modificational changes persist for a long period — over several years and even vegetative generations. Such forms of changes are called long-term modifications. Examples include facts regarding the influence of high vine vigor on yield. Therefore, the most precise and complete answer to this question can be obtained after studying one or two vegetative generations, which requires a considerable amount of time.

According to the methodological recommendations of N. I. Guzun, G. M. Karadzhi, and F. V. Kaisyn (1977), the isolation (selection) of a clone and its testing are carried out according to the following scheme:

1. Clone selection is carried out based on three-year data. Cuttings are harvested from the selected plants, from which nursery plants are grown.
2. These are used to establish a clone testing plot, usually including 30–50 selected clones. Each of them is planted in quantities of no fewer than 10 vines. Every 2–3 rows, plants of the "base" cultivar are planted as a control.
3. After the onset of fruiting, phenological observations, fruiting assessments, ampelographic analyses are conducted on the plantings for three years, and the plants' resistance to diseases, pests, and the quality of the produce are evaluated.

Based on the data obtained, the best clones are selected, from which cuttings are harvested and nursery plants are grown. These are used to establish a plot for the second stage of clone testing: in two to three replications with 25 plants in each. The clones are studied using the same methodology as in the first stage, also during three years of fruiting.

The result of this stage is the selection of the best clones for their production evaluation. For this purpose, each selected clone and the control "base" cultivar are planted in quantities of 50 to 1000 vines. Yield records, comprehensive analysis, and assessment of harvest quality are carried out per plot. The clones selected at this stage are transferred to state variety testing plots; cuttings are harvested from these clones for propagation and the creation of mother plantations.

To accelerate the breeding process, if the studied clone has clear advantages in economic traits, the duration of study at various stages can be reduced to two, and in some cases to one year.

According to the latest recommendations, clonal selection is conducted in parallel with phytosanitary selection, which allows for obtaining a clone on a healthy basis.

Clonal selection is practiced in most countries with developed viticulture, especially in France, Germany, Italy, Romania, and the USA. Productive work on clonal selection is also being carried out in the USSR. It is aimed mainly at obtaining high-yielding grape clones with large clusters and high fruiting indicators. The method of clonal selection also solves issues of obtaining cultivars with earlier ripening dates.

Clonal selection for yield is most widely carried out in our country in Moldova, Georgia, Ukraine, and Uzbekistan.

Using the mutagenesis method, a number of cultivars and clones have been obtained: VIV-6, Taisi, Saamo, Farashi, Sharabi, etc. Undergoing state testing are:

• Bayan Shirei Large-clustered;
• Dolgii Early-ripening;
• Mtsvane Kumsmtevana;
• Pinot Noir Large-clustered;
• Pilo Noir Productive.

One of the drawbacks of the clonal selection method is the "passive" waiting for spontaneous changes that occur in the grape plant found in the wild and in culture, and their very laborious search. Conducting this work, which is based to a significant extent on elements of chance, does not lend itself to planning.

Thanks to the achievements of biological science, as well as physics and chemistry, over the past decades, material has been accumulated for the development of the artificial mutagenesis method using physical and chemical mutagens. This new method, which is in the stage of development and refinement, is intended to to a certain extent compensate for the weaknesses of the traditional method of clonal selection.

To induce mutations in viticulture, the following are applied and tested:

• ionizing radiation (X-rays, gamma-rays, fast neutrons);
• a set of chemical mutagens: nitrosoethylurea (NEU), nitrosomethylurea (NMU), nitrosodimethylurea (NDMU), ethyleneimine (EI), dimethyl sulfate (DMS), ethyl methanesulfonate (EMS), diazoacetylbutane, colchicine, etc.

Introduction of cultivars: how to transfer grapes to a new region without errors

Quickly updating or expanding a vineyard's assortment without long breeding work is facilitated by introduction — the import and cultivation of cultivars in those regions where they were not previously grown. It is precisely thanks to such exchange that agronomists have obtained the modern diversity of grapes. Testing the same cultivars in different soil and climatic conditions allows for identifying their ecological plasticity and selecting the most reliable ones.

Before introducing a new cultivar to a farm, it is necessary to compare its biological characteristics with the local climate and soil. This allows for predicting the behavior of the vine in new conditions and assessing the feasibility of planting. When analyzing survival rates, it is important to evaluate the following environmental factors in detail:

• Sum of active temperatures by growing season phases and their intensity during specific periods.
• Duration of the growing season and frost-free periods.
• Magnitude of absolute minimum temperatures during the winter period.
• Water supply (total precipitation and hydrothermal coefficient).
• Illumination (radiation intensity and day length).
• Properties of the soil and underlying subsoil (mechanical and chemical composition).

Practice shows that the ability to adapt to new conditions heavily depends on the cultivar's origin. Knowing which ecological-geographical group a grapevine belongs to, one can estimate its chances of survival in advance. Based on the level of ecological plasticity, the groups are distributed as follows:

• Western European group — possess the highest plasticity and tolerate changing conditions the easiest.
• Black Sea Basin group — occupy an intermediate position in terms of adaptability.
• Eastern group — characterized by the lowest plasticity and require conditions as close to their native ones as possible.