Clonal selection is a specific form of breeding inherent exclusively to vegetatively propagated plants. A clone is defined as the vegetative offspring of a single individual. Although all plants within a clone are completely uniform in their hereditary (genetic) respect, individual differences always take place in vegetatively propagated perennial crops due to modification variance, prolonged modifications, and damage by pathogens.

Bush-by-Bush Variation and Its Causes

Experimental data indicate a high degree of phenotypic variance within a single variety-clone. Thus, based on Sartorius's materials, the bush-by-bush variation in yield for the Silvaner grape clone shows a coefficient of variation of V = 33%; according to Strickland and others, V = 33.4%. On commercial economic plantations of the Meslier variety, the coefficient of variation in the yield of individual bushes, according to individual accounting data by A. M. Negrul (1927), reaches V = 60,8%.

Academic science distinguishes several fundamental causes for such significant variation among individual specimens within a clone:

  1. Variation in soil fertility. Most researchers consider the variegation of soil fertility and the microrelief of the plot to be the primary factor behind the variability in bush productivity. According to data by Sax and Gowen (1935), who studied the variation of the Ben Davis apple variety, the total variation consists of 18% of causes directly related to the variation in soil fertility, and 62% of factors unrelated to the soil (which include 15% of permanent changes due to the influence of grafting, bud mutations, and unknown hidden factors).
  2. The influence of grafting and rootstock. The heterogeneity of tissue fusion at the site of the scion-rootstock union, as well as the diverse physiological influence of the rootstock material, cause severe variation in the development of the aboveground part (as pointed out by Sax, Gowen, Hedrick, Anthony, Dorsey, Knowlton, Gattoni, and many others).
  3. Agricultural practices and vine pruning. As demonstrated in the works of Winkler, Naumenko, Negrul, and Mishurenko, surgical intervention (pruning) fundamentally transforms the development of the plant, as well as the quality and quantity of individual yield elements. The unevenness of the individual approach of the master to pruning each specific bush enhances the variation of traits.

Theoretical Principles of Selection within a Clone

Classical biological research by Johannsen, Jodlos, Jennings, and many others demonstrates that the selection of extreme modificants in a pure line and in a stable clone does not lead to positive genetic results: the selected extreme specimens yield the same curve of variation in their vegetative offspring without its shift. However, these conclusions, obtained on protists and annual grain crops, cannot be mechanically transferred to vegetatively propagated woody plants.

In perennial crops, where each plant is individually accounted for, the breeder can precisely identify the specific causes of a beneficial modification deviation in a vine and, by artificially reproducing these conditions for the rest of the plantation, turn it into a permanent condition. Furthermore, during propagation by cuttings, the phenomena of nutrient after-effect and prolonged modifications, which depend on the physiological condition of the initial cutting, come into play.

Experimental field data on selection within a grape clone remained contradictory for a long time. Special experiments set up by Sartorius in Germany on a Green Silvaner clone regarding the selection of cuttings from the best and worst bushes yielded clear differences in the vegetative offspring. At the same time, experiments of a similar nature on the Muscat of Alexandria variety, conducted by Bioletti in California, yielded differences in the first years that subsequently faded away. Sartorius later explained these contradictions by the different state of the plasma and, mainly, by the unequal amount of storage starch in cuttings harvested from healthy and weak bushes. Under the highly fertile soil conditions of California, initial differences in the development of seedlings due to unequal starch content are quickly smoothed out, whereas under the poor soil conditions of Germany (where Sartorius worked), these differences have a stronger effect and are preserved in the vegetative offspring for a long time (the after-effect of nutrition).

The selection of cuttings from different parts of the plant, accounting for the age of the mother plantation, as well as the prevention of the phenomena of degeneration and the transmission of various infectious and functional diseases to the vegetative offspring, are of exceptional practical value. From the standpoint of modification selection theory, selection within a clone does not create new genotypes, but in practical viticulture, the regular selection of the best bushes, the special preparation of mother plantations for cutting, and the sorting of shoots are mandatory for every cultural farm. In the practical environment, this process is often called selection, but scientifically it is more accurately classified as "vegetative selection," performing the role of sorting and clearing the material.

Full-fledged clonal selection with a shift in the genotype becomes possible in two cases:

  • In the presence of vegetative mutations: spontaneous bud mutations can occur within clones, and in some unstable grape varieties, they proceed with high frequency.
  • When the variety is a mixture of clones: most old historical grape varieties represent not a pure clone, but a complex mixture of clones.

If several independent clones are included in the composition of an old variety, their economic value is obviously unequal. The scientific division of varieties into clones (performed by researchers Bonnet, Sartorius, Muth, Ziegler and Morio, Merzhanian, Lazarevsky, Paponov, etc.) revealed in the composition of standard plantations a huge percentage of undesirable mixtures: sterile, low-yielding, shedding, and small-berried clones. In this context, the timely removal of the worst clones or the selection and propagation of the best ones directly leads to a radical improvement and equalization of the indicators of the commercial variety.

Vegetative Selection

Vegetative selection refers to the meticulous sorting of the best cuttings and shoots from healthy plants (even within a single pure clone) in order to avoid the transmission of hidden diseases and to create the best initial conditions for the young plant's development during its early vegetative stages. From a strict theoretical standpoint, vegetative selection is not breeding in its narrow sense, as it does not lead to the creation of new genotypes. Its core essence lies in the phytosanitary cleaning of planting material and the selection of the best mother bushes. Due to the accumulated reserve of nutrients (the after-effect of nutrition), the offspring from these bushes demonstrate more vigorous growth and increased viability.

This type of sorting, as an essential agricultural practice, must be continuously carried out during cane harvesting across all specialized viticulture enterprises. Particular thoroughness must be exercised in regions where plantations are heavily affected by fungal and bacterial diseases, as well as in areas with poor soils where plants are prone to rapid physiological degradation and degeneration.

Clonal Selection by the Method of Mass Selection

The method of mass selection in clonal breeding consists of the annual field marking of the best bushes in the plantations (based on indicators of high yield, berry quality, and resistance) for the subsequent harvesting of planting material (cuttings) from these vines. In relatively pure-sorted and stable commercial grape plantations, it is often more profitable to mark vines not by their positive traits, but by their negative ones (negative selection). This approach makes it possible to timely identify and cull (up to uprooting and replacement with new seedlings) those bushes that remain sterile, low-yielding, or prone to critical flower shedding for several consecutive years.

Selection based on positive traits is conducted through annual markings of the best plants according to their primary characteristics based on observations carried out over several years. For marking purposes, builders use simple trunk whitewashing with lime, attach durable tags, or enter the coordinates into a book along with drawing up a precise plot map. Cuttings are harvested annually from these marked best bushes during the collection of planting material. It is self-evident that mass selection does not lead to the isolation of a single best clone. The collection of planting material is done bulk-style from the best plants; therefore, good clones could end up here along with extreme modificants of poor clones. However, in general, mass selection leads to the cultivation of a sum of the best clones, and due to this, its economic efficacy is significant in certain cases. Since mass selection is entirely compatible with vegetative selection and represents an agricultural practice that does not require particularly large expenditures but yields a great effect, both of these practices must be mandatory for collective and state farms harvesting planting material. In every viticulture region, there are directives from the Narkomzem regarding mass grape selection along with instructions from local experimental stations.

Clonal Selection by the Method of Individual Selection

Very often, the task is to improve the quality of a good, well-proven variety, increase its yield, or eliminate a certain biological defect. First and foremost, one should attempt to resolve this task through clonal selection using the method of individual selection. Indeed, it is highly probable that within the given variety there is a clone that, under a specific set of agricultural practices, will partially or completely satisfy the breeding target.

Study and Identification of Variations

First of all, exploratory work is undertaken to study and mark variations within the given variety. By examining individual plants in the vineyard, we study the variance of their properties. Morphological features, biological characteristics, and economic properties are all taken into account. Based on this tentative study of the variety, we ascertain that it contains many variations differing in the shape, color, and size of the flower and fruit, as well as high-yielding, low-yielding, and sterile forms (since it has not yet been established that all these forms correspond to distinct clones, we call them variations). If, on the contrary, the variety is more or less uniform and the desired deviations are not detected in it, then the rationality of conducting clonal selection remains open to question.

In the first scenario, the work can proceed in two directions: 1) a detailed study of individual variations is carried out on the mother plants to establish their adherence to different clones, or 2) the isolated variations are planted on a cloning trial plot to establish hereditary differences and select the best ones for propagation.

In the former case, the description and study of variations are performed in greater detail based on many botanical and economic traits. Observations are conducted several times during the summer. Individual traits are recorded with the greatest possible precision for each plant separately; an individual account of the crop weight is also kept, along with laboratory determinations of sugar content, acidity, starch content, and phenological observations. Based on the individual accounting of the entire aggregate of traits, variations are isolated. The method of isolating variations is based on knowledge of the variability of individual traits. Those traits that are least variable are the most reliable when isolating variations. If an isolated variation correlatively links several distinctive traits, this provides a greater basis for considering it a separate clone.

Observations for each plant are conducted over several years, with all attention directed toward establishing whether the discovered properties are indeed linked to a new clone, a new genotype, or if it is a modificational deviation. For this purpose, it is sometimes necessary to keep a detailed account of confounding circumstances: drawing maps of soil fertility, and accounting for the uneven impact of diseases, pests, and various accidental causes. Usually, 2 to 4 years are sufficient to isolate variations based on traits such as yield and resistance. Some morphological traits make it possible to isolate variations in a single year. The practical significance of this work is that the isolated variations within the variety are evaluated from the standpoint of their economic value; subsequently, the best and most useful variations that meet the target are propagated, while the worst, sterile ones are destroyed. This leads to an increase in the productivity of mother vineyards as well as young plantations.

Methodology of Cloning Trials

In the event that the variety represents a fairly uniform population, and the task is to isolate a higher-yielding clone, selection by studying variations is not possible, and therefore other methods must be resorted to. The radical pathway by which it can be definitively established whether a given variation is a new clone or a modification is a cloning trial—that is, a comparison by means of a field experiment of the vegetative offspring of the isolated high-yielding variations against the primary base variety.

The methodology of cloning trials is as follows. Within the plot of the variety undergoing selection, the best plants are isolated based on the primary traits of interest to the breeder. The isolated elites are vegetatively propagated; cuttings are taken from each bush strictly separately (individual selection) and planted on the trial plot. Cloning trials are established according to all the strict rules of variety testing. Uniformity of conditions, sufficient replication of plots, and standard plot sizes must be maintained in accordance with all the rules of a field experiment.

Cloning trials require many years and a large expenditure of funds and labor. Therefore, breeders usually try to reduce all clonal selection to the study and isolation of variations, turning to cloning trials only in rare cases. To accelerate cloning trials, various measures can be taken. For example, cuttings from each marked plant can be planted in a nursery during the very first year, and only after 2–3 years are the saplings planted in their permanent places. This makes it possible, as a result of a rough selection in the first year, to maintain a fairly large number of specimens in the nursery. Concurrently, for 2 years, observations can be conducted on the mother plants to trace how the traits of interest (yield, resistance, fruit quality, etc.) behave. After 2–3 years, the offspring of those plants whose properties turned out to be merely modifications are destroyed in the nursery, while the remaining stable numbers proceed to the cloning trial plot.

In the cloning trial, we finally verify the reliability of the clones (whether the variations correspond to true genotypes) and conduct their comparative evaluation. The best clones are propagated and enter mass commercial trials. In described form by individual selection, clonal selection of grapes is widely conducted under the supervision of specialized breeding stations (Rebenzüchtungsstation). Mass selection is carried out by the viticulturists themselves.