The morphology of the grapevine root essentially preserves all the typical features characteristic of the underground organs of higher flowering plants. However, the dynamics of its growth, the distribution of absorbing areas, and the anatomical modifications during maturation exhibit a distinct specificity that must be considered when evaluating the vital activity of the vine. A live, young grapevine root is clearly differentiated along its length into four functional zones (Fig. 5), which sequentially transition from one to another from the apex to the base.

The Root Apex and the Zone of Elongation

The very tip of the grapevine root is covered by a root cap. Its primary function is the mechanical protection of the delicate meristematic tissue of the apical growing point from damage during contact with solid soil particles. During the vigorous spring or summer growth of the vine, the root apex appears as a small tubercle that is clearly visible through the enveloping colorless cells of the cap. This entire terminal protective zone of the root extends only from 2 to 5 mm.

Immediately behind the growing point lies the zone of elongation. In this segment, young cells formed as a result of meristem division begin to elongate intensively and acquire their final permanent structure. It is precisely due to the colossal increase in cell size within the elongation zone that the root tip forces its way deep into the soil fractions. The length of the grapevine root's elongation area is also small, measuring literally a few millimeters.

The Zone of Absorption and Root Hair Development

Adjacent to the elongation zone is the zone of absorption, which is easily recognizable by its dense cover of root hairs. A root hair is formed when the outer membrane of an epidermal cell elongates into a long, thin, sac-like protrusion. The membrane of this hair is completely devoid of a protective cuticle. Due to this, solutions of soil salts and moisture diffuse unhindered into the plant cell.

The appearance of root hairs increases the total absorbing surface of the epidermis hundreds of times over and ensures the tightest possible contact between the root and the nutrient substrate. The lifespan of each individual hair is very short. Older hairs continuously die off, while new ones are formed in their place as the root grows in length. Because of this constant forward shift, the absorption zone of the grapevine root invariably maintains a stable length of several centimeters, regardless of how much the root itself has elongated.

The Conductive Zone and the Mechanism of Root Narrowing

From the upper boundary of the absorption zone to the very base of the root system stretches the conductive zone. It is easily distinguished visually by its characteristic brown coloration. The primary physiological function of this area lies in the two-way transportation of substances: soil water and mineral salts ascend through it to the leaves and shoots, while organic plastic substances synthesized by the above-ground parts descend to the growth points and the furthest ramifications of the underground system.

At the transition point from the absorption zone to the conductive zone, a unique anatomical feature can be observed—the grapevine root abruptly and suddenly narrows (Fig. 5). This occurs because the thick primary cortex, which supported the operation of the suction apparatus, dies off completely as it is no longer needed and sloughs off, sliding off the root in the form of a hollow tube. The secondary cortex replacing it in the conductive zone has a significantly smaller thickness, which leads to the visual narrowing of the root.

All the described zones form on the root gradually. Newly emerged young roots from a cutting possess only a cap and an elongation zone. Somewhat later, a zone of absorption is established on them, which initially reaches the very base. Subsequently, with the appearance of lateral ramifications, the base of the root transforms into a conductive zone. Over time, the main, most powerful perennial roots of mature grapevines completely lose all other sections and turn exclusively into conductive pathways.

Branching of the Grapevine Root System

The first adventitious roots that appear on a planted cutting begin to branch after reaching a length of several centimeters. For instance, in the Riesling variety, according to Kroemer's observations, this process is triggered when the root length reaches 10–12 cm. At the same time, lateral ramifications arise exclusively on the older, physiologically matured sections of the young root. Its very tip remains completely free of lateral branches for a distance of approximately 10 cm.

From the lateral ramifications of the first order, ramifications of the second order soon branch off, from them—the third order, and in some cases, the branching of roots in the grapevine reaches the fourth and fifth orders. Root ramifications of all orders are formed in the exact same strict sequence: they are first established on older sections of the root, and then gradually advance towards the younger ones.

All roots formed during the first year (the year the cutting is planted) are clearly divided into two groups according to their appearance, functions performed, and lifespan:

  • Main roots: These are few in number but are the longest and thickest. From the very beginning, they exhibit rapid and continuous growth in length. Their task is to ensure the wide distribution of the underground system in the soil and to serve as the main conductive pathways.
  • Absorbing rootlets: A vast mass of small root ramifications of subsequent orders (second, third, fourth, and further). They are capable of penetrating into the finest cracks and spaces between soil particles, taking on the primary function of absorbing water solutions of salts.

The first-order root ramifications branching off directly from the main roots have an intermediate character. Initially, they function as an absorbing apparatus, but as the small rootlets of higher orders grow, they cease this function and become exclusively conductive pathways.

The intensity of the underground system's work is confirmed by precise numerical studies. Kroemer recorded that in a single vine of the Riesling variety, by the end of the first year, 40 main roots, 1,112 ramifications of the first order, 2,772 of the second order, and 960 of the third order were formed (a total of 4,884 roots). Correspondingly, one of the main roots of this vine bore 43 ramifications of the first order, 207 of the second, and 212 of the third order.

Other ampelographers provide even more striking examples of the abundance of small roots in one- and two-year-old vines. A Muscat Blanc vine, grown from a long cutting with six nodes, had at the end of the first year: 78 main roots, 1,135 ramifications of the first order, 2,196 of the second, 939 of the third, 416 of the fourth, 121 of the fifth, and 23 of the sixth order (a total of 4,908 roots). In a two-year-old vine of the same variety, the total number of roots reached the huge figure of 10,415.

In subsequent years of the vine's life, a gradual decrease occurs in the number of main roots originating directly from the underground stem axis (trunk). In a mature, bearing grapevine, the number of these principal axes is always significantly smaller than in a one- or two-year-old plant. Similarly, the powerful roots of the first and second orders are completely cleared of small root ramifications over a large extent. The root system of an old grapevine consists of relatively few but exceptionally powerfully developed skeletal roots. They are capable of reaching extraordinary lengths: in 20-year-old vines, they frequently penetrate the soil to a depth of 4–5 meters, and in individual cases, their length exceeded 10 meters. At the same time, the longest roots that penetrate deeply into the subsoil are very poor in small absorbing rootlets. They function extensively, but during a severe summer drought, they play a critically important role, providing the vine with water from deep water-bearing strata.

The scientist Müller-Thurgau proved that the gradual decrease in the number of main roots as the vine grows is the result of a fierce internal struggle for existence. This competition arises between individual elements of the underground system, starting from the third year after the cutting is planted. The reason lies in the annual agrotechnical pruning of the above-ground part. Due to pruning, the total assimilating surface of the vine's leaves maintains approximately the same size for a number of years. Accordingly, the total amount of organic substances produced annually by the leaves remains at the same level. However, to ensure the normal growth of all the roots formed in the first years, an annually increasing amount of nutrition would be required. In the competition for plastic substances that arises between the roots, the stronger axes continue to grow, completely depriving their weaker neighbors of nutrition, which stop developing, dry up, and gradually die off.

Seasonal Periodicity of the Vine Root System

The youngest tips of large skeletal axes and the small absorbing rootlets of higher orders remain physiologically active only during a single vegetative period—from spring to autumn. With the onset of winter and the drop in soil temperature, root growth processes, as well as to a large extent the absorption of soil solutions by them, gradually cease. Consequently, in the development of the underground organs of the grapevine, there is the same clear seasonal periodicity as observed in the above-ground shoots and leaves.

Given the impossibility of continuous direct observation of the underground part of the vine, precise cycles are difficult to investigate; however, fragmentary data (in particular, studies by Wieber and Kroemer) show that in northern and temperate viticultural regions, the vine builds its absorbing root system from the beginning of April until the end of October. At the same time, the renewal and establishment of new absorption zones proceed most actively in the month of May.

Toward the end of the vegetative period, mass mortality of the primary cortex occurs. The main external sign of the root transitioning into a state of deep winter rest is the extension of a dense brown coloration to the very tip of the root axis. In winter, the cells of the root cap become completely suberized.

With the onset of spring and the warming of the soil, the active development of the grapevine root system resumes. According to Kroemer, this process takes place simultaneously in three directions:

  1. Formation of new roots directly on the root trunk (stem): This pathway is characteristic mainly of young vines and is extremely rarely recorded in old plants.
  2. Formation of new small ramifications on old skeletal roots: This process takes place intensively and is renewed annually, regardless of the overall age of the grapevine.
  3. Renewal of growth of roots formed in preceding vegetative periods: In spring, the activity of the apical promeristem of the root is restored. The growing tip quickly frees itself from the brown, suberized winter cover. In long roots, this newly grown white tip has a very characteristic, wedge-shaped expanded form (Fig. 6). At the same time, the growing tips of the smallest absorbing rootlets usually do not thicken, and their diameter remains equal to the diameter of the old part of the root.

External Conditions and the Character of Root Development in the Soil

The study of the root systems of grapevines grown in a natural environment proved that the total length of the roots and the character of their spatial distribution across soil horizons depend entirely on a complex set of external factors: the physicochemical properties of the soil, its moisture content, aeration, and temperature.

The grapevine root possesses a pronounced hydrotropism—the ability to grow in the direction of a moisture source. A close water table forces the grapevine root system to spread horizontally in the uppermost layers of the soil. On the contrary, the dryness of the upper horizons and the stoniness of the substrate condition an extremely deep, almost vertical penetration of the skeletal roots into the subsoil to more humid layers. On loose, humus-rich, and moderately moist lands, the root system is distributed evenly and, as a rule, does not go excessively deep.

Temperature conditions are of no less importance. An increase in soil temperature sharply stimulates the growth of the underground part: the best results and the maximum rate of tissue growth were obtained by Kroemer at a soil temperature of about 40 degrees Celsius (under greenhouse conditions).

Due to such plasticity, researchers' data on the depth of grapevine root penetration vary greatly depending on the region:

  • In vineyards with a subsoil of sandy gravel, the main mass of roots terminates at a depth of only 50–60 cm (Kroemer's data).
  • On the slate soils of Germany, the same varietal roots extended to a depth of 4.5–5 meters.
  • On the clay-slate soils of the Southern Coast of the Crimea, in old vines (aged 40–65 years), the root endings were located mainly at a depth of up to 3 meters, but individual axes deepened to 6 meters (data by N.P. Buzin).

A very illustrative example of the strong influence of soil structure on root architecture is provided by Professor Gogol-Janovsky. In the valley of the Qvirila River (Western Georgia), on clay soils with a close, impermeable subsoil, the roots of the Rupestris du Lot rootstock spread almost horizontally. However, on an adjacent plot located just a few meters from the first, where the soil was a river deposit of fine gravel with sandy layers, the roots of the same rootstock deepened almost strictly vertically.

The methods of cultivation apply a vast formative influence on the underground structure of the root system: the depth of the trenching layer (plantage) before planting, the planting density, timely katarovka, artificial irrigation, and fertilization. Grapevine roots possess a supreme evolutionary capacity to flexibly adapt to current environmental conditions, fundamentally changing the geometry of their development.