Structure and biological functions of the grapevine root system

How roots work: nutrition, synthesis and reserve accumulation

The root system of grapes provides the plant with water and mineral substances, which rise under pressure to the stems, leaves and inflorescences. Young roots (root hairs) not only absorb soil solutions but also take up carbon dioxide. The primary synthesis of protein occurs within them: carbohydrates from the leaves are converted into sugars and organic acids, which, upon interaction with ammonium salts, form amino acids. These compounds are then directed to the growing shoots and bunches to build new cells.

• Water content in roots — 40–60%
• Viability of the bush on skeletal root reserves — up to 6 years
• Depth of the taproot in the first year — from 1 m
• Starch proportion in roots in winter — more than 20%
• Sugar proportion in roots in winter — 1.5–3%

The roots release a portion of organic acids into the soil. Together with decaying remnants, they serve as nutrition for beneficial soil microorganisms — fungi and bacteria. The vital activity of this microflora converts hard-to-access soil nutrients into forms easily assimilated by the grape.

Grape roots contain more nitrogen, phosphorus and ash elements than the stem, but less potassium. The ratio of carbohydrates in tissues changes throughout the growing season. Starch accumulation intensifies in November, filling the cortical part and the medullary rays. In the late autumn and winter periods, the starch content exceeds 20%, while sugar levels decrease to 1.5–3%. Fats begin to accumulate in the cambium zone during the second half of summer (July) and reach their maximum in winter.

Types of root systems and their development under different propagation methods

The development of the root system of grapes depends on a complex of biological factors and growing conditions. The shape and depth of root penetration are influenced by:

• the plant's propagation method;
• species and cultivar traits;
• physical, mechanical and chemical properties of the soil;
• soil temperature and soil moisture;
• pre-planting tillage method.

In seed propagation, a taproot covered with root hairs develops from the embryonic radicle. After a few days, it begins to branch: first, 1st-order lateral roots emerge (at a right angle, then bending downward parallel to the main root), followed by 2nd and 3rd-order roots. In the first year, the taproot reaches a depth of more than 1 m, and during the first 2 years, the seedling's root system develops significantly more strongly than its aerial part.

In vegetative propagation (cuttings, grafting or layering), adventitious roots develop. They form from the stem pericycle cells at the nodes, and less frequently in the internodes. Upon reaching a length of 8–10 cm, they begin to branch, forming lateral roots of different orders with white root hairs at the tips. The nature of these roots' development depends on the planting method: with horizontal planting of short cuttings, roots grow in a bundle opposite the bud, while with vertical planting of long 3-, 4-, 5-bud cuttings, they appear at the nodes, predominantly on the lower part (the heel).

The anatomical structure of the root system depends on the origin of the nursery plant. It is important for an agronomist to distinguish the key zones of the plant's underground part:

• In a seedling: a taproot, lateral roots of various orders and the root collar in the transition zone to the stem.
• In a nursery plant of vegetative origin: an underground trunk, main (primary) roots, lateral roots and surface (dew) roots.

Unlike a seedling, the root system of a nursery plant (a plant grown from a cutting) has a fibrous character. The underground part of the nursery plant consists of a stem (underground trunk) with several tiers of roots extending from it. In the upper part of the underground trunk, at a depth of 10–15 cm in the annually tilled top layer of soil, a large number of surface (dew) roots, or "dew collectors," develop. These roots are thin and short, and if they are not removed in time by pruning, the plant will completely transition to developing its root system in the surface soil layer, which hinders root development in deeper layers and leads to the weakening of the bushes and their total death. This is observed during prolonged summer droughts and after severe winter frosts. In grafted vineyards, surface roots must be removed especially carefully; otherwise, the bushes will switch to the rootstock's roots, and the purpose of grafting as a method of plant protection against phylloxera will not be achieved. In the middle part of the underground trunk, mainly at the site of former nodes, lateral roots develop, and at its very base — the main (heel) roots, which are the thickest, longest and penetrate deep into the soil. By the age of three to four years, a grape bush develops a powerful root system.

The root grows by the division of cells in the meristematic tissue — the meristem — located at its distal end in the growing tip. In both taproots and adventitious roots, the root tip, measuring a few millimeters, is covered on the outside by a hard, sharp, and very durable yellow root cap. In aerial roots, which readily develop under conditions of high temperature and humidity, the cap is brown.

Beneath the root cap lies the zone of elongation. This is the most delicate part of the root, 2–5 mm long, white in color. Its cells, by elongating, push the root cap forward, under the cover of which the root tip carves its path between soil particles. This explains the ability of grapes to grow in stony, skeletal, and other hard soils. The outer cells of the root cap gradually dry out and flake off, and in their place, new cells are continuously formed at the root tip from the growing tip.

Following the zone of elongation is the absorption zone. It is thicker, yellowish-white, and 2–7 cm long. Its outer cells (epidermis) elongate, forming numerous root hairs. The cell wall of a root hair lacks a cuticle, which is why the water with dissolved salts absorbed from the soil easily penetrates the cells.

Root hairs are short-lived, surviving for 10–20 days. As the root grows and elongates, old hairs die off, and new ones form in their place, causing the absorption zone to shift.

Following the absorption zone is the conducting zone. On the outside, this zone is covered with a thick layer of brown cork, which transforms into bark as the roots age. At the transition point between the absorption zone and the conducting zone, a thin constriction is visible on the root. Up to this point, the root is considered young, and beyond it — skeletal.

In terms of anatomical structure, a young root differs significantly from an old, skeletal one. In a cross-section of a young root taken in the absorption zone, tightly packed, uniform epidermal cells with root hairs are visible on the outside. Beneath the epidermis lies the first layer of the cortex — the intercutis. This is followed by 20–25 layers of cortical parenchyma cells, and then a layer of cells with thickened walls — the endodermis.

In the center of the root lies the central cylinder, which in a young root consists of two to three primary vascular bundles arranged in a circle. Peripherally to the bundles lie several layers of parenchymal cells called the pericycle, which gives rise to lateral roots, which is why it is called the root-forming layer.

By the end of the first growing season, a number of changes occur in young roots at the transition point between the absorption and conducting zones. The root becomes covered with a brownish-brown cork, ages, becomes skeletal, and acquires a secondary anatomical structure.

Fig. 17. Anatomical structure of a young root in the absorption zone (primary structure):

1 — root hairs; 2 — epidermis; 3 — intercutis; 4 — endodermis; 5 — primary xylem; 6 — primary phloem; 7 — cortical parenchyma; 8 — cells containing raphides; 9 — pericycle.

Fig. 18. Anatomical structure of a two-year-old root (secondary structure):

/ — first-year wood (xylem) ring; 2 — second-year wood ring; pd — second-year periderm; ph — phloem; cam — cambium; mr — medullary rays; r11 — radial rays formed in the spring of the first year of the root's life; r12 — radial rays formed in the spring of the second year; rl3 and rl4 — rays formed in the summer of the second year; v1 — first-year wood vessels; v2 and v3 — second-year vessels (according to Baranov).

In the central cylinder, between the phloem and xylem and in the gaps between them, meristematic cambium tissue forms in a circle. Cambium cells constantly divide and deposit secondary xylem (wood) inward and secondary phloem (bast) outward. Wood and bast are intersected radially by medullary and radial rays. As a result of annual cambium formation, the root grows in thickness, and thus annual wood rings are formed. In the secondary phloem, the cambium forms alternating thick-walled and thin-walled bast elements of hard and soft bast. Along with this, the endodermis cells become suberized, and the entire primary cortex dries out and gradually sloughs off. From the pericycle cells, rings of secondary meristem — the phellogen (cork cambium) — subsequently form. On the outside, all its tissues dry out, forming bark. Every year, new rings of phellogen and new layers of bark appear within the thickness of the secondary bark.

Anatomical characteristics of roots and resistance to phylloxera

The structure of the root system directly affects the resistance of grapes to phylloxera and the ability to tolerate soil drought. The roots of European cultivars (V. vinifera) are fleshy, thick, and loose, with large xylem vessels and lateral branches extending at a shallow angle. In contrast, the roots of American species, such as V. vulpina (V. riparia), are thinner, grow actively in length, and have a compact tissue structure. Their lateral branches emerge almost vertically — at an angle of about 80°.

The anatomical advantage of American species lies in the structure of their protective tissues. They possess a two-layered intercutis, small conducting vessels, poorly developed parenchyma, and thickened cell walls of the cork tissue. Due to this, their roots are capable of rapidly isolating areas affected by phylloxera with a dense layer of cork. European cultivars lack this ability; therefore, wounds from the pest do not heal and become prone to decay.

Soil Temperature Regime and Root Depth

The grape root system has no dormant period and is capable of growing year-round in the presence of heat, moisture, and nutrients. During the growing season, two waves of most active growth are observed: spring and autumn. In spring, roots begin to grow when the soil at the depth of their location warms up above 6–8 °C, and in autumn, growth intensifies after shoot growth stops at a soil temperature above 7–8 °C amid heavy rainfall. In dry years, the autumn wave of growth may not occur.

Active root vital activity begins before bud burst at a soil temperature of 5–7 °C, which coincides with the spring "bleeding" phase. Processes intensify during the period of intensive shoot and leaf growth at a temperature of 18–24 °C. Maximum daily growth and branching are observed in the first half of the summer before flowering, when the temperature in the root zone reaches optimal levels. If the soil overheats, growth weakens, and roots are damaged and die.

• Temperature for the start of sap flow in spring — 5–7 °C
• Optimum for root growth and branching — 28–30 °C
• Maximum daily growth — up to 12 mm
• Temperature of root suppression and death — 34–40 °C
• Temperature of growth cessation in autumn — below 7–8 °C

When the soil temperature drops below 7–8 °C, growth stops, and in autumn, the roots become covered with a crust and suberize up to the root cap. At the same time, a portion of the young non-suberized roots dies off — a mass root drop occurs. This process is observed all year round, but it proceeds most actively during dry periods and in winter. Only old skeletal roots remain long-lived and hardy to unfavorable conditions.

In winter, nutrients (mainly starch, sugars, and fats) accumulate in the roots, which are necessary for the spring start of the vine. At the same time, the resistance of the root system to frost is lower than that of the above-ground part. The roots of European-Asian grape cultivars die at a soil freezing temperature of 8 °C below zero. The roots of Amur grapes and American phylloxera-resistant rootstocks withstand temperature drops down to 12 °C below zero.

The development of the root system largely depends on the age of the vine and cultivation conditions. The bulk of the roots forms in the first 2 years after planting, when the number of skeletal roots and first-order branches reaches 60–70%. With age, small first-order branches die off, their number decreases to 5–6, but the quantity of third-order feeder roots increases. The depth of root penetration increases over the years: in the second year, the bulk lies in the 20–80 cm layer, in the third — 20–130 cm, and by the fourth year, it reaches 180 cm.

In the horizontal direction, the vine's roots can spread at a distance from 2–5 to 8 m. The bulk of the absorbing roots is always concentrated in the horizon of deep tillage (at a depth of 20–40 and up to 60 cm), where the best conditions for nutrition, aeration, and soil moisture are created. Deep tillage of inter-row spaces stimulates the root system to move into deeper soil layers. On terraces, the root system of the vines develops unevenly.

The development of the grape root system on terraces directly depends on the relief of the surface. Due to the uneven distribution of moisture in the soil, the roots are distributed across the area unevenly.

Root regeneration and vine development management

The root system of grapevines possesses a high regenerative capacity. When broken or pruned, roots easily regenerate, forming a dense network of fine branches. This ability forms the basis of an important agrotechnical practice — plantation renewal via deep loosening of vineyard inter-rows.

For rapid and successful root system regeneration, it is necessary to maintain optimal conditions on the plot. Root recovery is most intensive when combining two factors:

• sufficient soil moisture;
• application of mineral and organic fertilizers.

The vigor of the underground part of the plant is closely linked to cultivar characteristics. High-vigor grape cultivars that require a large feeding area also develop a more powerful root system. To assess its condition under field conditions, an excavation method is used, which allows for the precise determination of the depth of active roots.

By knowing the depth and boundaries of the distribution of the bulk of absorbing roots, an agronomist can effectively manage vine development. Proper care allows for the creation of an optimal water-air and nutrient regime in the root-inhabited layer. For this purpose, commercial vineyards employ a complex of agrotechnical practices:

• appropriate tillage;
• application of fertilizers;
• irrigation;
• protection of the root system from low temperatures.