The biological role and anatomical structure of the grapevine stem

The role of the stem and its anatomical features

The grape stem serves as the main transport artery and the primary storage of nutrients for the bush. Through it, the ascending flow of water and mineral salts travels from the roots to the leaves, and the products of photosynthesis are directed back to the root system. The starch and other compounds stored in the stem are consumed during critical periods of the plant's life. These include spring bud burst, active growth of young shoots, and inflorescence differentiation.

Unlike classic trees and shrubs, a grapevine does not have axial symmetry or a permanent shape. In the wild, the plant behaves as a liana — it climbs trees or trails along the ground. In cultivation, viticulturists artificially give the bush an optimal shape. To do this, they form an upright or slanted trunk, an enlarged "head" of the bush, and perennial arms on which annual fruit-bearing shoots develop.

With age, the bush's stem thickens. On average, its diameter reaches 8–12 cm, although under favorable conditions and given the longevity of the bush, these parameters can be significantly larger. A telling example is the famous century-old grape bush, the parameters of which are recorded in the table.

Annual fruit-bearing shoots have a jointed structure and consist of nodes and internodes. All primary organs — leaves, inflorescences, lateral shoots, tendrils, and buds — are formed exclusively at the nodes. Inside each node is a diaphragm consisting of starch-rich cells with thick, lignified walls, which are separated from the dead pith by layers of suberized cells. On nodes with inflorescences or tendrils, this partition is solid and contains the maximum amount of nutrients, whereas on nodes without tendrils, it remains incomplete.

• Diameter of the perennial stem — from 8–12 cm
• Maximum internode length — at the 5th node
• Number of lateral shoots on one shoot — up to 5–6 pcs.
• Appearance of tendrils on the main shoot — from the 3rd–5th node
• Appearance of tendrils on a lateral shoot — from the 2nd node

Patterns of shoot growth and lateral shoot development

The stem grows in length due to the division of cells in the apical meristem (apical growth) and cell elongation in the internodes (intercalary growth). At the same time, cell division at the tip of the shoot provides only insignificant elongation — growth occurs mainly due to stretching. The first 2–3 internodes at the base do not grow for long and remain shortened, while the 5th internode reaches maximum length. The parts of the shoot bearing tendrils grow most intensively. In thickness, the stem increases in the lower part due to the cambium, which forms layers of phloem and xylem, and this growth proceeds faster than in the root.

Bush branching is ensured by lateral shoots — shoots of the first and subsequent orders that develop from precocious axillary buds in the same year. With a high level of nutrition and favorable weather, lateral shoots grow quickly and can provide a harvest for the current season. The growth and maturation of lateral shoots proceed faster than the main ones, and inflorescences on them are initiated at an accelerated rate. Lateral shoots develop most vigorously at the lower nodes, while closer to the apex of the main shoot, their growth fades. If the apex of the main shoot is damaged or broken, the upper lateral shoot continues its growth.

Anatomically, lateral shoots differ from main shoots. At their base, only 1 underdeveloped scale-like leaf is formed, whereas 2 are initiated on the main shoot. Tendrils on lateral shoots appear earlier — starting from the 2nd node, while on the main shoot they develop only from the 3rd–5th node. In the leaf axil of the lateral shoot, an overwintering bud is formed, which will give rise to the main shoot in the following season.

How to manage bush growth polarity and harvest initiation

The direction of tying directly determines the uniformity of grape bush development. With a vertical position of the stem, shoots from the upper buds grow first and most actively, while the middle and lower zones lag significantly behind. To balance growth and achieve uniform bud burst, perennial arms and fruit-bearing shoots are tied horizontally. This simple technique allows for leveling the growth vigor of shoots along the entire length of the vine.

The growth vigor of a vine largely depends on its origin. Cultivars from the Eastern ecological-geographical group are characterized by greater growth vigor than Western European cultivars or those from the Black Sea basin. There are also clear differences within these groups. For example, Cabernet Sauvignon grows more vigorously than Aligoté and members of the Pinot group, while the latter surpass the Chasselas and Pearl of Csaba groups in growth vigor.

Before the vine enters into fruiting, all its shoots perform only a vegetative function. In fruit-bearing plants, growth and fruiting functions are combined on a single shoot. However, some shoots always remain barren. This depends on the genetics of a specific cultivar, as well as internal and external conditions during the period of winter bud initiation and the formation of inflorescence primordia within them.

Branching anatomy and types of shoots in the vineyard

Vine shoot growth has its own cycles. Initially, up to the 2nd node, the main axis grows only by the apical meristem — this is monopodial growth. Starting from the 3rd node, it is replaced by sympodial growth: the apical point shifts to the side and transforms into an inflorescence or a tendril, and shoot growth continues from a new lateral tubercle. At the next node, no shift occurs, and the growth point continues straight again, after which the cycle repeats.

The alternation of growth types determines the structure of the vine and the order of node succession. It sets a strict pattern for the arrangement of inflorescences or tendrils on one side of the shoot, and suckers and buds on the opposite side. Growing green shoots remain herbaceous, brittle, and have a characteristic downward-curved tip. Inflorescences or tendrils are initiated at nodes within the 3rd node from the base and above.

• Change of monopodial growth in mature vines — from the 3rd node
• Change of monopodial growth in seedlings — from the 6th node
• Interval of leaf initiation in a seedling — 10–15 days
• Number of inflorescences on a green shoot — from 1–2 or more

When growing grapes from seed, development follows a different scenario. The germinating stem reaches the soil surface due to the elongation of the hypocotyl, which brings out the embryonic bud. Approximately one month later, the primordium of the first true leaf is formed, after which new leaves are initiated every 10–15 days on opposite sides of the stem in a spiral pattern.

The growth of the seedling stem continues until the end of the growing season, after which the terminal bud dies. From the second year, shoots develop sympodially from dormant axillary buds (eyes). All shoots developing on the vine are divided into four groups according to their biological role:

• Fruit-bearing — shoots that carry inflorescences.
• Barren — shoots on which there are no inflorescences.
• Water sprouts (from the base) — shoots that develop from dormant buds on the underground trunk.
• Water sprouts (or vigorous shoots) — shoots growing from dormant buds on the above-ground trunk, cordons, or the head of the vine.

Water sprouts and basal shoots are usually thicker, distinguished by vigorous growth, have a looser tissue structure, and typically do not fruit in the year they develop. The winter buds initiated on them can yield a harvest in the following year. Water sprouts and basal shoots are used for forming new cordons and rejuvenating vines.

Often, in the growth and development of both primary and lateral shoots, various anomalies can be observed in the form of fasciation, i.e., the shoot becomes flat and wide. In some cases, a double tip is formed: the shoot appears to consist of two fused ones.

Towards the end of the growing season, shoots slow their growth and their maturation begins. The first 3–7 internodes mature slowly, over 10–15 days, after which the rate of maturation increases sharply. Higher located internodes (8th and above) mature within three days. Shoot maturation is a process of anatomical and physiological transformation that occurs after the initiation of the periderm. During this period, nutrient reserves are stored in the stem cells, the starch content increases, and the water content decreases. By the end of the growing season, mature shoots form a ring of cork cambium (phellogen), consisting of several layers of cork, which separates the primary cortex from the secondary phloem. After this, the bark begins to dry out, forming a crust. In grapes, it peels off in long strips. As a result of the formation of cork cambium and crust, shoots become light brown to dark brown depending on the cultivar. Well-matured shoots are called canes.

Fig. 20. Mature shoot in cross-section:

/ — dorsal side; 2 — ventral; 3 — grooved; 4 — flat; 5 — overwintering bud (eye); 6 — tendril.

Mature annual shoots have a sharply defined rounded shape with clearly expressed asymmetry of the sides — dorsoventrality. The widest side of the shoot is the ventral one. Collenchyma, pericycle fibers, bast, xylem, medullary rays, etc., are most developed on it. Opposite the ventral side is the dorsal side. It is more convex and darkly colored. The elements of collenchyma, xylem, and others are less developed on it, however, growth processes here proceed significantly more actively than on the ventral side. The lateral sides — the grooved one and especially the flat one — are less developed. A leaf is located on the grooved side, and a lateral shoot and an overwintering bud (eye) are in its axil. The lateral shoot faces the dorsal side, and the eye faces the ventral side. A tendril or cluster is located on the flat side.

Anatomical structure of the stem (according to Baranov). A vegetative shoot in the upper part has a primary anatomical structure, in many respects similar to the anatomical structure of a young root. The outer cells of the primary apical meristem of the growing cone differentiate and form the outer skin — the epidermis, while the inner layers form the parenchymal cells of the primary cortex: the endodermis and the central cylinder. A young shoot differs from the root by a thinner primary cortex (8—10 layers of denser parenchymal cells rich in starch), a more developed pith, and an epidermis with stomata, outgrowths of cobweb-like hairs, bristles, and "pearl" glands. Collenchyma is formed in the primary cortex. The central cylinder consists of bundles of primary phloem and xylem, arranged radially. Above them are sclerenchymal strands, which are called pericycle fibers or pericambial strands. There are some differences in the anatomical structure of the node and the internode. In the latter, the parenchyma and collenchyma are more developed, the vascular system is weaker, and the diameter of the pith is larger.

Fig. 21. Anatomical structure of the stem at the end of the first growing season:

/ — bark; 2 — cork; 3 — cork cambium (phellogen); 4 — periderm; 5 — bast fibers; 6 — sieve tubes; 7 — bast parenchyma; 8 — bast (phloem); 9 — cambium; 10 — porous vessels; 11 — septate libriform; 13 — wood (xylem); 14 — pith (according to Baranov).

The secondary anatomical structure of the stem appears after the formation of cambium between the primary phloem and xylem. As a result of cambium activity, the shoot grows in thickness.

In this process, secondary phloem is formed with a characteristic layered structure, where several layers of soft and hard bast cells alternate, consisting of:

• sieve tubes;
• companion cells;
• bast fibers;
• bast parenchyma.

The walls of the hard bast cells are heavily thickened. The hard bast fibers perform the role of mechanical tissue, which gives strength to the thin-walled tissue of the soft bast.

Secondary xylem consists of vessels — tracheae and tracheids, mechanical tissue — septate libriform and wood parenchyma. Medullary rays in the stem have a smaller width than in the root and serve for the storage of plastic substances, mainly starch.

In the center of the stem is the pith, consisting of large, pentagonal-shaped dead cells.