Plant Physiology - Musiyenko M. M. 2001

Physiology of Plant Reproduction
Vegetative Reproduction

Vegetative reproduction is a form of asexual reproduction that is very common among plants. Many plant species propagate exclusively through vegetative means. This process can occur using either specialized or unspecialized plant body parts.

In many plants, vegetative propagation takes place through the detachment of body parts that subsequently develop into independent organisms. For instance, a segment of an Elodea plant can be separated and will continue its growth independently. Duckweed propagates in a similar manner, rapidly producing massive populations that carpet the Water surface of various aquatic habitats.

Plants frequently form structures specifically adapted for vegetative reproduction: bulbs, rhizomes, stolons, and tubers (Fig. 198). Some of these serve to store nutrients, enabling the plant to survive unfavorable environmental conditions. Most plants possessing specialized Organs of vegetative propagation practically lose The ability to reproduce via seeds, delegating both reproductive and nutrient-storage Functions entirely to these organs. Morphologically, flowers, tubers, and bulbs are all modified shoots.

Propagation by bulbs. A bulb consists of a very short stem and fleshy leaves laden with reserve nutrients. Externally, it is covered by the remnants of previous years' leaves, whose nutrient reserves have already been depleted.

A bulb contains one or more daughter bulbs (cloves), each capable of producing a SHOOT that forms a new bulb by the end of the growing season. If multiple such shoots develop, vegetative propagation is achieved. Roots emerge directly from the stem of the bulb; thus, a true primary ROOT is absent.

Vegetative reproduction via bulbs is specifically driven by The formation of these daughter bulbs, which are initiated during the winter dormancy period. Kinetin, gibberellic, and indoleacetic

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Fig. 198. Types of vegetative reproduction in plants

acids, when applied to the basal plate of the bulb—much like high-Temperature treatments during storage—significantly enhance the efficiency of their formation. Breaking the dormancy of the bulb is accelerated by exposure to low temperatures, a process known as vernalization.

Propagation by tubers. Tubers have a lifespan of only one year. The process of tuberization begins with the formation of stolons, upon which the tubers subsequently develop. According to M.Kh. Chailakhyan (1984), the induction of tuber formation is controlled by phytohormones (Fig. 199).

Initially, Gibberellins and Abscisic acid are transported from the leaves to the lower stem buds. Later, tubers are formed, their growth being governed by high concentrations of Cytokinins relative to Auxins. Tuber formation, optimal growth, and development are sustained by a flow of photoassimilates from the photosynthetic aerial organs. Tubers can be of root or stem origin, and either subterranean or aerial. For example, dahlias possess tubers of root origin, whereas potatoes have tubers of stem origin. In dahlias, tubers form on the roots and do not produce stem shoots.

Fig. 199. Hormonal Regulation of tuberization in short-day plants: D — long-day photoperiod, K — short-day photoperiod

Other Methods of vegetative reproduction.

Some plants reproduce through cuttings. This involves The Use of winter leafless cuttings, summer cuttings with leaves, or green leaves (Fig. 200).

Fig. 200. Regeneration in Begonia: a — isolated leaf with adventitious shoots, b, c — dedifferentiation of an epidermal Cell into a meristem from which an adventitious shoot develops

Fig. 201. Willow branch with a ring of bark removed. Roots have formed from the calluses primarily above the ringed area

Winter cuttings are harvested from one-, two-, or three-year-old branches. Younger, one-year-old cuttings are most suitable; they are gathered when the plant enters its dormancy period—following leaf fall—and are prepared up to 30 cm in length (Fig. 201).

Root cuttings are also utilized.

Green cuttings are harvested during active stem elongation, before they become fully woody. A protective film forms on the cut surface, beneath which Cells actively divide to subsequently form a callus. Roots develop from the callus, while shoots emerge from the buds of the cutting. The callus can be of diverse origins, potentially arising from the cambium, pericycle, secondary cortex cells, medullary rays, and wood parenchyma. Synthetic growth-promoting substances that accelerate root formation—such as auxins, indolebutyric, indolepropionic, triiodobenzoic, α-naphthylacetic, and other acids—are widely applied in cutting propagation.

There is a group of viviparous plants. In the axils of their leaves and within inflorescences, tiny leaf-covered shoots develop in place of flowers; upon detaching from the parent plant, these shoots take root.

A number of dicotyledonous plants (such as carrots, parsnips, radishes, and turnips) develop a well-formed taproot. Together with the buds located at the Base of the old stem directly above it, these roots form overwintering structures that simultaneously serve as organs of vegetative reproduction. Fleshy taproots are characteristic of biennial plants, which undergo vegetative growth during their first year, survive the winter relying on an underground storage organ, and produce flowers and seeds in their second year before dying off at the end of the season.

Certain species propagate via rhizomes (such as couch grass and peppermint). Rhizomes are underground stems that grow horizontally and consequently bear small, scaly, brown leaves (as seen in couch grass), buds, and adventitious roots. Rhizomes function both as overwintering organs and as units for vegetative propagation.

There is also reproduction by means of above-ground stolons (such as yellow star-of-Bethlehem, stone bramble, etc.). Stolons are creeping horizontal stems that grow along The surface of the ground. While they do not serve as overwintering organs, their nodes produce adventitious roots. These roots, along with a portion of the stem, eventually separate from the parent plant, allowing it to propagate.

Above-ground creeping shoots, known as runners, represent an intermediate form between typical vertical stems and rhizomes. This type of propagation occurs in strawberries, wild strawberries, buttercups, and others (Fig. 202).

Fig. 202. Formation of buds and adventitious roots in strawberries

The Development of root suckers is very common among plants and occurs in many woody species. This mode of propagation is characteristic of cherry, plum, lilac, hawthorn, alder, birch, and other plants. Suckers can arise not only from the roots but also from the stumps of felled trees, originating from so-called adventitious buds following canopy damage.

Various methods of grafting, or transplantation, are widely used in fruit tree cultivation (Fig. 203).

Fig. 203. Methods of grafting: a — whip-and-Tongue grafting (copulation), b — cleft grafting, g — budding (oculation)

The plant onto which the graft is placed is called the rootstock, while the plant being grafted is the scion. Rootstocks are carefully selected for specific economically valuable traits, such as frost resistance, vigorous or dwarf growth, longevity, and Disease resistance. For instance, cultivated apple varieties are typically grafted onto wild forest apple seedlings. Such rootstocks are referred to as vigorous rootstocks. In addition, apple trees are sometimes grafted onto dwarf rootstocks, such as doucin and paradise apple. The advantage of this approach is that plants on dwarf rootstocks begin bearing fruit much earlier, although their lifespan is shorter.

There are various types of grafting (Protsenko, 1978). Bark grafting in woody plants is performed in spring when cambial activity is high and the bark easily separates from the wood of the stem. A horizontal cut is made on the rootstock just below a stem node within the internode. The bark is then slit vertically downward from the cut, and the edge of the bark is carefully folded back. The scion is prepared as a cutting with 3–4 internodes. A semi-conical cut, shaped like a tongue, is made at the lower end of the scion cutting and inserted under the bark of the rootstock so that the convex side of the tongue faces outward. The folded edges of the bark are then pressed firmly against the scion, and the graft union is wrapped with plastic film.

Cleft grafting is used when the rootstock is significantly thicker than the scion. In this method, the rootstock is split, and the scion cuttings, with their lower ends trimmed into wedges, are inserted into the cleft. The graft union is sealed with grafting wax and securely tied. This technique is applied to both woody and herbaceous plants.

Budding (oculation), or grafting with a single bud (eye), is very frequently used. In this method, a bud is excised along with a small piece of bark and wood and grafted onto seedlings. A T-shaped incision is made in the rootstock, and the bud is inserted with its "shield"—a small piece of wood and bark. Once inserted under the bark, the shield is held in place by the flaps of bark and tied securely. When the scion and rootstock are of equal thickness, whip-and-tongue grafting (copulation) is used. Sloping cuts are made on both the scion and rootstock so they fit snugly together, which also maximizes the contact surface between them. The union is then tied and coated with grafting wax. Grafting is typically performed in spring and autumn, though it can be done at other times depending on the method. For example, budding is often carried out in summer using a dormant bud, which unites with the rootstock by winter and begins active growth the following spring.

The union of the rootstock and scion occurs as follows: living cells near the cut—particularly those of the cambium, bark, and vascular bundles—begin to divide rapidly, bridging the existing gap. After a certain time, plasmodesmata form between the Cells of the scion and rootstock, connecting their vascular systems.

Overall, methods of vegetative propagation have become widespread in horticultural and agricultural practice.