Vegetative and artificial propagation – Reproduction in Plants (Senior 4 Biology) | Ganzaa

Reproduction in Plants

Lessons

Senior 4 · Biology

Reproduction in Plants

5 lessons

Lesson 2 of 5

Vegetative and artificial propagation

25 min

A thousand bananas from one shoot tip

At the Rwanda Agriculture and Animal Resources Development Board (RAB) station at Rubona, technicians working in a sterile cabinet cut a shoot tip from a healthy banana plant, smaller than a grain of rice, and place it on a jelly of nutrients in a glass jar. A few weeks later it has become a cluster of shoots; each is divided and grown again. RAB uses this method, plant tissue culture, to mass-produce pest- and disease-free planting material of cassava, banana, sweet potato, potato and hybrid coffee, because, as RAB explains, the conventional way of propagating gives few planting materials and a high risk of passing pests and diseases from the mother plant.

Lesson 1 showed how plants reproduce asexually in nature. Farmers and scientists copy and speed up these natural methods to keep good varieties true and to multiply them fast. This lesson compares the methods and asks when each is worth using.

Definition

Artificial propagation: deliberate asexual reproduction of plants by people, using cuttings, layering, grafting, budding or tissue culture.

Cutting: a piece of stem (sometimes root or leaf) that is planted and grows roots and shoots of its own.

Layering: making a stem form roots while it is still attached to the parent plant, then cutting it off.

Grafting: joining a scion (a shoot of the chosen variety) onto a stock or rootstock (a rooted plant) so that they grow as one plant. Budding is grafting with a single bud as the scion.

Micropropagation (plant tissue culture): growing many plants from tiny pieces of tissue, called explants, on a sterile nutrient medium. It depends on totipotency, the ability of a single plant cell to divide and develop into a whole plant.

Hardening off (acclimatisation): gradually exposing plants raised in a protected place to normal outdoor conditions.

Natural and artificial vegetative propagation

Natural vegetative propagation	Artificial propagation
Happens without human help	Done deliberately by farmers, gardeners and laboratories
Uses organs the plant already makes: tubers, bulbs, corms, rhizomes, runners, suckers	Uses cuttings, layering, grafting, budding, tissue culture, or divides natural organs (cutting a seed potato into pieces each with an eye)
New plants appear slowly and close to the parent	Can be very fast and produce huge numbers (tissue culture)
Offspring are clones of one plant	Offspring are clones; grafting can combine two different plants in one

Cuttings and layering

Cassava is grown from pieces of mature stem 20 to 30 cm long, each with several nodes; buds at the nodes produce shoots and the cut base forms roots. Sugarcane is planted as stem pieces (setts), sweet potato as lengths of vine, and tea as single-node cuttings. Many cuttings root faster when the cut end is dipped in a rooting powder containing an auxin (S4 Unit 8). In layering, a low branch is bent down and pegged under soil, or a ring of bark is removed and wrapped in moist soil and plastic (air-layering); roots form while the parent still feeds the branch, so success is high, but few plants are made.

Cassava stems cut into pieces for planting. Each piece carries several nodes, the raised scars where leaves were attached; buds at the nodes will grow into shoots and the buried end will form roots. Photo: Judethedeus, CC0, via Wikimedia Commons

Grafting and budding

Grafting joins the best top of one plant to the best roots of another. The stock is usually a vigorous seedling of a type that resists soil diseases or drought; the scion comes from a variety with high-quality fruit. The key is the vascular cambium: the thin layer of dividing cells just under the bark. Where the cambium of scion and stock touch, both produce new cells (callus) that link up, and new xylem and phloem form across the join.

Grafting works only between closely related plants (usually the same species or genus), because unrelated tissues do not join: they are incompatible. Avocado, mango, citrus and many other fruit trees are grafted, because seedlings do not come true to the parent variety and take longer to start bearing fruit.

Worked example 1: a grafting trial

A nursery grafts 60 avocado scions of an export variety onto local seedling stocks; 45 grafts "take" and grow (illustrative figures). (a) Calculate the success rate. (b) Suggest three reasons why some grafts failed. (c) Why does the nursery not simply plant seeds from the export variety?

(a) $\frac{45}{60} \times 100\,\% = 75\,\%$ .
(b) The cambium of scion and stock was not lined up, so no vascular link formed; the scion dried out before the union healed; fungi or bacteria entered the cut; the cut surfaces were dirty or crushed; the tie was too loose, so the parts moved.
(c) Avocado flowers are cross-pollinated, so a seed carries alleles from an unknown second parent and its fruit will differ from the export variety. A grafted tree is a clone of the scion variety, so its fruit is predictable, and it bears fruit sooner than a seedling.

Micropropagation: plants by the thousand

Two plant hormones control what the explant does: a high ratio of cytokinin to auxin makes it form many shoots; a high ratio of auxin makes shoots form roots. If the explant is the apical meristem, the tiny dome of dividing cells at a shoot tip, the plants are usually free of viruses, because viruses spread through the vascular tissue and the meristem has none yet. This is how virus-free banana, potato and cassava stocks are cleaned and kept. Plantlets can also be stored in jars in a gene bank, saving space and keeping old varieties safe.

Using propagation to improve crops

Artificial propagation does not create new varieties; it multiplies them. Improvement happens in two stages:

Create variation and select: breeders cross plants sexually (or screen many seedlings) and pick the rare individual with high yield, good taste or disease resistance.
Multiply it asexually: cuttings, grafting or tissue culture copy that one plant thousands of times without losing its combination of alleles, which sexual reproduction would shuffle.

Grafting adds a third possibility: combining a disease-resistant or drought-tolerant rootstock with a high-quality scion, getting the advantages of both in one tree.

Worked example 2: how fast is tissue culture?

A laboratory starts with one banana shoot tip. Every 4 weeks each shoot is divided into 4 shoots (illustrative). (a) How many shoots are there after 6 subcultures, and how long does this take? (b) If 80 % of plantlets survive hardening off, how many plants reach the field? (c) How many subcultures are needed to reach 10 000 shoots?

(a) $N = 4^{n}$ , so $N = 4^{6} = 4096$ shoots after $6 \times 4 = 24$ weeks.
(b) $0.80 \times 4096 = 3276.8$ , so 3276 plants (a fraction of a plant cannot survive).
(c) $4^{n} \ge 10\,000 \Rightarrow n \ge \frac{\log 10\,000}{\log 4} = \frac{4}{0.602} = 6.64$ , so 7 subcultures (28 weeks). A banana plant in a field produces only a few suckers a year, so tissue culture is hundreds of times faster, and every plantlet starts clean.

Evaluating the methods

Method	Strengths	Limitations
Cuttings	Cheap, simple, uses material farmers already have	Spreads any disease in the parent (cassava mosaic virus travels in stems); some species root poorly
Layering	High success, since the parent feeds the new plant until it roots	Slow; only a few new plants from each parent
Grafting and budding	Combines the best roots and the best fruit; early, predictable fruiting	Needs skill; only works with compatible plants; viruses can pass through the graft
Micropropagation	Huge numbers fast; disease-free plants; year-round; small space	Expensive laboratory, power and skilled staff; contamination can destroy whole batches; rare genetic changes in culture (somaclonal variation); all plants identical

DrBananaGarcia: "Tutorial – Banana Micropropagation". Watch for the sterile working conditions, the small piece of shoot tip placed on a nutrient medium, and the many shoots that develop from it.

Warning

Safety and common mistakes

Use a sharp, clean knife and cut away from your body when taking cuttings or grafting; wipe blades with alcohol between plants so that you do not carry viruses or bacteria from one plant to the next.
Saying tissue culture creates new varieties. It copies an existing plant; new varieties come from breeding or genetic modification.
Saying the grafted tree has the genotype of the stock. The fruit and leaves above the union come from the scion; the stock provides only the roots.
Forgetting that a clone carries the parent's diseases. Clean (tested, disease-free) planting material is the first rule of propagation.

Tip

Memory hook. In tissue culture, "Cytokinin for Crowds of shoots, Auxin for Anchors" (roots). In grafting, "Cambium to cambium, or nothing."

✦

Practice: vegetative and artificial propagation

Distinguish between natural vegetative propagation and artificial propagation, giving one example of each.
Define stock and scion, and explain why their cambium layers must touch.
Explain why grafting usually fails between plants of different families.
Explain why plants grown from apical meristems are usually free of viruses.
In tissue culture, each shoot gives 5 new shoots every 3 weeks. Starting from 2 explants, how many shoots will there be after 12 weeks?
Of 1 500 cassava cuttings planted, 1 320 established. Calculate the establishment rate.
A cooperative wants to replace a field of virus-infected cassava. Evaluate two methods it could use: cuttings from neighbours' fields or tissue-culture plantlets.

Show answersHide answers

Natural vegetative propagation happens without human help using the plant's own organs, for example new potato tubers forming on stolons. Artificial propagation is done deliberately by people, for example planting cassava stem cuttings or grafting avocado.
The stock is the rooted plant that provides the root system; the scion is the shoot or bud of the chosen variety joined onto it. The vascular cambium is the only tissue that divides to make new xylem and phloem; where the cambia touch, callus from both links up and new vascular tissue forms across the join, carrying water up and sugars down.
Plants of different families are too distantly related: their tissues are incompatible, so callus does not unite and no continuous xylem and phloem form; the scion dies.
Viruses move through the plant mainly in the vascular tissue (phloem) and from cell to cell; the apical meristem has no vascular tissue yet and its cells divide fast, so it is usually virus-free. Plants grown from it start clean.
12 weeks is 4 subcultures: $2 \times 5^{4} = 2 \times 625 = 1250$ shoots.
$\frac{1320}{1500} \times 100\,\% = 88\,\%$ .
Neighbours' cuttings: cheap, quick and familiar, but if the area has cassava mosaic disease the cuttings will probably carry the virus, so the new field will be infected too. Tissue-culture plantlets: certified disease-free and of a known good variety, but cost more, need careful hardening off and a nursery, and must reach the cooperative from a laboratory such as RAB's. The best choice is clean plantlets (or cuttings from a certified clean multiplication plot), combined with removing infected plants so that the virus does not return.

Quick check

Put the steps of micropropagation in order.

Tap the cards in the right order.

Key points

Artificial propagation (cuttings, layering, grafting, budding, micropropagation) is deliberate asexual reproduction: it keeps a good variety true and multiplies it.
Grafting joins a scion (chosen variety) to a stock (good roots); the cambium layers must touch, and only compatible plants unite.
Micropropagation uses totipotency: explant → sterilise → multiply (cytokinin) → root (auxin) → harden off. Meristem culture gives virus-free stock. Numbers grow as $N = N_0 \times r^{n}$ .
Crop improvement = create and select variation (sexual breeding), then multiply the best plant asexually. Clean planting material is essential, since clones carry the parent's diseases.

Sources and credits

6.3.4: Asexual Reproduction (section "Artificial Methods of Asexual Reproduction"), from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for grafting, cuttings, layering and micropropagation. Adapted by Ganzaa.
Rwanda Agriculture and Animal Resources Development Board, "Plant and Microbial Biotechnology" (tissue culture at Rubona and Musanze).
Figures 1 and 2, the tables, worked examples (the grafting and tissue-culture figures are illustrative), practice and quick check: written by Ganzaa.
Photos: Judethedeus, CC0; Porto Neto, CC BY-SA 4.0; both via Wikimedia Commons.
Video: "Tutorial – Banana Micropropagation", DrBananaGarcia, embedded from YouTube.

1A thousand bananas from one shoot tip

2Natural and artificial vegetative propagation

3Cuttings and layering

4Grafting and budding

5Micropropagation: plants by the thousand

6Using propagation to improve crops

7Evaluating the methods