Plant propagation: sexual and vegetative
Seed and vegetative propagation compared, clones, juvenility, polyembryony, which crops are propagated how, and a sum for planting material needed.
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A mango orchard planted from seed would contain hundreds of different trees, most bearing fruit unlike the variety the farmer wanted, and only after many years. Planted with grafted trees, every tree bears the same fruit within a few years. Choosing how to multiply plants, by seed or vegetatively, is one of the most consequential decisions in horticulture, because it determines uniformity, time to bearing, cost and plant health.
Plant propagation is the foundation of nursery work and of horticulture courses and exams.
Sexual propagation: seed
Seeds come from fertilisation, so each seedling has a new combination of its parents' genes.
Advantages
- Cheap, easy to store, transport and sow in large numbers.
- Seedlings usually start free of most viruses, which are rarely transmitted through seed.
- Seedlings have strong tap roots and long lives, useful for rootstocks and forest trees.
- The only practical method for most field crops and many vegetables and flowers, and the source of new varieties in breeding.
Disadvantages
- In cross-pollinated and highly heterozygous species, such as mango, apple, avocado and most fruit trees, seedlings vary and rarely match the parent variety.
- Long juvenile period: seedling fruit trees may take five to fifteen years to bear.
- Seedling trees are often tall and vigorous, harder to manage and harvest.
- Some crops produce no viable seed: banana, seedless grape, pineapple, garlic.
Seeds of many woody plants need treatment to germinate: scarification (breaking a hard seed coat, as in many legumes), stratification (moist chilling for several weeks, as in apple, pear and stone fruits), or soaking.
Polyembryony
In many citrus and some mango varieties, seeds contain several embryos, most of them nucellar embryos that develop from maternal tissue, not from fertilisation. Nucellar seedlings are genetically identical to the mother tree and free of most viruses, which made them valuable for citrus rootstocks and for some polyembryonic mangoes in South-East Asia.
Vegetative (asexual) propagation
Vegetative propagation uses a part of a plant, such as a stem, root, leaf, bud or specialised organ, to grow a new plant. All plants derived from one original plant by vegetative propagation form a clone, genetically identical except for rare mutations (bud sports).
Advantages
- Plants are true to type: every tree of a variety is the same.
- Earlier bearing, because the propagated material is already mature (adult phase).
- Grafting allows the use of rootstocks that control tree size, resist soil-borne pests and diseases, or tolerate salinity, drought or poor drainage.
- Propagates plants without viable seed.
Disadvantages
- More costly, skilled and slower to produce in large numbers (except tissue culture).
- Viruses and other systemic pathogens pass from the mother plant to every offspring, so clean, certified mother plants are essential.
- Planting a single clone over wide areas reduces genetic diversity, increasing the risk of epidemics.
Main vegetative methods
- Cuttings: pieces of stem, root or leaf that form roots; grape, sugarcane, cassava, rose, tea, many ornamentals.
- Layering: rooting a stem while still attached to the parent; litchi, guava and many ornamentals.
- Grafting and budding: joining a shoot or bud (scion) of the desired variety onto a rootstock; apple, mango, citrus, avocado, rubber, rose.
- Specialised organs: bulbs (onion, garlic, tulip), corms (taro, gladiolus), tubers (potato), rhizomes (ginger, turmeric), runners (strawberry), suckers (banana, pineapple), crowns and slips (pineapple).
- Tissue culture (micropropagation): multiplying plants from small pieces of tissue in sterile culture; banana, potato seed tubers, date palm, orchids, sugarcane.
Choosing a method
| Crop | Usual commercial method |
|---|---|
| Cereals, pulses, oilseeds, most vegetables | seed |
| Mango, citrus, apple, avocado | grafting or budding onto rootstocks |
| Grape | cuttings, or grafting onto pest-resistant rootstocks |
| Banana | suckers or tissue-cultured plantlets |
| Potato | seed tubers (often from tissue-cultured stock) |
| Pineapple | suckers, slips and crowns |
| Litchi, guava | air layering, grafting or cuttings |
| Coconut, papaya | seed |
Juvenility and maturity
Seedlings pass through a juvenile phase in which they cannot flower, however well they grow, followed by an adult phase. Juvenile tissue roots more easily as cuttings, while adult tissue flowers and fruits sooner. Vegetative propagation from adult shoots skips the juvenile phase, which is why grafted fruit trees bear early.
How many seeds for a grafted orchard?
A farmer will plant 1 ha of mango at 10 m × 10 m spacing, needing 10,000 ÷ 100 = 100 trees, plus 10 per cent extra for gap filling: 110 grafted plants.
The nursery raises rootstock seedlings from seed and grafts them:
- Seed germination: 70 per cent.
- Grafting success: 80 per cent.
- Seeds needed = 110 ÷ (0.70 × 0.80) = 110 ÷ 0.56 ≈ 197 seeds, so about 200 stones.
At closer, high-density spacing of 5 m × 5 m, the same hectare needs 400 trees, 440 with gap filling, and about 440 ÷ 0.56 ≈ 786 seeds.
Key points
- Seed is cheap and usually virus-free but gives variable offspring and long juvenility in fruit trees.
- Vegetative propagation produces clones that are true to type and bear early, but carries systemic diseases and reduces diversity.
- Main vegetative methods: cuttings, layering, grafting and budding, specialised organs, and tissue culture.
- Nucellar embryos in polyembryonic citrus and mango give true-to-type, virus-free seedlings.
- Rootstocks add size control, pest and disease resistance and stress tolerance.
What the exams ask
- School-leaving and entrance papers: sexual versus asexual propagation, advantages and disadvantages, and examples of propagation by organs.
- Degree and research-fellowship entrances: polyembryony and apomixis, juvenility, seed treatments, rootstocks, and the propagation methods of major fruit crops.
- Professional and nursery examinations: planning planting material, certification of mother plants, and choosing methods for crops.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why can't I grow the same mango from its seed?
Most mango varieties are cross-pollinated and heterozygous, so seedlings differ from the parent. Only polyembryonic types produce some seedlings that are true to the mother.
Are tissue-culture plants better?
They are uniform, can be produced in large numbers, and can be free of pests and diseases if started from clean material. But they are clones, and errors in culture can occasionally cause off-types.
Why do grafted trees fruit sooner?
The scion comes from a mature tree that has already passed its juvenile phase, so it can flower as soon as the young tree is big enough.
Sources
- Hartmann, H. T., Kester, D. E., Davies, F. T., Geneve, R. L. and Wilson, S. E. Hartmann and Kester's Plant Propagation: Principles and Practices, 9th edition. Pearson, 2018.
- Macdonald, B. Practical Woody Plant Propagation for Nursery Growers. Timber Press, 1986.
- Janick, J. Horticultural Science, 4th edition. W. H. Freeman, 1986.
- Litz, R. E. (ed.) The Mango: Botany, Production and Uses, 2nd edition. CABI, 2009.
- FAO. Quality Declared Planting Material: Standards and Protocols for Vegetatively Propagated Crops. FAO Plant Production and Protection Paper 195, 2010.
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