7.2.2 Vegetative options for reproduction

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We can think of vegetative options for reproduction as a trade-off where a plant forfeits long-term advantages of genetic variability generated via the sexual process and invests in short-term gains inherent in a particular genotype–environment combination. Thus, sexual reproduction may allow a plant to acquire and sustain genetic variability for adaptation to a particular environment, which is then exploited by large numbers of clonal individuals. Many primary coloniser species use vegetative reproduction, for example Hieracium and Taraxacum, both members of the Asteraceae, where maternally produced (apomictic) seeds grow into plants genetically identical to the mother.

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Figure 7.15 Apomixix pathways in angiosperm ovules.  Three possible routes are diplospory, apospory and adventive embryony.  All lead to diploid embryos with genotypes identical to the parent plant.

(Based on Koltunow et al. 1995)

Adventive embryony is one of three types of apomixis leading to genetically identical seeds without fertilisation (Figure 7.15). In this process, somatic embryos develop from maternal ovarian tissue (e.g. nucellus) instead of from the egg cell of the embryo sac, so resultant plants are genetically identical to their mother. Adventive embryos are also called maternal or nucellar embryos. More than one adventive embryo often develops leading to polyembryonic seeds (Figure 7.16).

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Figure 7.16 Germinating seed of polyembryonic Poncirus trifoliata (a relative of Citrus used as a rootstock) showing multiple roots, each arising from a separate embryo.

(Photograph courtesy J.A. Plummer)

In some genera, such as Citrus, fertilisation of the sexual (zygotic) embryo is an essential prerequisite to development of adventive embryos, so the sexual embryo may be present among the maternal embryos within the mature seed. More commonly, however, the sexual embryo does not compete successfully with the maternal embryos. This natural pheno-m-enon is used commercially in clonal multiplication of rootstocks of some genotypes of citrus and mango.

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Figure 7.17 pt 1 Many different parts of plants - stems, leaves, roots - have become adapted as vegetative propagules, often incorporating storage tisues.  (a) Haemodorum spicatum bulb showing swollen leaf basis. (b) Oxalis pes-caprae life cycle showing daughter bulbs (DB) pulled underground by contractile roots (CR); note also parent bulb (PB) and root tuber (RT). (c) Stylidium petiolare corm (CO) pulled underground by contractile roots (CR). (d) Triglochin procera, a swamp species with root tubers buried in mud below water layer (stippled). (e) Potato (Solanum tuberosum) stem tuber surface showing multiple shoot meristem buds.

((a)-(d) Based on Pate and Dixon 1982; (e) photograph courtesy C.G.N Turnbull)

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Figure 7.17 pt 2

There is also a wide range of vegetative reproduction mechanisms that do not involve floral structures. Examples of some of these are shown in Figure 7.17. Several species considered as weeds employ very effective vegetative repro-duction strategies (Figure 7.18), which illustrates the ability of plants to clone themselves repeatedly.

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Figure 7.18 Many 'weed species' make use of vegetative reproduction.  Often the vegetative propagules are a greater problem than any seed produced. (a) Couch grass (Cynodon dactylon) has long slender rhizomes, rooting at nodes. (b) Nutgrass (Cyperus rotundus) is particularly hard to eradicate because of its combination of vertical and horizontal rhizomes and small bulbs. (c) Protasparagus aethiopicus showing a large number of root tubers.

(Photographs courtesy C.G.N Turnbull)

(a)  Runner

A runner is an aerial side shoot from a leaf axil of a rosette plant such as strawberry. Runners produce leaf clusters and adventitious roots at their tips, thus forming new plants, which can be exploited in commercial clonal multiplication. Runners are typically long structures which thrust daughter plants away from the mother, and attain independence when the runner stem rots and the connection is severed.

(b)  Bulb

A bulb is a shortened stem with thick fleshy leaf scales (Figure 7.17a), and like all leaves these scales have buds in their axils. Some of the axillary buds form new bulbs by developing their own fleshy leaf scales, and these daughter bulbs eventually separate from the mother plant (Figure 7.17b). The fleshy leaf scales are storage organs which fuel early growth and flowering in spring after winter dormancy. Photosynthesis by current-season leaves is essential to replenish reserves depleted in spring, and also to develop daughter bulbs. This method of vegetative propagation is characteristic of many ornamentals, such as amaryllis, daffodil and hyacinth, and the fleshy scales are sometimes edible, as in the onion family. Daughter bulbs may take some years to flower if they contain insufficient reserves to support flower and seed formation. In warm climates, temperate bulbs are often stored refrigerated for several weeks, otherwise inadequate winter chilling may result in poor flowering.

(c)  Corm

Corms are thick fleshy shortened stems, with a storage function analogous to the leaf scales of a bulb. Flowers form from buds in the axils of highly reduced scale leaves. After flowering, the base of the flower stem forms a new corm. Some corms form contractile roots (Figure 7.17c), an unusual example of plant tissue shrinking, which pull the corms down into the soil, affording better protection from severe winter weather. Many ornamental species have corms, including crocus and gladiolus, and an edible example is water chestnut.

(d)  Rhizome

Rhizomes are horizontally growing underground stems, some-times mistaken for fleshy roots, which are often swollen with stored reserves. Leaves are reduced to scales and rhizomes generally contain no chlorophyll, but buds do form in the leaf axils, allowing underground stem branching. Examples include Iris, ginger, banana and some lawn and turf grasses, especially couches, for example Cynodon dactylon in Australia (Figure 7.18a) or Agropyron repens in Europe.

(e)  Offshoot

Offshoots are known by a number of different names including offsets, suckers, crown divisions, ratoons and slips. Typically a lateral shoot forms on the stem, develops roots and then separates from the mother plant. Pineapple is a plant which produces offshoots, as do some palms. Sugar cane is clonally multiplied from short stem pieces which produce offshoots.

(f)  Stem tuber

Stem tubers are swollen underground stems, with reduced scale leaves and axillary buds (Figure 7.17e). They are distinct from rhizomes because their terminal shoot apex stops growing and development is entirely radial and lateral, whereas rhizomes also continue to grow apically. After developing adventitious roots, either the whole tuber or individual buds generate a new plant. Potato is a starch-accumulating tuberous plant. Tubers generally do not contain chlorophyll unless exposed to light. The underground ‘nuts’ of nutgrass (actually a sedge, Cyperus rotundus) are small stem tubers that in combination with this species’ network of wiry rhizomes make it a tenacious, spreading weed (Figure 7.18b).

(g)  Root tubers

Tuberous roots superficially resemble stem tubers, but morpho-logically are quite distinct. As the name suggests they are fleshy swollen roots (Figure 7.17d) and store reserves such as starch. They are somewhat unusual in that they readily form shoot buds which develop into new plants. Sweet potato, yam and cassava are major root tuber staple foods in many tropical areas and ornamental dahlias are propagated commercially by root tubers. Some species such as Protasparagus aethiopicus were originally cultivated for their ornamental foliage and berries, but have escaped into native vegetation where their survival is assisted by root tubers (Figure 7.18c).

(h)  Root sucker

A root sucker is an adventitious shoot which develops on a plant’s root system, then emerges above ground and becomes a new plant. Many members of the Rosaceae family reproduce by this means, including raspberry and gooseberry. An important Australian native example is the genus Acacia, with several root-suckering species, such as A. melanoxylon (blackwood) and A. decurrens.

(i)  Conclusions

Advantages of vegetative propagation include exploitation of conditions unsuited to reliable sexual reproduction. Vegetative reproduction is usually seasonal and often is combined with production of a dormant structure incorporating storage reserves for early spring growth. Most plants which reproduce vegetatively can also reproduce sexually, and allocation of resources to each strategy allows the best of both worlds: maintenance of genetic variability and clonal multiplication. Many natural vegetative reproductive structures are con-veniently exploited for commercial propagation of economic species.

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