8.1.2  Seed dormancy

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For most plants, seeds are the primary means of reproduction. Dormancy allows seeds to separate from their mother plant and survive dispersal over distance and time before growth recommences. Developing embryos are growing tissues but enter dormancy late in maturation and seeds then dehydrate. This state of suspended animation enhances chances of survival. The torpedo-shaped seed of a mangrove (Rhizophora maritima) is an exception that germinates while still on the mother plant. When they fall, seed penetrate securely into soft mud flats. This adaptation aids speed of establishment in the unstable tidal zone.

Plant breeders often select seed for uniform, rapid germination but these characteristics are rare in nature. If all seed from a species or population germinated synchronously but was subsequently destroyed, say, by frost, the genome would be lost. Instead, we find that germination is usually staggered over a season or over years. Sometimes it is possible to harvest seeds or embryos before dormancy is induced and thereby germinate otherwise difficult species.

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Figure 8.1 Long-lived seeds of species typical of Australian sclerophyll forests. (a) Eucalyptus erythrocorys radicle emerging from capsule; (b) Acacia coriacae with fleshy aril still attached.

(Photographs courtesy P.T. Austin and J.A. Plummer)

There are two main reasons why a seed does not germinate: it may be dead (not viable) or dormant (Mott and Groves 1981; Langkamp 1987). Vital stains can confirm viability of embryos (Bewley and Black 1982). Embryos may never develop due to post-zygotic incompatibility (Section 7.2.4), may abort during development or may die after seed dispersal. Endodormant or paradormant seed may be viable, but may not germinate even when supplied with water and O2 at an appropriate temperature.

Seed longevity often relates to a species’ natural environ-ment. In climates favourable for germination, many species have seeds which remain viable for only a few days, for example the Queensland umbrella tree (Schefflera actinophylla), which originates in subtropical rainforests, or a few months, for example water gum (Tristania laurina) and myrtle beech (Nothofagus cunninghamii), which come from cooler rainforests. In contrast, seed from sclerophyllous forests, such as Eucalyptus and Acacia (Figure 8.1), remain viable for many years.

There are two categories of seed, recalcitrant and orthodox, and appropriate storage can vastly extend longevity of both. Many tropical and subtropical species, such as Citrus, mango and rambutan, have recalcitrant seeds; these are not desiccation tolerant and survive best if stored at high water content (30%) and warm temperature (usually >15°C). Orthodox seeds, such as Eucalyptus and Brassica, are usually stored below 10% water content and below 10°C. Between these extremes are many intermediates, and optimum con-ditions for several important crop species have been deter-mined by empirical experiment. For example, wheat is best stored at 14.5% seed water content, peas at 14.0% and clover at 11.0%.

Cells of some testas have hard, thick walls and a waxy layer which prevents imbibition (uptake of water) and sometimes even gas exchange. Dormancy persists in the absence of water or O2 essential for germination. Seed-coat-imposed dormancy is a special case closely related to paradormancy of perennials. Seed coats resist embryo expansion but plant tissues can exert substantial turgor pressure, so mechanical resistance is not a common form of dormancy. Roses have a very hard seed coat with several sclerified (stony) cell layers and great pressure is required to break them. Hard seeds are found in many families and are particularly common in legumes such as Fabaceae (e.g. clover (Trifolium) and lucerne (Medicago)), Mimosaceae (e.g. Acacia) and Caesalpiniaceae (e.g. Cassia). The seed coat exerts force on the strophiole, a plug-like valve structure near the hilum with elongated malphigian cells that separate to permit water entry. These seed coats need to be weakened physically or chemically to permit imbibition. This may occur naturally as a result of temperature fluctuations, abrasion and microbial or insect damage. Artificial scarification is often achieved by scratching, nicking or by rotating seeds in barrels containing an abrasive. Alternatively, seed can be chemically scarified with concentrated H2SO4, which mimics the effect of acid in the stomach of animals. In many parts of Australia spontaneous fire is common and destroys most living tissue but enables germination of many hard-seeded native species (Table 8.1; Bell et al. 1993). In these plants, brief seed boiling is commonly substituted to effect break of dormancy. Heat from fires will damage the testa, but smoke, perhaps via ethylene and/or sulphur compounds (Dixon et al. 1995), is also effective in overcoming other dormancy mechanisms. In serotinous plants, such as Hakea, Banksia and Eucalyptus, seeds are stored on the mother plant until fires open the woody fruits, dispersing the seeds into the nutrient-rich ash bed when competition for light from other plants has also been reduced (Chapter 19).

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Germination inhibitors can be present in the embryo, endosperm, testa or the surrounding fruit tissues. Inhibitors present in seed of Iris, freshly harvested hazelnut (Corylus avellana) and desert ephemerals, and in fleshy fruit such as tomato, Persoonia and Lomandra, must be removed or in-activated before germination can proceed; this often happens inside an animal gut or by rain leaching.

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Many species germinate in response to light, but usually only become light sensitive after imbibition. Germination of Grand Rapids lettuce (Lactuca sativa), the weed species Bidens pilosa, some Australian daisies and many other small-seeded species is promoted by red light (R; 660nm) but inhibited by subsequent exposure to far-red light (FR; 730nm) — a classic photoreversible phytochrome response (Table 8.2 and see Section 8.4). Sunlight has a high R:FR ratio which signals to a seed that it is located in an unshaded position. However, chlorophyll in leaves filters out red light so that under a canopy there is relatively more far-red light; that is, a low R:FR ratio prevents germination where light quantity is likely to be insufficient for most species. These seeds use light spectral composition as an indicator of likely total photosynthetic radiation. This is an example of secondary dormancy because it is induced only after seed dispersal (seed that is dormant when shed from the mother plant has primary dormancy). Seeds may lie dormant for months or years, germinating only when a tree falls in a forest or after a disturbance such as ploughing a field. In the latter case, phytochrome is being used mainly to sense light quantity. Deep burial in soil prevents germination of small seeds with inadequate resources to grow to the surface. In contrast, germination of Spinifex hirsutus, which grows on sand dunes, is inhibited by light. Dark con-ditions exist deeper in the dune where there is likely to be more moisture, nutrients and stable sand.

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Many seeds will not germinate unless water content has been reduced by dry storage. This is a common adaptation in desert annuals, which experience a seasonal rhythm of water availability. In cereals such as barley and wheat, alternative treatments can be substituted (Table 8.3). Some seeds, for example Ranunculus and orchids, contain rudimentary embryos that must develop further before germination can occur. Symbiosis with a fungus supports embryo growth of many orchids, and inoculation is incorporated into in vitro propagation methods.

Stratification, or pre-chilling, the exposure of seeds to cool moist conditions, is in many ways similar to chilling of buds (see below). The optimum temperature is usually about 5°C for temperate species such as peach (Prunus persica) and apple (Malus sylvestris). Embryos removed from freshly harvested fruit can germinate but growth is slow and abnormal. Normal growth is restored by chilling or exposure to long photo-periods, conditions which seeds in nature would eventually experience. In Australia and New Zealand, many alpine species require stratification. Eucalyptus pauciflora seeds col-lected from high altitudes respond to chilling but those of coastal populations do not, suggesting that natural selection has occurred, creating two ecotypes. For tropical species, chilling may operate at a higher temperature range, usually above 10°C.

Single or multiple dormancy mechanisms can ensure germination at an appropriate time, depending on the species (Table 8.3). Despite all the complex entrainment to en-vironmental cues, many seeds will eventually germinate even without their normal signals, a failsafe mechanism ensuring some attempt at establishment before the seed’s longevity expires.

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