The Biology of Triticum aestivum L. (Bread Wheat)



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4.4 Seed dormancy, germination, seed banks and persistence

4.4.1 Dormancy and germination


Seed dormancy inhibits the germination of viable seeds under optimal conditions (Hilhorst & Toorop 1997). It is desirable for seeds of crop species to have a certain degree of dormancy to prevent sprouting if wet and moist conditions occur before harvest, but it can also restrict the timely elimination of volunteer cereals. Most of the commercial cultivars of wheat have been selected against dormancy to achieve quick and uniform germination, and thereby good stand establishment.

Pickett (1993) provided the following definitions of various forms of dormancy:



  • Innate Dormancy: environmental conditions favour germination however seeds do not germinate.

  • Enforced Dormancy: present in seed in dry storage or deep in soil or where seed does not germinate as environmental conditions are not correct.

  • Induced Dormancy: seed is no longer able to germinate even when conditions favour germination, the inability to germinate may be the result of environmental conditions.

Wheat may be capable of extended dormancy, but reported survival times vary widely depending on variety and environmental conditions. Australian wheats have a low level of dormancy that it is easily broken down, allowing germination to begin. By contrast, red wheats widely grown in Europe and North America have higher levels of sprouting tolerance and typically are dormant for longer periods after harvest than the white wheats. In dry regions, wheat seed can survive in the soil beyond two years (Anderson & Soper 2003; Beckie et al. 2001; De Corby et al. 2007; Harker et al. 2005; Nielson et al. 2009; Pickett 1989; Pickett 1993; Seerey et al. 2011; Willenborg & Van Acker 2008) and surveys from Western Canada indicated that, under certain conditions, there may be seed survival of up to five years (Beckie et al. 2001), but the mechanism of persistence is not known and may be due to reseeding (De Corby et al. 2007).

Under Northern European conditions, seed that is buried too deeply in soil for germination can be imbibed but remain metabolically inactive in a state of enforced dormancy. Pickett (1993) claimed that the seed coat is responsible for an inhibitory effect in developing a harvest-ripe grain. This inhibition of germination can be caused by the inner layer of the green pericarp of wheat. In latter stages of maturation the outer pericarp layer exercises similar control.

Classical studies involving burial of wheat seed in retrievable containers have shown persistence of less than 1 year, but field data suggests survival and emergence of wheat seedlings up to 2 years post-harvest (Anderson & Soper 2003). One concern related to the classical studies was the high density of wheat seed which may provide conditions conducive to disease and thus reduce seed survival (see review by Anderson & Soper 2003). Pickett (1993) mention reports of wheat seeds surviving in the soil for five years, but note that these claims are unproven. Pickett (1989) also noted earlier reports of germination inhibitors found in the seed coat of 18 red-grained varieties of wheat. In Australia, under a ‘no-till’ system and dry conditions, some seed remained viable for 17-18 months post-harvest (Wicks et al. 2000).

Induction of secondary dormancy of buried hexaploid wheat has not been reported under field conditions (De Corby et al. 2007), although secondary dormancy has been induced in laboratory settings (King 1976). Komatsuzaki and Endo (1996) found that Japanese cultivars with greater primary dormancy remain dormant for longer and exhibit greater persistence in the soil. The longevity of seeds in unthreshed ears was longer than that of loose seeds in the soil at depths of 3 to 21 cm. Similarly, Seerey et al (2011) found that for wheat scattered at the soil surface, wheat spikes are less likely to emerge as a volunteer in the following growing season than their threshed counterparts.

Failure of seed due to unfavourable conditions is referred to as enforced dormancy (Pickett 1993). Ploughing can bury a high proportion of seeds to a depth where this occurs or at which germinating seeds will be unable to reach the surface and develop into plants, however, it may also affect the conditions which could release seed from enforced dormancy (Pickett 1993).

Minimum moisture for germination in wheat is 35 to 45 % of kernel dry weight (Evans et al. 1975). During germination, the seminal root extends first, followed by the coleoptile. Adventitious roots are produced in association with the coleoptile node. When the coleoptile emerges from the soil its growth stops and the first true leaf pushes through its tip. The seedling is dependent upon energy and nutrients provided by the endosperm until its first leaf becomes photosynthetically functional (Simmons 1987).


4.4.2 Seed banks and persistence


Dormancy can affect the persistence of seeds in soil, but as discussed above, wheat seeds are generally short lived in the soil, with red wheats typically showing longer dormancy than white wheats, which should limit persistence in seed banks.

Seed lost at harvest could potentially persist and develop a seed bank which could lead to the dispersal of wheat from the field site (or other areas where it may grow as a volunteer) over many years. The amount of seed lost at harvest would depend heavily on the yield. Wheat yields can vary greatly between countries, ranging from 780 g/m2 (United Kingdom, Denmark, Germany, France and Egypt) down to the low­yield range of 100 - 220 g/m2 in countries such as Australia. The average worldwide yield is approximately 300 g/m2 (FAO 2015b), or 7500 seed/m2, assuming a weight of 0.04 g per seed. Seed loss during harvest is also variable; sometimes reaching more than 10 % (Clarke 1985), but 3 % loss is considered acceptable (Clarke 1985; Huitink 2014). In the United Kingdom, harvest losses of wheat averaged 2 % of yield, with 95 % of the surveyed farmers recording losses of less than 6 %. These results were probably for winter wheat, but this was not clearly stated. A 2 % loss from a yield of 3,000 kg/ha leaves about 240 seeds/m2 whereas a 6% loss would leave more than 700 seeds/m2 (Anderson & Soper 2003).

In Australia the average yield for 2012-13 was 1.76 tonnes/ha (see Table 2), which is 176 g/m2 or about 4400 seeds/m2. Assuming a 3 % loss, about 132 seeds/m2 would have remained post-harvest. This amount is approximately the sowing rate for wheat (assuming anticipated yield of 2 tonne/ha and stand of 100 plants/m2 (Anderson et al. 2000).

A three year study of volunteer hard red spring wheat emergence across the Canadian prairies found volunteer wheat emergence in approximately half the sites. Wheat seeds were dispersed in the autumn (post-harvest) at a density of 190 seeds/m2. The overall volunteer wheat emergence rate in continuous cropping fields, in the spring following dispersal, was 3.3 plants m2. At the end of the three year monitoring period none of the wheat dispersed at the start of the trial was detected in the soil seed bank (Harker et al. 2005). Another Canadian study examined post-harvest emergence and persistence of hard red spring wheat varieties, which were broadcast in late autumn at a rate of 500 seeds/m2 (to simulate pot-harvest seed loss). Emergence of volunteer wheat ranged from 0.9 to 13 % (average 4.3 %) the following spring. Wheat that did not recruit (i.e. germinate and emerge) rapidly degraded in the soil and did not persist past 12 months (De Corby et al. 2007).

Based on the Canadian studies (above) volunteer densities in the spring following the autumn dispersal of seed were 3.3 and 21.5 plants/m2, which represents average emergence rates of 1.7 and 4.3 %, respectively. There is little in the literature regarding emergence of volunteer wheat in Australia. An Australian study reported mid-fallow volunteers of wheat (i.e. about 10 weeks after harvest) of 0.7, 5.6 and 5.3 plants/m2 for no-till, stubble-retained & cultivated and stubble-burned & cultivated treatments, respectively. From the data, it is unclear what these losses represent relative to the yield (Wicks et al. 2000). At early fallow (i.e. five weeks after harvest) the volunteer wheat was a greater problem than at mid-fallow (Wicks et al. 2000). The authors also suspected that self-sown wheat would be a greater problem under experimental conditions because small-plot harvesters were less efficient than commercial harvesters. Where the harvest of buffer, or border plots was delayed, volunteer wheat was always increased in the early fallow period. Viable seeds persisted later in dry seasons in no tillage plots; at Winton (northern NSW; 1983) viable seeds persisted until June and at Warialda (northern NSW; 1986) viable seeds persisted until May, after harvest in the preceding Australian summer. Although no densities were provided, it is reasonable to assume that the density of wheat volunteers was greater at early fallow compared to mid fallow.

The Canadian and Australian studies (above) examined persistence of volunteer wheat under a number of different farming systems (e.g. no-till, retained stubble & cultivation, chemical fallow etc.) and demonstrated that under normal farming practices, volunteer wheat would not persist beyond three years. Wheat seed dispersed along roadsides or other non-cultivated areas is unlikely to emerge and thrive (due to predation and germination at wrong time of year) and seed production per unit area is likely to be considerably less than that under crop conditions due to suboptimal germination and growth conditions (e.g. moisture and nutrients) and competition by other plants.


4.5 Vegetative growth


Bread wheat is a cereal of temperate climates. Its various growth stages and their durations are listed in Table 4. Spring wheat varieties, which are grown in Australia, do not require cold weather to form inflorescences or spikes. In Australia, spring wheats can be planted in May and June, ideally before the middle of June, to maximise vegetative growth and to ensure that flowering does not coincide with late frosts.

Table 4. Duration of growth stages of wheat.


Plant growth stage

Temperature requirements (C)

Duration (days)

Germination

3-4 (minimum);

12-25 (optimum)



4-10

Flowering

14 (minimum)

4-15

Vegetative: winter




280-350

Vegetative: spring




120-145

4.5.1 Root development


One or more nodes may develop below the soil surface depending on the depth of sowing, each bearing roots (Hadjichristodoulou et al. 1977). Root axes are produced at predictable times in relation to shoot development, and the total number of roots formed is associated with the number of leaves on a tiller (flowering stem) and the degree of tillering (Klepper et al. 1984).

Roots originating from tillers generally develop after a tiller has formed three leaves. The relationship between root growth and plant height has been debated. Some have stated that root growth of a genotype is proportional to its top growth (MacKey 1973) and that more extensive root growth was seen in semi-dwarf cultivars of winter wheat than in taller cultivars (Lupton et al. 1974). Others compared tall and semi-dwarf winter wheat genotypes and concluded that no correlation existed between cultivar height and rooting depth (Cholick et al. 1977).


4.5.2 Leaf development


After germination the vegetative shoot apex initiates additional leaf primordia. The number of leaf primordia can vary from seven to 15 (Kirby & Appleyard 1983) and is affected by genotype, temperature, light intensity, and nutritional status of the plant. Temperature has a major influence on leaf appearance and extension. The minimum temperature for leaf extension is approximately 0 C, the optimum 28 C and the maximum greater than 38 C (Kirby & Appleyard 1983).

4.5.3 Stem development


Stem elongation coincides with the growth of leaves, tillers, roots and the inflorescence (Patrick 1972). Elongation of the stem begins when most florets on the developing spike have initiated stamen primordia, which corresponds closely to the formation of the terminal spikelet. In spring wheat the fourth internode is the first to elongate, possessing nine leaves, while the lower internodes of the stem remain short (Kirby & Appleyard 1987).

When an internode has elongated to half its final length, the internode above it begins to elongate. This sequence continues until stem elongation is complete, usually near anthesis. The peduncle is the final segment to elongate (Evans et al. 1975). The height of the wheat plant ranges from 30 – 150 cm and is determined by the genotype and the growing conditions. Differences in plant height are mostly attributable to variation in internode length rather than internode number (Austin & Jones 1975).


4.5.4 Tiller development


The first tillers to emerge are those formed between the axils of the coleoptile and the first true leaf. In general, three phyllochrons (the interval between two successive leaves) separate the emergence of a leaf and its subtended tiller (phyllochron is the interval between two successive leaves; (phyllochron is the interval between two successive leaves; Kirby & Appleyard 1983).

In winter wheat, a few tillers may form in autumn or winter if conditions are mild. A rapid increase in tiller number occurs with warmer spring temperatures. The main shoot and early formed tillers complete development and form grains in winter or spring wheat (Kirby & Appleyard 1983). Later formed tillers usually senesce prematurely.


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