Most Acacia species have a seed coat which is impervious to water. This causes seed dormancy so that germination may extend over months or years. In order to propagate acacias efficiently in the nursery it therefore is necessary to apply some form of presowing treatment to ensure not only a high final germination percentage but rapid and uniform germination after sowing.
The cause and nature of the seed coat impermeability are not fully understood, but it has been found that under natural conditions and after most artificial treatments the first site at which water penetration occurs is the strophiole. This is the weakest and least reinforced area of the seed coat and is seen as a small raised area close to the hilum but on the side opposite the micropyle (Cavanagh 1980a).
Numerous techniques have been used to render Acacia seeds permeable. In Australia treatment of the seeds with boiling or hot water was practised last century (Brown 1881) and elsewhere sulphuric acid was commonly used (Ford - Robertson 1948). Germination usually proceeds rapidly once the seed coat is made permeable. In all techniques used there is danger of injury to the embryo if the treatment becomes too severe. The most successful treatments fall essentially into two major classes Cavanagh (1980a):
Dry: use of dry heat, microwave energy, impaction, percussion, and manual or mechanical scarification.
Soaking Acacia seed in water below about 40°C is effective in promoting germination only in those seeds which already have a permeable seed coat (soft seeds). It is common to find a small fraction (<10%) of soft seeds in acacia seed lots but some species have a high proportion of soft seeds if they are harvested before the pods have dried out. However, most seeds will develop impermeability as they mature on the tree or in subsequent storage (e.g. A. senegal, Kaul and Manohar 1966).
A frequently-used technique is to immerse the seeds in 4–10 times their volume of boiling water (100°C), remove the heat source, and allow the seeds to soak in the gradually cooling water for 12–24 h. This method is widely applied but can give erratic results. The volume or weight ratio of seeds to water is critical, and optimum soaking time may vary between species. The rate of cooling is greatly influenced by the scale of the operation and the nature of the container used, so precise control is difficult to achieve. The method appears to give better results for Australian acacias than for the majority of African species (Delwaulle 1979). Pretreatment with concentrated sulphuric acid is frequently more effective for African acacias.
Boiling acacia seeds in water removes the cuticle and sometimes part of the pallisade layers of the seedcoat and can effectively break dormancy. The range of response to boiling is illustrated by the following examples: A. sieberiana (an African acacia) given 60% germination after boiling for 1 h (Larsen 1964). Boiling A. melanoxylon for 3 minutes is usually effective but can be very detrimental if the seedlot is of poor quality, i.e. lower than average viability (Zwaan 1978); A. acuminata and A. pycnantha will withstand 100°C for a maximum for 5 seconds (Harding 1940) and A. terminalis for a maximum of 30 seconds (Clemens et al. 1977). For many acacias immersion in boiling water for more than 30 seconds is detrimental.
Boiling usually promotes germination to a critical point beyond which there is a decline in the final germination percentage. Soaking in water within the range 60–90°C is often as effective as soaking at 100°C but there is less chance of damage at the lower temperatures. For several Australian acacias soaking the seed at 80°C for 1–10 minutes is effective (Clemens et al. 1977).
Advantages: When suitable prescriptions have been determined hot and boiling water methods are reasonably effective for many species, little or no special equipment or chemicals are required, the cost is negligible and with minor precautions the technique is safe for the operator.
Disadvantages: The technique may be unsatisfactory because wet, swollen seeds are difficult to handle and cause problems in mechanical seeders or in pelleting.
Soaking in concentrated sulphuric acid is the most common method of treating acacia seeds. The effect on the seed coat is similar to that of prolonged boiling and the seed coat is left dull and shallowly pitted. It is a more effective method than boiling water for many African acacias. This scarification technique requires a supply of commercial grade sulphuric acid (95%, 36N), acid resistant containers, wire containers and screens, and an abundant supply of water for rinsing the seeds after treatment.
Safety precautions require strict attention as concentrated sulphuric acid is dangerous to people and materials. It should be handled at all times with great care. When mixed with water it produces a violent exothermic reaction. Water should never be added to the acid or it may boil explosively. If a dilute mixture is required the acid must be allowed to trickle slowly into stirred water. All operators should wear acid-resistant protective clothing, gloves and eye protection. A solution of sodium or potassium bicarbonate should be kept readily available as an antidote to accidental skin contact with the acid (FAO 1974a).
A procedure for acid scarification, based on Bonner et al. (1974), is as follows:
Allow seeds to come to air temperature and make sure the seed surface is dry.
Completely immerse the seeds in undiluted acid (1200 ml per kg of seed) for the required period. The treatment is best carried out at 20°–27°C; lower temperatures require longer soaking times.
Remove the seeds from the acid, immediately wash them thoroughly in cool running water for 5–10 minutes to remove all traces of acid. Use a large amount of water at the start of the washing and stir carefully.
Spread the seeds in a thin layer for surface drying, unless wet sowing is preferred.
The optimum soaking period will depend on the species. It is usually in the range 20–60 minutes but soaking for 120 minutes has given very good results for A. caven, A. farnesiana, A. nilotica and A. tortilis in tests at the CSIRO Seed Centre, Australia. A less severe acid treatment can be applied by pouring acid over a pile of seeds (approx. 1 litre of acid per 35 kg of seed), distributing the acid throughout the seeds by turning with a shovel, and then washing the seeds thoroughly (Bonner et al. 1974).
Advantages: Acid treatment is effective for many species and can be carried out with simple equipment and at a low cost for materials as the acid is reusable. The treated seeds are dry and unswollen and suitable for mechanical sowing or short-term storage.
Disadvantages: The prescription for the treatment must be carefully defined and a pilot test is desirable. The temperature of the acid during the treatment must be carefully controlled. The greatest disadvantage is the risk to personnel in using the acid and the need to rigidly enforce safety precautions.
Ethanol, methanol and acetone have been used on a laboratory scale to treat acacia seed but the results have been variable (Ford-Robertson 1948, Cavanagh 1980b) and as they have little advantage over hot water tretment it seems unlikely that they will be used extensively.
Scarification aims to abrade the seed coat so as to permit water absorption. Physical scarification may be performed by hand, especially for laboratory purposes, or by the use of specially designed machines.
Piercing, chipping, nicking or filing the testa of individual seeds with a mounted needle, knife, handfile or abrasive paper is a technique especially suitable for small quantities of seed. Scarification on the shoulder of the seed one quarter of the way round the circumference from the micropyle (ISTA 1981) or the removal of one square millimetre of seed coat at the cotyledon end (Magini 1962) is sufficient. This is usually considered to be the most reliable method of pretreatment and the percentage germination following this operation probably approximates closely to the germination capacity (Moffett 1952). Hand scarification is recommended as a method for pre-treating acacia seeds before germination tests (ISTA 1981). However, instances have been recorded when chipping the seed coat has proved detrimental to germination (Clemens et al. 1977).
A number of commercial machines are available which operate on the principle of tumbling or blowing the seeds against an abrasive surface in a drum or mixer. The machines may be portable hand-operated models or larger less mobile machines driven by an electric motor. Various models, e.g. the Forsberg scarifier, are advertised by seed equipment companies.
Advantages: Mechanical scarification requires no temperature control, it is safe for the operator, the seeds remain dry and so are suitable for machine sowing, and with appropriate machines large quantities of seed can be processed.
Disadvantages: The larger machines are expensive; machine-scarified seeds frequently sustain considerable damage and their storage life is reduced.
Acacia seeds have been pretreated by applying dry heat, frequently by placing the seed in an oven maintained at the desired temperature (e.g. Aveyard 1968). Dry heat has generally been less effective than hot water or scarification pretreatments but work with agricultural legumes (Mott et al. 1982) suggests that germination of seed briefly exposed to very high temperatures (e.g. 155°C for 15–20 seconds) may be improved.
This is a recently-developed technique in which the seeds are heated by microwave energy. Large quantities of seeds can be treated with exposure times from 20 seconds to 4 minutes. The technique has an effect similar to that of boiling water but the seeds remain dry. It has the disadvantage of requiring special equipment. This technique is being further developed (Tran 1979; Tran and Cavanagh 1979).
Table 3 Recommended pre-sowing treatments for some dry-zone acacias
|A. albida||Fresh seeds with soft seed coats require no pretreatment or can be soaked in cold water for 24 hours before sowing. More mature seed can be soaked in conc. sulphuric acid for 20–60 minutes (2)||West (1950), Wickens (1969), Giffard 1971, FAO (1974a, 1975), Elamin (1975) and Turnbull (unpublished).|
|A. aneura||Dip seeds in boiling water (100°C) for 5–30 seconds or pour boiling water on the seeds and allow them to remain in the water until it cools to room temperature.||Preece (1971b), Hall et al. (1979), Turnbull (unpublished)|
|A. cambagel||Fresh seed frequently does not require pretreatment, more mature seed can be treated as A. aneura.||Hall et al. (1975)|
|A. caven||Soak in conc. sulphuric acid for 120 minutes||Galleguillos (personal communication), Turnbull (unpublished)|
|A. farnesiana||Soak in conc. sulphuric acid 40–120 minutes Optimum length of soaking period will vary with individual seedlots||Kumar and Purkayasha (1972) Turnbull (unpublished)|
|A. holosericea||Treat as for A. aneura||Delwaulle (1979), Turnbull (unpublished)|
|A. nilotica||Fresh seeds with soft seed coats require no pretreatment. Immersion in boiling water as for A. aneura is reasonably effective but where hardseededness is well developed soaking in conc. sulphuric acid for 60–120 minutes is recommended.||FAO (1974a), NAS (1980) Turnbull (unpublished)|
|A. pruinocarpa||Treat as for A. aneura||Hall et al. (1981b)|
|A. senegal||Fresh seeds with soft seed coats require no pretreatment. More mature seed can be immersed in conc. sulphuric acid for 3–15 minutes, or dipped in boiling water for 5 seconds.||Kaul and Manohar (1966), Cheema and Qadir (1973), Giffard (1975), NAS (1980), Turnbull (unpublished)|
|A. tortilis||Soak in conc. sulphuric acid 20–120 minutes. Optimum soaking period will vary with different seedlots. Immersion in boiling water as for A. aneura may also be effective.||Karschon (1975), Carr (1976), Pathak et al. (1980), NAS (1980), Turnbull (unpublished)|
|A. victoriae||Treat as for A. aneura||Hall et al. (1981a)|
Footnote (1) Hand scarification is the most effective pretreatment for all species.
(2) There are conflicting prescriptions for presowing treatment of A. albida in the literature. It is suggested that this has arisen because the state of development of the seedcoat strongly influences the effectiveness of a particular treatment in this species.
Within any acacia seedlot not all the seeds are equally ‘hard’. The proportion of hard seed in a sample depends on the environmental conditions during the growth of the plant, the degree of maturation of the seeds when collected and the length of the storage period. Mature seeds and those that have been stored for several months or years usually have less than 10% of seeds that will germinate rapidly without pretreatment. A study of A. aneura seed of several provenances and ages found the percentage of ‘soft’ seeds to be within the range 0.7–4% (Preece 1971b). The degree of seed polymorphism may influence the relative success of pretreatment techniques. Interaction between the treatment and seed coat sensitivity results in differential germination, the degree of which depends on the severity of the treatment. Different responses may be obtained when the seeds are divided into different size classes (Moffett 1952; Pathak et al. 1980). Variation in hardseededness will occur within a sample, between samples of the same species and between species. This makes it impossible to prescribe a standard technique which will be optimum for all seeds. When treating large or valuable seed lots with the more severe treatments such as mechanical and sulphuric acid scarification, or boiling water it is recommended that a preliminary test be made to determine the optimum prescription. This can be done by subjecting small samples to a range of conditions, e.g. different periods of immersion in acid, and then germinating the seeds.
Seeds dried after boiling water treatment can be safely stored for at least short periods (e.g. 1 yr) with no loss in viability (Osborn and Osborn 1931, Harding 1940, Moffett 1952, Isikawa 1965, Sherry 1971). This also applies to acid-scarified seeds (Cheema and Qadir 1973), microwave treatment (Cavanagh 1980a) and to mechanically scarified seeds providing they remain undamaged by the pretreatment. Storage may be beneficial for good quality seed but can be detrimental if the seed is of poor quality (Zwaan 1978). There is evidence (Goo et al. 1979) that pretreatment may be detrimental to seeds stored for several years. The seeds will probably store for longer periods without deterioration if kept at low temperatures.