The expense and effort required to assemble a comprehensive range of provenances of widely distributed species makes it essential that the seed be safely and effectively stored until it is required for distribution and sowing.
In general, acacias, by virtue of their hard seed coat which minimises moisture exchange and the loss of stored reserves through respiration, retain their viability well for many years and present few storage problems. Seeds of some species can remain viable in relatively uncontrolled laboratory storage for at least 57 years (Holmes and Buszewicz 1958) and 68 years (Ewart 1908), and even more remarkable, for at least 50 years in the field (Moffett 1952, Farrell and Ashton 1978).
Data for the longevity of several acacia species tested by the Division of Forest Research, CSIRO, Canberra are given in Table 2. The seeds were stored at room temperature (20–25°C) in closed containers. While there was some loss in germination capacity with time, most seedlots had retained their viability well and even the oldest lots (A. leptopetala and A. victoriae, 18 yr) showed excellent germination energy. A similar experience has been reported for Israel, where seed in sealed containers retains high germinability for at least 7–8 years in the existing store which is protected from high temperatures (FAO 1980).
While a general recommendation for acacia seeds is to store them dry, in moisture-proof containers in a cool secure location, some species require special treatment. A. harpophylla (brigalow) is a good example of the latter. Given routine treatment, brigalow loses all viability within one year of harvest (Johnson 1964), while seeds held at -20°C retain viability and remain soft for up to five years (Coaldrake 1971). If the seeds are collected before they have reached maturity, as described in 4.23, the evidence suggests that cool storage (5°C) is necessary to arrest the on-set of seed coat dormancy (Isikawa 1960).
There appears to be little information on the relations between maturity, seed characteristics, conditions of storage, and retention of viability in acacia seeds.
Table 2 Longevity of acacia seedlots in storage
Before storage, acacia seed should be fumigated or dusted with insecticidal powder to kill insect pests. This is extremely important in bruchid infested seedlots, the viability of which may be quickly lost through ongoing attacks by successive generations of the weevil.
Carbon disulphide is frequently used as a fumigant for this purpose, and must be applied in a sealed container with provision for safe injection and release of the gas so as to protect peronnel. The chemical is used at a rate of 240 g per cubic metre of air space. The length of exposure required depends on temperature: 3.5 h at 10°C to 0.7 h at 30°C. Afterwards, the seed should be spread out and well aerated (e.g. for about 3 h) to remove the gas before storage. Larvae living within the hard seed coat of acacias may survive the initial treatment, so careful observation of the seedlots and repeat fumigation may be required.
The dusting of the seed coats with insecticidal powder is also in frequent use to arrest damage by seed insects. A commercial powder known as ‘Phostoxin’ is successfully used for this purpose in Israel (FAO 1980), while commercial preparations of insecticides based on malathion, pyrethrins and benzene hexachloride are in use in Australia.
The ‘skin-packaging’ of tree seed and their preservation by the carbon dioxide exchange method (CEM) is under evaluation by the Division of Forest Research, CSIRO, Canberra and is showing encouraging early results (J.W. Turnbull unpublished). In this technique, seed with an appropriate volume of CO2 gas is sealed in a bag made of plastic laminated film of low gas-permeability and (Fig. 26). Head-space is reduced by the seed absorbing the CO2 gas which may give a tightly sealed, relatively inflexible package. The seed is thus locked away from damaging agencies. The technique is cheap, easily applied and is in regular use for the safe preservation and transport of grains and foods (Mitsuda et al. 1973). It does, however, require more testing before adoption for routine storage of acacia seed.
For long-term storage acacia seed should be kept in airtight containers. Glass or plastic bottles with screw tops and tin-plate containers with tight-fitting lids are suitable (Fig. 27). They should be as full as possible to reduce the amount of included air. The seed may also be placed in sealed plastic bags within the containers. A desiccant, such as silica gel, placed in the container may be a useful precaution in humid conditions.
Meticulous comprehensive records of each seedlot are essential. The seed store record system which was used successfully by the CSIRO Division of Forest Research, Canberra for many years prior to the recent adoption of a computerized system (Wolf and Turnbull 1982) is described in Appendix 4.
|Fig. 26||Skin-packaging and preservation of seed by carbon dioxide exchange, Division of Forest Research, CSIRO, Canberra.|
|Fig. 27||Airtight metal containers used for storing seed, Division of Forest Research, CSIRO, Canberra.|