Seed longevity

Many tropical trees and shrubs have short-lived seeds in soil, as indicated by the soil seed bank data. Commercial seed companies and researchers use various types of storage conditions to maintain seed viability, with the majority using cold rooms (CRs). Seed storage properties vary with species, provenance, seed processing techniques and many other factors. Ashwath et al. (2003) carried out an extensive study to test germination, storage and responses to smoke treatment of >800 Central Queensland species/provenances. Seeds were collected from the field, tested for germination soon after collection and representative samples of these seed lots (102 species representing 83 genera of the Central Queensland flora) were stored in cloth/paper bags contained in metal tins in a: (i) garden shed (GS) (8-60°C, 48 species); (ii) air-conditioned (AC) laboratory (8-29°C, 51 species); (iii) CR (8-10°C, 800 species/provenances); and (iv) freezer (F) (-18°C, 53 species).

Germination tests were conducted following storage for 12—24 months, with the majority of species tested around 18 months. The CR germination data were compared with initial germination data to determine which species showed an increase in germination, associated with the breaking of dormancy. The germination data of GS, AC and F were compared with those of CR to reveal the effects of different storage conditions on subsequent germination. Selected seed lots (84) were also treated with smoke water/aerosol (SW) following storage, and the effects of SW treatment were determined by comparing the SW data with those of CR. The resulting ratios were grouped into four classes: (i) treatments showing a substantial improvement in germination compared with initial germination (>125%); (ii) those having little effect; (iii) those showing a substantial reduction (<75%); and (iv) those completely inhibiting germination while their counterparts germinated successfully in other treatments.

Among the 92 species tested for dormancy, 31 species showed improved germination, 19 species showed little change and 35 species showed reduced germination (Fig. 43.2a). Seven species showed complete loss of viability during storage. Overall, >50% of the tested species either improved or remained unchanged in their germination when stored in a CR.

Garden shed Air-conditioned

Freezer

Fig. 43.2. The number of central Queensland species showing improved, unaffected, reduced or inhibited germination: (a) when stored in a cold room (CR) for 12-24 months, as compared with initial germination at collection (92 species); and (b) when stored in a garden shed (GS), air-conditioned room or freezer, as compared with germination following storage in a CR (i.e. >125% indicates a proportionate increase in germination percentage of >25% compared with that of the comparative germination result).

Of the 53 species stored in different conditions (Fig. 43.2b), 20 species showed improved germination in all three storage conditions compared with CR storage and the overall responses were similar among the three storage conditions. The seeds were air-dried and contained <10% moisture and hence they were not adversely affected by storage in a freezer. Storage in warm conditions, such as in a GS, may be essential for after-ripening some species, such as grasses (Fesuk and Ashwath, 2005), whereas storage in a freezer may be needed to prevent ageing of certain other species. For short-term storage (up to 2 years), all storage conditions appear to yield similar results when we consider a wide range of tropical native species. Testing of stored seeds at 5 and 10 years will be required to assess the best long-term storage conditions.

The responses of 84 species to smoke treatment (after being stored in a CR for 12—24 months) were also assessed. Smoke treatment improved germination in 37 species, but reduced in 29 species, and there was little change in 16 species. Two species showed no germination. Smoke improved germination in >40% of the tested species and it either reduced or inhibited germination in about one-third of the species. The response to smoke treatment often mimicked the effects of storage (e.g. freezer or GS; Fig. 43.2) suggesting that the storage conditions may also induce similar germination responses as smoke treatments.

The responses of tropical native species to storage were variable and no single storage condition is optimal for all or the majority of tropical native species, at least for 2 years of storage. Therefore, it is necessary to undertake species-specific stor age and/or seed dormancy studies to maximize either storability or germinability of native plant seeds.

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