Peter G. Gosling

Loss and induction of conditional dormancy in seeds of Sitka spruce maintained moist at different temperatures

Pretreatment of moist seeds at certain temperatures can reduce seed dormancy, but such observations represent the net effect of pretreatment. Seeds of Sitka spruce ( Picea sitchensis [Bong.] Carr.) were raised to 30% moisture content and pretreated at five different temperatures between 5 and 30°C for up to 24 weeks (168 d). Subsequent ability to germinate at 10°C and viability were then determined in order to investigate the effects of pretreatment on seed dormancy and survival. There was a curvilinear, negative semi-logarithmic relationship between seed longevity and pretreatment temperature, such that Q 10 for loss in viability increased from 2.6 between 10 and 20°C to 2.8 between 20 and 30°C. Simple multiplicative models combining cumulative normal frequency distributions for each of loss in viability and loss in dormancy were able to describe the changes in ability to germinate at 10°C, after pretreatment at 5, 10 and 30°C. However, in order to quantify the changes in ability to germinate observed at 10°C after pretreatment at 15°C, it was necessary also to invoke a model of dormancy reimposition, while for the results at 20°C it was necessary to postulate both dormancy reimposition and the further loss of this reimposed dormancy. It is concluded that moist seeds of Sitka spruce held at 15 and 20°C cycle between the dormant and non-dormant condition.

Reimposition of conditional dormancy during air-dry storage of prechilled Sitka spruce seeds

Prechilling seeds of Sitka spruce ( Picea sitchensis [Bong.] Carr.) at 4°C with 30% moisture content for 12–14 weeks (84–98 d) removed conditional dormancy (i.e. they were then able to germinate at 10°C). The non-dormant status was preserved after redrying to 6% moisture content. However, conditional dormancy was gradually reimposed during subsequent air-dry storage at 4°C and 6% seed moisture content in all five seed lots tested. Further investigations with one seed lot showed that reimposition was reversed by a second prechill treatment, but was reimposed again during subsequent air-dry storage. The trend of dormancy reimposition within seed lots over time was quantified by negative exponential relations between ability to germinate at 10°C and duration of air-dry storage. The progress of dormancy reimposition was influenced by seed storage moisture content and was most rapid at 4–10%. At higher moisture contents (15 and 20%) the rate of the reimposition of conditional dormancy was much reduced, while at moisture contents of 25 and 30% further loss in dormancy occurred. Thus it is clear that dormancy reimposition occurred during storage at low water potential rather than solely during desiccation from high to low water potential.

Dormancy in Sitka Spruce Seeds

Sitka spruce (Picea sitchensis [Bong.] Carr.) seeds exhibit conditional (relative) dormancy, whereby conditionally-dormant seeds will not germinate in tests at 10°C but will germinate at 20°C. Such dormancy can be removed in Sitka spruce seeds by using a moist cold treatment (prechill) at about 3°C. The conditional dormancy is removed progressively by extending the prechill period. Following a suitable prechill period, Sitka spruce seeds acquire the ability to germinate in tests at 10°C and also germinate faster at the optimal test temperature of 20°C. Variation in these two variables among seed samples are correlated and can be used to determine the level of conditional dormancy in Sitka spruce seedlots. The effects of seed development on both the ability of Sitka spruce seeds to germinate and their level of dormancy was also studied, as was the effect of a commercial seed extraction procedure on dormancy. Implications for the commercial collection of, processing of, and presowing treatments to Sitka spruce seeds are discussed.