Discussion

Recalcitrant seeds are metabolically active (Farrant et al., 1997), initiate germination-associated developmental events (Berjak et al., 1984; Farrant et al., 1986, 1989) and often germinate in storage. Successful extension of medium-term storage would require a reduction in the rate of this metabolism and development. One possible approach is to reduce the water content to levels inhibiting germination (King and Roberts, 1980; Hong and Ellis, 1996). However, previous work has indicated that this is not a successful approach (Corbineau and Côme, 1986, 1988; Drew et al., 2000). The data presented in this chapter confirm this. In every case, partial drying before storage led to a decline in storage lifespan, or a loss of vigour in the case of T. dregeana, relative to seeds stored at their initial water contents. The effect of temperature on the seeds was complex: partial drying before storage induced a chilling sensitivity in P. henkelii ; S. cumini seeds survived marginally better at 25°C; but partially dried seeds of other species survived better at 16°C than at 25°C (Table 8.2). Although the greatest loss of viability occurred in the species that had been most severely dehydrated (i.e. T. emetica), the species that was least dehydrated before storage (i.e. S. cumini ) showed the next greatest loss of viability. Generally, there were no apparent relationships between the extent to which axis water content was reduced by pre-storage partial drying, the rate of germination of fresh seeds and the performance in storage. Seeds were not stored at their initial water content until viability was lost, so the suggested relationship between storage lifespan and germination rate (Berjak et al., 1989; Pammenter et al., 1994) could not be assessed.

When the effect of drying on immediate post-dehydration germination was studied, it was observed that mild dehydration actually enhanced the rate of germination (Table 8.3). This observation has been reported before (Fu et al., 1994; Pammenter et al., 1998; Kioko et al., 1999; Rodriguez et al., 2000), although we suspect that it is an observation more often made than reported. Thus, seeds that are partially dried before storage could lose viability through one of the two-related causes: overdrying could lead to the accumulation of desiccation damage; or mild drying could stimulate germination events, which would likely lead to loss of viability in the absence of exogenous water.

The underlying processes leading to the enhancement of germination by mild dehydration are not yet understood. However, this phenomenon may have ecological implications. Partial dehydration is a risk to which most recalcitrant seeds on the soil surface are likely to be exposed. If this enhances germination, it could lead to the extending root accessing water deeper in the soil, thereby increasing the probability of survival of seeds exposed to a dry atmosphere at the soil surface. Many recalcitrant seeds are large (Daws et al., 2005), providing them with the resources necessary for rapid germination.

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