Seed recalcitrance - current perspectives

The phenomenon of non-orthodox seed behaviour is presented in the framework of the probability of being much more common than might be expected, considering that scientific knowledge about seeds is largely drawn from what has been established for a mere handful of cultivated crop species. The scattering of species producing recalcitrant seeds across most angiosperm families (and among the gymnosperms) appears to have no taxonomic basis, although recalcitrance might be the ancestral seed condition. Recalcitrance is considered not as deviant, but rather as one extreme of a continuum of seed behaviour based on the response to dehydration, the other extreme being manifested by those orthodox seeds that are able to tolerate almost total desiccation. Post-harvest seed responses to dehydration are presented in the context of developmental events that characterise pre-shedding development. Those that acquire desiccation tolerance and will survive lengthy periods in the desiccated state do so as a result of the full development and interaction of a suite of mechanisms enabling this competence. Highly recalcitrant seeds which consistently remain metabolically active, are considered not to possess, or express, most or all of these, while in between the extremes of recalcitrance and orthodoxy, seeds that might be considered as non-orthodox (but also not recalcitrant) will manifest these mechanisms and their interactions variably thus accounting for a continuum of seed behaviour in the context of relative desiccation tolerance. Emphasis is laid on the fact that the degree of dehydration tolerated by seed tissues is a function of the rate of dehydration: the more rapidly water can be lost, the less time is available for metabolism-linked damage, and the lower the water content that can be attained without lethal injury. However, recalcitrant seeds cannot lose structure-associated (non-freezable) water without sustaining lethal injury, which is considered as desiccation damage sensu stricto. Finally, the limited potential for storage of recalcitrant seeds is discussed, and the alternative biotechnological approach of germplasm conservation by means of cryostorage of embryonic axes, is presented.