A review of recalcitrant seed physiology in relation to desiccation-tolerance mechanisms

A suite of mechanisms or processes that together have been implicated in th e acquisition and maintenance of desiccation tolerance in orthodox seeds is discussed in the context of the behaviour of desiccation-sensitive seeds, and where appropriate, parallels are drawn with the situation in vegetative plant tissues that tolerate dehydration. Factors included are: physical ch aracteristics of cells and intracellular constituents; insoluble reserve ac cumulation; intracellular de-differentiation; metabolic 'switching off'; pr esence, and efficient operation, of antioxidant systems; accumulation of pu tatively protective substances including LEAs, sucrose and other oligosacch arides, as well as amphipathic molecules; the presence and role of oleosins ; and the presence and operation of repair systems during rehydration. The variable response to dehydration shown by desiccation-sensitive seeds is co nsidered in terms of the absence or incomplete expression of this suite of mechanisms or processes. Three categories of damage are envisaged: (i) reduction in cell volume whic h can lead to mechanical damage; (ii) aqueous-based degradative processes, probably consequent upon deranged metabolism at intermediate water contents . This is termed 'metabolism-induced damage' and its extent will depend upo n the metabolic rate and the rate of dehydration; and (iii) the removal of water intimately associated with macromolecular surfaces leading to denatur ation: this is referred to as desiccation damage sensu stricto. The effects of drying rate and the maturity status of seeds are considered in relation to the responses to dehydration, leading to the conclusion that the concep t of critical water contents on a species basis is inappropriate. Viewing s eed postharvest physiology in terms of a continuum of behaviour is consider ed to be more realistic than attempting precise categorization. Rapid dehydration of excised embryonic axes (or other explants) from desicc ation-sensitive seeds permits retention of viability (in the short term) to water contents approaching the level of non-freezable water. This opens up the possibility of long-term conservation, by cryopreservation techniques, of the genetic resources of species producing non-orthodox seeds.