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

Abstract

A suite of mechanisms or processes that together have been implicated in the 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 characteristics of cells and intracellular constituents; insoluble reserve accumulation; intracellular de-differentiation; metabolic ‘switching off’; presence, and efficient operation, of antioxidant systems; accumulation of putatively protective substances including LEAs, sucrose and other oligosaccharides, 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 considered 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 which 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 upon the metabolic rate and the rate of dehydration; and (iii) the removal of water intimately associated with macromolecular surfaces leading to denaturation: this is referred to as desiccation damagesensu 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 concept of critical water contents on a species basis is inappropriate. Viewing seed postharvest physiology in terms of a continuum of behaviour is considered to be more realistic than attempting precise categorization.Rapid dehydration of excised embryonic axes (or other explants) from desiccation-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.

Sementes recalcitrantes: intolerantes a baixas temperaturas? Embriões recalcitrantes de Inga vera Willd. subsp. affinis (DC.) T. D. Penn. toleram temperatura sub-zero

“…Portanto, os resultados apresentados reforçam a hipótese da existência de níveis de recalcitrância (Berjak & Pammenter 1994, Pammenter & Berjak 1999, Pammenter et al 2003, demonstrando a diversidade de respostas das sementes enquadradas em apenas um ou dois grupos (reclacitrantes e/ou Tabela 3. Germinação de embriões de Inga vera subsp.…”

“…Esse conceito é aceito, ainda, por muitos autores, acrescentando-se a categoria das intermediárias, que também seriam intolerantes às temperaturas de congelamento e, ainda, intolerantes à secagem até 10% de água (Ellis et al 1990, Mai-Hong et al 2006, Usberti et al 2006. Por outro lado, há autores que consideram a existência de níveis de recalcitrância (Berjak & Pammenter 1994, Pammenter & Berjak 1999, Pammenter et al 2003, demonstrando a diversidade de respostas das sementes enquadradas em apenas um ou dois grupos (recalcitrantes e/ou intermediárias). Essas formas de classificação ou agrupamentos são importantes, por exemplo, para que se definam estratégias e técnicas diferenciadas de conservação de germoplasma.…”

Armazenabilidade de sementes de cafeeiro colhidas em diferentes estádios de maturação e submetidas a diferentes métodos de secagem

“…Na secagem rápida foi empregado um secador estacionário experimental, de pequena escala, conforme modelo descrito por Navratil e Burris (1982). O secador foi regulado para funcionar na temperatura de 35 o C e fluxo de ar de aproximadamente 20m 3 .min -1 .t -1 .…”

Drying and soaking pretreatments affect germination in pedunculate oak

“…Another potential barrier to germination is lack of sufficient water within the acorn (Pammenter and Berjak, 1997). Newly shed recalcitrant seeds are metabolically active and undergo germinative processes that require water.…”