Development of the understanding of seed recalcitrant and implications for <I>ex situ</I> conservation

Abstract

Recalcitrant seeds are shed at high water content, are desiccation sensitive and cannot be stored under conditions conventionally employed for desiccation-tolerant orthodox seeds. Recalcitrant seeds are metabolically active when shed, with high rates of respiration and a high degree of intracellular differentiation. Development grades gradually into germination, with no punctuating quiescent period, and many recalcitrant seeds will germinate if aintained at their shedding water content. Consequently, storage in the hydrated condition is strictly a short-term option only. Although recalcitrant seeds are desiccation sensitive, if excised embryonic axes are dried very rapidly they will survive to lower water contents than those dried slowly. When axes are dried they are initially subject to aqueous-based oxidative degradative processes. Axes dried slowly are exposed to these conditions for an extended period of time, accumulate considerable damage and die at high water contents. Rapidly dried axes accumulate less damage over a shorter period and so survive to lower water contents. The response of excised axes to rapid drying opens up the possibility of cryo-storage. Partial drying reduces the chances of ice formation on subsequent exposure to cryogenic temperatures, and if the drying is rapid, will reduce oxidative damage. Rapid cooling can induce the remaining intracellular water to vitrify, permitting cryo-storage without damaging ice crystal formation. However, because of the highly variable physiology of recalcitrant seeds and axes, detailed protocols have to be established on an individual species basis, guided by these concepts.Key words: Cryopreservation, damage, desiccation, recalcitrant seeds, storage