Cryopreservation of Orthodox (Desiccation Tolerant) Seeds

Although there are several methods of ex situ plant conservation, seed banking is the most efficient for many species, particularly for ease of application and the amount of diversity conserved (Linington and Pritchard 2001). Indeed, seed storage is the main form of ex situ plant genetic resources (PGR) conservation globally, representing about 90% of all collections, the vast majority of which are crops, including cultivars (FAO 1996). More than half the world’s PGR accessions are held in medium-term or long-term storage conditions. For long-term storage, the international standards are drying at 10–25°C and 10–15% RH to 3–7% moisture content, followed by storage at about –18°C (FAO/IPGRI 1994). Whilst less than expected seed longevity at about –20°C is known for “intermediate” or Type II seeds (see Pritchard 2004), ‘orthodox’ Type I seeds can also age quicker at seed bank temperatures than predicted by the seed viability equations (for explanation see Pritchard and Dickie 2003). This was revealed by an elegant experiment in which orthodox Hordeum vulgare ssp distichium cv. Proctor seed, ageing at warm temperatures, was interrupted by transfer to –20°C, which indicated longevity parameters associated with storage at –6°C (Roberts and Ellis 1977). This observation, combined with comparisons between actual performance and extrapolation of longevity to sub-zero temperatures, suggests that the benefits of all sub-zero storage temperatures may be less than previously thought (Dickie et al. 1990; Pritchard 1995; Pritchard and Dickie 2003; Walters et al. 2004). Although the modelling of seed longevity at sub-zero temperatures is a challenge, such cooling generally enhances dry seed longevity (Dickie et al. 1990; Pritchard and Seaton 1993; Walters et al. 2004). Consequently, cryopreservation may be of particular importance for the long-term (10-100s years) storage of otherwise inherently short-lived orthodox seeds (Pritchard 1995; Pritchard and Seaton 1993; Pritchard et al. 1999b; Walters et al. 2004; Pritchard 2007).