Cryopreservation of Seeds and Seed Embryos in Orthodox, Intermediate, and Recalcitrant Seeded Species

Seeds are one of the preferred and most used sources of germplasm for the ex situ preservation of plant genetic resources. They are generally stored dry at -20°C in seed banks following international standards. However, some seeds do not tolerate drying and/or storage at -20°C, or present short lifespans at these conditions. For them, cryopreservation is indicated for long-term preservation. When seeds tolerate desiccation (i.e., orthodox seeds), they can be dried at about 32% ± 3% relative humidity at 18°C and stored in the vapour phase of liquid nitrogen. This is the method followed in the Millennium Seed Bank of the Royal Botanic Garden`s, Kew for wild species with short life spans in the standard conditions of seeds banks. When seeds do not tolerate desiccation (i.e., recalcitrant seeds) or their tolerance to desiccation and/or -20°C storage is limited (i.e., intermediate seeds), drying and cooling procedures must be adjusted, and often, antioxidants and/or cryoprotection are required to avoid physical and metabolic lesions. Some methods are detailed for diverse species of temperate and tropical origin.