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Do ‘Intermediate’ Seeds Exist? Towards a Mechanistic Understanding of Cold Stress

Interaction between drying and cooling is a feature of Carica papaya (Caricaceae) seeds

This project is one of a series in the theme the 'Science of Seed Ageing'.

Orthodox seeds are fully desiccation tolerant and can be stored at –20°C and lower, recalcitrant seeds are desiccation sensitive and are thus held at ambient temperatures, whilst intermediate seeds have limited desiccation tolerance and a sensitivity to dry storage at 0°C and –20°C. The occurrence of this third seed storage group has profound implications for the long-term safety of gene bank collections. But do ‘intermediate’ seeds really exist? Contrary to the definition of intermediate seeds, evidence is growing that the optimum sub-zero temperature for seed storage is species-specific and that cold stress can be rapid (i.e. shock). For example, seeds of some orchids are sensitive to –30°C and –50°C within 1 month of storage and some palm seeds are intolerant of –20°C within 1 week. We have now observed that cold shock in dry and wet seeds of Carica papaya can impact in days, but the cause of such responses is unclear.  See Annex 1 for data and references that frame, or emanate from, this project.

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