Acquisition of cryotolerance in maize embryos during seed development.

A desiccation-based cryopreservation protocol was employed to study the development of cryotolerance and desiccation tolerance in maize embryos from 23 to 50 days after pollination (DAP). Tolerances were acquired gradually and concomitantly. Maize embryos had low desiccation tolerance at 23 DAP when assessed by survival (embryo elongation) and emergence (root and shoot growth) after dehydration. Desiccation tolerance increased progressively, reached its maximum at 38 DAP, and remained constant afterwards. Cryotolerance, assessed by survival and emergence of post-thaw embryos, however, was nil until 26 DAP. Embryos at 29 DAP withstood cryoexposure within a very narrow moisture range only. Throughout development cryotolerance increased gradually, reached a maximum at 44 DAP and then remained at this level. The correlation between moisture content and cryopreservation success was notably influenced by the maize embryo's development stage. As seeds developed, the moisture content allowing 90% dehydrated embryos to survive and to emerge decreased, while the upper moisture content allowing 50% post-thaw embryos to survive and to emerge increased. Moisture contents of c. 14% allowed no less than 50% post-thaw embryos to emerge at the later development stages (e.g. c. 44 DAP); but no embryos within the same moisture range survived cryoexposure at 29 DAP, although they could withstand desiccation to this moisture level without impairment of survival and emergence. The relationship between cryotolerance and desiccation tolerance during maize seed development is discussed.