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item Wesley-smith, James
item Walters, Christina
item Berjak, Patricia
item Pammenter, Norman

Submitted to: Cryo-Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2003
Publication Date: 12/1/2004
Citation: Wesley-Smith, J., C. Walters, P. Berjak and N.W. Pammenter. 2004. Non-equilibrium cooling of Poncirus trifoliate (L.) embryonic axes at various water contents. CryoLetters 25:121-128.

Interpretive Summary: The rate at which cells are exposed to liquid nitrogen temperatures determines whether lethal ice crystals form. In very small systems (1 to a few cells), sufficiently fast cooling is relatively easy to achieve. For most plant germplasm, propagule size is about 500,000 cells and this geometry limits how quickly heat can be dissipated. This paper examines how mechanisms of heat transfer (convection and conduction) and variables affecting thermal mass affect cooling rate of a desiccation-sensitive embryo. By slightly adjusting water content and exploiting convective cooling, cooling rates of hundreds of degrees (C) per sec can be achieved.

Technical Abstract: The present study investigated the rate of temperature change within axes of Poncirus trifoliata during cooling and warming by various methods. Cooling rates ranged between 0.1 and 1700C/s-1, and warming rates of 1.25 and 600C/s-1 were measured when axes were warmed at room temperature air or in water at 40C, respectively. Partial drying increased the cooling rate within axes in direct contact with the cryogen, but did not affect the cooling or warming rates within axes enclosed in a double layer of lightweight Al foil. The procedures described illustrate the orders of magnitude that separate extremes of the range of cooling or warming rates attained using methods commonly employed in cryopreservation studies. Quantifying these rates allows the relationship between cooling rate, water content and survival of hydrated embryonic axes to be explored.