Ultrastructural Evidence for the Effects of Freezing in Embryonic Axes of Pisum sativum L. at Various Water Contents

This study investigated the effects of rapid drying (in an air stream) and rapid freezing (in sub-cooled liquid nitrogen) on the survival and ultrastructural preservation of pea embryonic axes that had been imbibed for 4 h (desiccation tolerant) and 24 h (desiccation sensitive). Maximum survival of all axes in the absence of freezing was attained. Similarly, 100% survival was obtained if freezing was preceded by rapid drying. Axes imbibed for 24 h and not dried were more sensitive to freezing than undried, 4 h imbibed axes. Ultrastructural examination showed no organellar or cytomatrical deformations in axes from any of the treatments. Some cells of the 24 h imbibed axes showed localized plasmalemma abnormalities after railed dehydration. Subsequent to freezing, irregular nuclei were observed and plasmalemma vesiculation occurred. If these axes were not dried prior to freezing, plasmalemma vesiculation became prominent, clumping of the cytoskeleton occurred and some wall abnormalities became apparent. Rapid drying probably increases intercellular solute concentrations, and sub-cooled liquid nitrogen will increase the rate of heat exchange between tissue and cryogen. A combination of rapid drying and rapid freezing may obviate, or reduce, the requirement for cryoprotectants on freezing of desiccation sensitive tissue.Copyright 1995, 1999 Academic Press