Dehydration of Onion Cells: A Comparison of Freezing vs. Desiccation and Living vs. Dead Cells

Measurements were made of the amount of liquid water present in the epidermal cells of onion at various degrees of dehydration caused by slow extracellular freezing and by desiccation. This was achieved by using a pulsed NMR spectrometer during freezing stress and by weighing the epidermal pieces during desiccation. Measurements were made on the extent of cell survival by direct microscopic observation (plasmolysis and protoplasmic streaming). Onion epidermal cells (Allium cepa L. cv. Downing Yellow Globe) were found to survive freezing temperatures as low as –20°C and an equivalent desiccation stress. This equivalence opposes the reports by others on Hordeum vulgare and on Solanum sp. of greater injury by freezing than by an equivalent dehydration due to desiccation. The discrepancy ‐has been explained in terms of the limitations of the conductivity method used by those authors to evaluate the injury. The freezing and desiccation curves correspond to the equation: L t=L0Δtm/t+Lu where Lt and L0 are the amounts of liquid water at temperature t and O°C respectively. Δtm is the freezing point depression of the cell sap and Lu is the amount of liquid water which does not freeze. These results demonstrate that the dehydration of onion cells during both freezing and desiccation duplicates the dehydration of ordinary aqueous solutions. This was equally true for living and dead cells, and suggests that the negative turgor invoked by others is not significantly involved in the dehydration of living Allium cepa epidermis cells. An explanation is proposed for these contradictory results.