Low temperature induced ultrastructural alterations in tomato (Lycopersicon esculentum Mill.) shoot apex cells

- Following osmoprotection tunica cells showed a similar structure to control cells.
- Tunica cells displayed regular ultrastructure pursuing liquid nitrogen exposure.
- After 3 weeks of post-thawing tunica cells displayed a damaged epidermal layer.
- Ultrastructure after acclimatization brings added value to the existing studies.

The ultrastructural changes in tomato shoot apices and the severity of cumulative effects related to the successive stages of a cryopreservation protocol and ex vitro acclimatization of recovered plants have been assessed using imaging methods. Even though after osmoprotection, features like a dense cytoplasm, numerous plastids with starch grains in tunica (L1, L2) cells or the presence of Golgi complex in corpus (L3) cells suggest an intact cell structure, dehydration in the vitrification solution left traces especially in some basal corpus (L3) cells. Shortly after thawing following liquid nitrogen exposure tunica (L1, L2) cells displayed a regular structure with dense cytoplasm, while in basal corpus (L3) cells high vacuolation and signs of plasmolysis were observed. The findings presented in this study have shown that the injuries occurred at cellular and subcellular level cannot be attributed with certainty to a single stress element but rather they may act cumulatively and become evident some time later. As a consequence after 3 weeks of post-thawing on recovery culture medium tunica (L1, L2) and corpus (L3) cells exhibited cellular heterogeneity. To our knowledge this is the first report on ultrastructural observations on shoot apices of acclimatized plants regenerated from cryostored plant material.