Cytological and physiological changes in orthodox maize embryos during cryopreservation

Cytological and physiological changes during cryopreservation were studied in maize embryos at 35 days after pollination (DAP). Both dehydration and freezing caused cytological damage, such as plasmolysis, swelled mitochondria, increased heterochromatin, and nuclear shrinkage. Dehydration alone slightly impaired plasma membrane integrity while a drastic increase in electrolyte leakage was observed after freezing of embryos with moisture content above 23%. Damage to cellular ultrastructure and plasmalemma integrity was negatively related to moisture content in unfrozen embryos and positively related in frozen embryos. The pattern of changes in activity of antioxidant enzymes differed from one another during dehydration and/or freezing-thawing treatment. Dehydration increased activity of ascorbate peroxidase (APX) and glutathione reductase (GR) but decreased activity of superoxide dismutase (SOD) and dehydroascorbate reductase (DHAR). Freezing further decreased GR and SOD activity and resulted in extremely low DHAR activity. Embryos at intermediate moisture contents had low catalase (CAT) activity before freezing but highest CAT activity after freeze-thaw. Both dehydration and freezing promoted membrane lipid peroxidation which resulted in an approximately threefold increase at most in the malondialdehyde content in postthaw embryos. Changes in viability of postthaw embryos can be closely related to damage in cellular ultrastructure and plasmalemma integrity but directly related neither to antioxidants nor lipid peroxidation levels.