Cold-hardening and plant developments are associated with many changes in cellular and molecular levels that lead to an improvement and then decrease of freezing tolerance. Exposure of the plant to low non-freezing temperature can induce various protective mechanisms (cold acclimation). These protective mechanisms include both alterations in gene expression, post translation modifications and metabolic re-adjustments. LT acclimation during the vegetative growth stages may lead to accumulation of some LT-responsive proteins, compatible organic solutes (e.g. glycinebetaine, glutamine, alanine, inositol and proline) and solute carbohydrate (e.g. sucrose and trehalose) in crown and leaves tissues. Accumulation and activity of proteins involved in carbohydrate metabolism, cryoprotection, redox adjustment, cytoskeletal rearrangements, scavenging of reactive oxygen species, defence/detoxification significantly increase by LT acclimation, whereas severely down-regulated by transition from vegetative to reproductive development. Genomic analysis have been revealed that expression of photosynthetic genes during the LT acclimation down-regulated. However the genes involved in the synthesis of osmoprotectants-organic compounds, enzymatic antioxidant, anti-pathogenic polypeptides, cytoskeletal proteins (alpha-tubulin), Late Embryogenesis Abundant proteins (dehydrins) and others significantly up-regulate during overwintering. Evaluation of intracellular calcium has been shown to be sufficient and necessary to promote the expression of low-temperature responsive genes. These modifications are common adaptation reactions and it seems that play critical role in a successful overwintering. The analysis suggests that there are close relationship between phonological development and improvement of freezing tolerance. Winter plants which required vernalization can regulate their development through adaptive mechanisms that are responsive to low but non-freezing temperatures.
