Biochemical changes after cold acclimation in Nordic red clover (Trifolium pratense L.) accessions with contrasting levels of freezing tolerance.

The ability to tolerate low freezing temperatures is an important component of winter survival and persistence of red clover. Cold acclimation (CA) allows plants to acquire higher levels of freezing tolerance. However, the biochemical responses to cold and the importance of such changes for the plant to acquire adequate freezing tolerance have not been investigated in red clover of Nordic origin, which has a distinct genetic background. To shed light on this, we selected five freezing tolerant (FT) and five freezing susceptible (FS) accessions and studied the effect of CA on the contents of carbohydrates, amino acids, and phenolic compounds in the crowns. Among those compounds which increased during CA, FT accessions had higher contents of raffinose, pinitol, arginine, serine, alanine, valine, phenylalanine, and one phenolic compound (a pinocembrin hexoside derivative) than FS accessions, suggesting a role for these compounds in the freezing tolerance in the selected accessions. These findings, together with a description of the phenolic profile of red clover crowns, significantly add to the current knowledge of the biochemical changes during CA and their role in freezing tolerance in Nordic red clover.