Abstract
Many land plants increase freezing tolerance upon exposure to low, non-freezing temperatures, the phenomenon known as cold acclimation. To understand basic mechanisms of plant cold acclimation, we determined the effect of cold acclimation on freezing tolerance, cell structure and compatible solute composition of a green alga (Charophyceae) Klebsormidium flaccidum, which occurs in cold region and is thought to be a phylogenetic ancestor of land plants. Cold acclimation resulted in a significant increase in freezing tolerance, an increase in the size and number of starch grains in chloroplasts, a decrease in the size of vacuoles, and the thickening of cell wall. Osmotic concentration of cells increased after cold acclimation, partly due to the accumulation of soluble sugars (such as sucrose and unidentified compounds) and amino acids (such as glutamine and alanine). These changes are thought to be associated with the increase in freezing tolerance of K. flaccidum during cold acclimation.
