Physiological and Biochemical Responses Induced by Cold Acclimation in Relation to Freezing Tolerance for Perennial Ryegrass.

Compared to other cool-season turfgrasses, perennial ryegrass (Lolium perenne L.) can exhibit significant freezing injury in northern climates of the United States.  Recently, field trials and controlled-freezing tests were conducted on breeding populations of perennial ryegrass that showed significant variability in freezing tolerance among genotypes.  The physiological mechanisms underlying differences in winter hardiness among these ryegrass accessions, however, is unknown.  Therefore, the objectives of this study were to (i) quantify major physiological and biochemical changes occurring during the cold acclimation process for perennial ryegrass genotypes varying in freezing tolerance, and (ii) obtain data on useful traits that may be utilized to screen for improved freezing tolerant perennial ryegrass germplasm.  The experiment was conducted in a controlled environment chamber, using two freezing tolerant and two freezing intolerant genotypes. Plants were subjected to an acclimating temperature regime of 2 °C for 21 d. During the acclimation period, visual turf quality and photochemical efficiency were measured weekly. A group of plants (4 replicates per genotype) were harvested at 0, 7, 14, and 21 d for controlled freeze tests at -12 °C to determine differences in rate of acclimation. A separate group of plants (4 replicates per genotype) were harvested at 0, 7, 14, and 21 d of acclimation for biochemical analysis of crown tissue, including lipid composition, proline content, and water soluble carbohydrate content.  In general, differences were found in rate of acclimation among the four genotypes, with the more tolerant genotypes acclimating more rapidly than the freezing intolerant genotypes.  Proline and water soluble sugars significantly increased throughout the acclimation period for all four genotypes; however, the magnitude and rate of increase was greater for the two freezing tolerant genotypes.  Changes in the lipid composition in response to acclimation will also be discussed.