Physiological Changes During Cold Acclimation for Perennial Ryegrass Accessions Differing in Freeze Tolerance.

Perennial ryegrass (Lolium perenne L.) is a cool-season turfgrass species that can exhibit significant freezing injury in northern climates of the United States.  Recent breeding efforts have identified novel perennial ryegrass germplasm with superior freezing tolerance compared to many commercially available cultivars. The underlying factors responsible for these differences in freezing tolerance, however, require further investigation.  Therefore, the objectives of this study were to quantify major physiological and biochemical changes occurring during the cold acclimation period for four perennial ryegrass accessions varying in freezing tolerance (two freezing tolerant accessions, TOL-1 and TOL-2, and two freezing susceptible accessions, SUS-1 and SUS-2).  Plants were subjected to a cold acclimation period of 2 °C for 21 d in a controlled environment chamber. Crown tissues were harvested (4 replicates per accession) at 0, 7, 14, and 21 d of acclimation for determination of proline, water soluble carbohydrates (WSC), and lipid composition. Leaf photochemical efficiency (Fv/Fm) was also measured weekly prior to harvest.  In general, proline levels increased and Fv/Fm decreased in response to 2 °C; however, there were no differences in proline accumulation or Fv/Fm among the accessions in response to cold acclimation.  Water soluble carbohydrates significantly increased in response to 2 °C, with TOL-1 and TOL-2 accumulating greater WSC than SUS-1 and SUS-2 by 21 d of acclimation. There were also significant changes in major phospholipids classes at 2 °C. Specifically, TOL-2 exhibited a higher ratio of membrane stabilizing lipids and unsaturated fatty acid content compared to SUS-1, SUS-2, and TOL-1 by 21 d of cold acclimation.  The results from this research suggest that enhanced freezing tolerance of accessions TOL-1 and TOL-2 may be due to a greater capacity to accumulate protective compounds during cold acclimation.