Freezing tolerance of selected perennial ryegrass (Lolium perenne L.) accessions and its association with field winterhardiness and turf traits

Many winter stresses affect the ability of a perennial grass to overwinter in cold, temperate climates. Freezing tolerance is one of the major component traits affecting winterhardiness. Although effective freezing tolerance assessment procedures have long been available for perennial ryegrass (Lolium perenne L.), no work has been done to characterize plant collections in the United States that may possess novel variation for freezing tolerance and winterhardiness. In this study, 21 accessions sampled from a larger set of 300 accessions with known winterhardiness and 3 check varieties were subjected to environmentally-controlled, low temperature acclimation, followed by controlled freezing using a programmable laboratory freezer. Eight freezing treatments from -2 to -16°C were used to determine survival across the known spectrum of freezing tolerance in perennial ryegrass. LT₅₀ values were estimated based on whole plant survival, as well as tiller survival at each of the temperature treatments in the evaluation. LT₅₀ values for the accessions and checks ranged from -10.31 to -13.95°C, with 3 accessions possessing significantly greater freezing tolerance than the most freeze-tolerant check, 'NK200'. LT₅₀ values were well correlated with winterhardiness in St. Paul during the winters of 2004-2005 and 2005-2006. Lower LT₅₀ values in this experiment were associated with greater spring growth following the stressful winter of 2004-2005 and upright growth habit. The lack of negative biological association between freezing tolerance and turf quality components indicates that freezing tolerance testing could be a useful tool in breeding and selection with landrace germplasm.