Frost Tolerance, Deacclimation and Reacclimation Traits in Perennial Ryegrass

The ability of perennial grasses to harden and maintain frost tolerance throughout the winter is crucial for winter survival. This includes the ability to resist deacclimation during transient mild spells in winter, and the ability to reacclimate when cold temperatures return. The latter traits are especially critical in regions with cycles of freezing and thawing, and lack of a stable, insulating snowcover that can protect the plants from extreme air temperatures. Such conditions are typical for many coastal areas in Northern Eurasia and America, such as the southwestern coast of Norway. The climate is changing and one of the consequences for Norway will be milder winter temperatures. This might open up for increased use of perennial ryegrass (Lolium perenne L.) in Scandinavian forage grass production systems at the expense of timothy (Phleum pratense L.), the most commonly used forage grass today. However, there are still important questions about the winter survival of perennial ryegrass under future climate conditions that needs to be addressed before a wider use of this grass species can be recommended. One is related to the risk of frost injury connected with more fluctuating temperatures at plant level in winter and spring. Thus, simulation studies using a grassland model parameterized for current winter-hardy cultivars of perennial ryegrass indicated an increased risk of frost injury in winter and spring in many areas of North Europe including Norway (Höglind et al., 2013). The increased risk was associated with a reduced snow cover, and earlier onset of spring growth followed by frosts. The simulation results indicate that cultivars that can resist deacclimation and/or that can reacclimate to a substantial degree will be needed for successful grass production under the projected future climate conditions. However, more information is needed about the genetic variation with respect to deacclimation resistance and reacclimation capacity. The aim of the present work was to compare three cultivars of perennial ryegrass with respect to their resistance to dehardening and ability to reharden under fluctuating winter temperatures. The plants differed widely with respect geo-climatic origin. Plants were first hardened under controlled conditions, and then subjected to a period of mild temperatures followed by decrease to pre-dehardening temperatures. Frost tolerance was estimated by freezing tests after completed hardening, twice during the mild episode and twice after the return to pre-hardening conditions. Our hypothesis was that the deacclimation and reacclimation characteristics would differ largely between the cultivars given their contrasting geo-climatic origin