Deacclimation response to winter temperature fluctuations in Lolium perenne

Studies on climate change have shown an increase in not only the average global surface temperature but also an increase in extreme weather events. One such extreme weather event is the occurrence of winter temperature fluctuations, which can induce loss of cold acclimation. Sudden temperature fluctuations have occurred in the United States in the early spring of 2007 and the late winter of 2009. Both events caused significant freezing damage to plants. However, plants that were able to maintain a portion of their original freezing tolerance, by a process called deacclimation resistance, did not suffer significant damage. Different phenotypic assays were conducted to determine frost tolerance and deacclimation resistance of selected genotypes. Field trial exhibited deacclimation resistance of freezing tolerance loss during a warming trend and sudden freezing in February 2009. Because of this extreme weather event, the number of cultivars in the field trial assay was reduced by 92%. This extreme weather event was simulated in a growth chamber assay. In this assay cultivars from the field trial were exposed to similar temperature fluctuations but under controlled temperature settings. Another assay used to determine plants freezing tolerance during different lengths of deacclimation was the ion leakage assay. This assay indirectly measures freezing tolerance under the premise that ion leakage is negatively correlated with freezing tolerance. However, repeatability of the ion leakage assay was too low, to determine freezing tolerance during deacclimation. By comparing the phenotypic assays (field trial, growth chamber and ion leakage assay) with frost tolerance and deacclimation resistance, the in vivo assay in the growth chamber was best reflecting field conditions. Transcriptome expression profiling was used to find candidate genes for freezing tolerance and/or deacclimation resistance, complementing the phenotypic assays. Expression profiles during cold acclimation, freezing and various lengths of deacclimation were compared. The results indicated that 14 ESTs were significantly differentially expressed during cold acclimation, freezing and/or deacclimation. Further analysis by RT-PCR is necessary to verify, if these 14 ESTs are candidate genes for frost tolerance or deacclimation resistance