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Diel and seasonal oscillations are two major environmental changes in nature. While organisms cope with the former by the well-characterized mechanism of the circadian clock^,, there is limited information on the molecular mechanisms underlying long-term responses to the latter^{, –}. Histone H3 lysine 27 trimethylation (H3K27me3), a repressive histone modification, imparts stability and plasticity to gene regulation during developmental transitions^{, , –}. Here we studied the seasonal and diel dynamics of H3K27me3 at the genome-wide level in a natural population of perennial Arabidopsis halleri and compared these dynamics with those of histone H3 lysine 4 trimethylation (H3K4me3), an active histone modification. Chromatin immunoprecipitation sequencing revealed that H3K27me3 exhibits seasonal plasticity and diel stability. Furthermore, we found that the seasonal H3K27me3 oscillation is delayed in phase …