| Summary: | The Antarctic Peninsula has rapidly warmed up in past decades, and global warming has exhibited an asymmetric trend; therefore, it is interesting to understand whether nocturnal or diurnal warming is the most relevant for plant cold deacclimation. This study aimed to evaluate the effect of diurnal and nocturnal warming on Antarctic vascular plant’s freezing resistance under laboratory conditions. This was studied by measuring the lethal temperature for 50% of tissue (LT<sub>50</sub>), ice nucleation temperature (INT), and freezing point (FP) on <i>Deschampsia antarctica</i> and <i>Colobanthus quitensis</i> plants. Additionally, soluble carbohydrates content and dehydrin levels were analyzed during nocturnal and diurnal temperatures increase. Nocturnal warming led to a 7 °C increase in the LT<sub>50</sub> of <i>D. antarctica</i> and reduced dehydrin-like peptide expression. Meanwhile, <i>C. quitensis</i> warmed plants reduce their LT<sub>50</sub> to about 3.6 °C. Both species reduce their sucrose content by more than 28% in warming treatments. Therefore, nocturnal warming leads to cold deacclimation in both plant species, while <i>C. quitensis</i> plants are also cold-deacclimated upon warm days. This suggests that even when the remaining freezing resistance of both species allows them to tolerate summer freezing events, <i>C. quitensis</i> can reach its boundaries of freezing vulnerability in the near future if warming in the Antarctic Peninsula progress.
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