The importance of the circadian system for adaptation to heat wave stress in wild barley (Hordeum spontaneum)

Acute heat episodes (heat waves) present some of the most significant challenges for plants growing in both natural and agricultural settings. Circadian (similar to 24 h) systems are molecular time-pacing mechanisms responsible for synchronizing a wide range of diverse physiological processes, including abiotic and biotic stress responses. Our aim in this paper was to determine whether circadian systems may also contribute to the capacity of plants to withstand acute heat episodes. We used chlorophyll fluorescence (F) to analyze circadian rhythms and plant vitality under non-stress and heat wave conditions in a panel of wild barley (Hordeum spontaneum) accessions from diverse environmental sites across the Southern Levant. Using average linear multi-models we identified correlations between rhythm attributes, plant vitality and eco-climate environment at the sites of origin of the wild barley accessions. Under non-stress conditions, the environment explained a significant portion of the variance in period time and amplitude. By contrast, in acute heat stress conditions while there was a shortening of period and decrease in amplitude, circadian traits did not directly correlate with eco-climate origins. However, plant vitality showed significant correlations with circadian plasticity in response to increasing temperatures. Unexpectedly, optimal circadian traits for heat stress resilience were not primarily associated with barley accessions from any particular climate, suggesting that thermotolerance capacity for consistently higher temperatures is not necessarily correlated with the capacity to withstand heat wave stress.