The summer-deciduous habit of Daphne pseudomezereum is a response to warm summer as cooling converts it to an evergreen

This study intended to clarify whether low irradiance, high temperature or both acted as a trigger inducing summer dormancy in Daphne pseudomezereum (Dpm), a wintergreen shrub native to Japanese mixed forests. A 2-year growth chamber experiment was conducted in Otsu, Japan where, in the first year, 3 provenances of Dpm were raised in two irradiance levels by two temperature regimes to observe their effects on leaf phenology. The results showed a clear effect of temperature, but not irradiance, on leaf phenology. In the second year, the experiment was repeated using different Dpm plants and applying only the two temperature regimes. The results confirmed those obtained in the first year, where the temperature regime that tracked normal field condition induced summer dormancy similar to field populations. In both years, when plants were kept over the summer under a cool temperature regime mimicking field conditions in April, most plants did not undergo summer dormancy (i.e., becoming “evergreen”). In contrast to phenology, leaf morphology (i.e., LMA), and photosynthetic capacity (e.g., A max ) did respond to irradiance levels consistent with shade adaptive changes, but were not affected by temperature. Simulated carbon gain using previously determined parameters for Dpm and the chamber microclimate data found that only plants in the warm treatment experienced carbon deficit in mid-summer. These findings suggest that summer temperature alone and the attendant rise in respiration can alter internal carbon balance and trigger the onset of summer dormancy in Dpm.