UV-B and temperature enhancement affect spring and autumn phenology in Populus tremula

Perennial plants growing at high latitudes synchronize growth and dormancy to appropriate seasons by sensing environmental cues. Autumnal growth cessation, bud set and dormancy induction are commonly driven by the length of photoperiod and light quality, and the responses are modified by temperature. However, although ultraviolet (UV)-B radiation is well known to affect plant growth and development, information on the effects on bud phenology is scarce. We examined the separate and combined effects of enhanced temperature and UV-B on autumnal bud set and spring bud break in female and male clones of Populus tremula in an outdoor experiment in Joensuu, Eastern Finland. Enhancements of UV-B and temperature were modulated to +30% and +2 degrees C, respectively, from June to October 2012. Enhanced UV-B accelerated bud set, while increased temperature delayed it. For both UV-B and temperature, we found sex-related differences in responsiveness. Temperature increase had a stronger delaying effect on bud maturation in male compared with female clones. Also, male clones were more responsive to UV-B increase than female clones. Increasing autumnal temperature enhanced bud break in spring for both sexes, while UV-B enhanced bud break in male clones. In conclusion, we found that UV-B affected phenological shifts in P.tremula, and that temperature and UV-B affected genders differently. As light quality is recognised to affect phenological shifts in several perennial plant species, Ultraviolet B (UV-B) has not yet, to our knowledge, been shown to affect any such processes. In our study, increased UV-B radiation significantly affected phenological shifts in the dioecious tree species Populus tremula. Furthermore, we found different responsiveness to treatments between sexes. UV-B and temperature enhancement treatments were applied in the field during autumn, and male plants displayed earlier bud set in autumn and bud break in spring following UV-B enhancement.