Ecotypic Differences in the Flowering of Pimelea ferruginea (Thymelaeaceae) in Response to Cool Temperatures

Pimelea ferruginea Labill., a spring-flowering woody perennial from Western Australia, is distributed coastally from Albany (latitude 35 degrees 11'S) to just below Geraldton (latitude 29 degrees 07'S). Clones developed from cuttings, taken from single plants of Pimelea ferruginea at sites covering the range of its distribution, were grown vegetatively for up to 6 months in a naturally lit glasshouse and then transferred to a range of temperatures for induction of flowering. Selections from cooler, southerly sites (33-35 degrees S latitude) flowered with a low temperature optimum (12-15 degrees C average) but remained vegetative at a 3 degrees C higher temperature (18 degrees C). By contrast, northerly selections (29-30 degrees S latitude) tolerated 3-5 degrees C warmer conditions for their flowering. Daylength effects were of minor importance relative to the response to cool temperature with short (10 h) days being marginally more favourable than long days (16 h). In the wild, Pimelea ferruginea flowers in spring (September-October) after experiencing winter temperatures comparable to those effective in controlled environments. To define field flowering response, at the beginning of winter, vegetative six-month-old plants of the various selections were transferred to a southerly, intermediate or northerly field nursery site in Western Australia. The differences in flowering response across sites and selections broadly matched the temperatures needed for flowering of the selections in controlled environments. These physiological differences in regulation of flowering indicate ecotypic adaptation to temperature over the latitudinal range of distribution of this species of Pimelea. The field transfer experiments highlight the overriding inductive effect of cool winter temperatures in determining flowering time. Furthermore, any rise of 2-4 degrees C in winter temperature due to global warming could have serious consequences for floral initiation and for survival of P. ferruginea.