EFFECT OF LOW-TEMPERATURE ON FLORAL INDUCTION OF EUCALYPTUS-LANSDOWNEANA MUELL,F. AND BROWN,J. SUBSP LANSDOWNEANA

Transferring seedlings of Eucalyptus lansdowneana from a heated glasshouse (24/19-degrees-C) to a cold glasshouse (15/10-degrees-C) for 5 or 10 weeks and back to the heated glasshouse was sufficient to induce floral buds. Bud production was further enhanced when seedlings were transferred to cold conditions during periods of high solar radiation. Under low levels of solar radiation and short duration of cold, 0-5 weeks, plants reverted to vegetative development, suggesting a low floral induction stimulus. Seedlings that produced a visible floral inflorescence had fewer leaves than seedlings grown under similar conditions that had not produced an inflorescence. This was more noticeable under high-radiation conditions. Plants grown under outside conditions in Canberra and transferred to a heated glasshouse (25/18-degrees-C) during winter initiated inflorescences 7-9 weeks earlier than plants grown continuously outside. The early initiation enabled buds to develop and flower before the onset of the following winter. More buds were initiated in plants transferred to the glasshouse in September compared with 16 June or 28 July. Plants transferred on 16 June initiated few buds or none at all. These plants may have been in a juvenile or transitional stage of development, experienced insufficient cold for full induction or been limited by the low winter irradiances. Floral response occurred under both long days (phytotron) and short days under outside conditions in Canberra, suggesting that E. lansdowneana may well be relatively insensitive to day length. These results are discussed in relation to controlled breeding programs which aim to manipulate flowering time and duration to decrease the generation interval.