Is the time of floral initiation in Ptilotus nobilis affected by temperature, daylength or light intensity?

Sybille Orzek
University of Queensland
Received the Australian Flora Foundation Young Scientist Award for her talk at The National and Trans-Tasman Horticultural Science Conference “Smart Science for Innovative Horticultural Enterprises”; at the Gold Coast International Hotel, Surfers Paradise 21 – 23 July 2008:

S. Orzek, M.E. Johnston, R.R Williams and M. Perkins
Centre for Native Floriculture, School of Land, Crop and Food Science The University of Queensland ,Gatton Campus, 4343, QLD

Ptilotus nobilis is an Australian native plant recently released into the Australian horticultural market as the Outback Princess ‘Purity’, ‘Passion’ and ‘Poise’. To supply markets efficiently, manipulation of flowering is essential. Environmental conditions affect floral initiation in other species therefore P. nobilis plants were subject to 2 temperatures (25/10 ºC, 35/20 ºC), 2 daylengths (11h, 16h) and 2 light intensities (low light/high light) in a research greenhouse. Preliminary experiments showed that initiation occurs early and rapidly, therefore harvesting commenced 16 days after germination and was repeated every 3rd day for 3 weeks. Each harvest, 5 plants were assessed for leaf number, stem height and the stage of the shoot apical meristem (SAM). Four meristem stages were identified: 1. small (red) vegetative meristem; 2. enlarged (translucent) vegetative meristem; 3. bract initiation; 4. macroscopic bud appearance. Plants under low light did not initiate bracts and only 6 plants reached the vegetative translucent stage so only plants under high light were investigated further. On days 16 and 19 all plants remained in stage 1 and the first signs of transition were observed on day 25 in 50% or 30% of plants under high or cool temperatures respectively. In some plants subsequent development was faster under cool conditions; by day 28, 10% of plants under both temperatures had reached stage 3 but 30% of those under high temperatures were still at stage 1 whereas the other 90% of plants under cool temperatures were at stage 2. By day 37, 90% of plants under cool temperatures had microscopic buds (stage 4) while only 10% of those under high temperature had reached this stage. Similarly short days increased the number of plants reaching stage 4 (60% compared to only 40% under long days). Under long days, 60% remained vegetative (stage 2). These results indicate that light intensity is the main factor affecting floral initiation followed by temperature. However, while daylength or temperature may increase floral initiation at the shoot apical meristem level it remains to be seen whether this translates into more flowering buds.