Ptilotus nobilis is a short-lived perennial wildflower, native to semi-arid and arid areas of Australia. Propagation by vegetative means is constrained by the early onset and a continuous flowering habit. Despite being defined as the main barrier for vegetative propagation no published research on floral induction and initiation was found. The aim of this study is to provide the first insights into floral evocation in P. nobilis, with the general objectives being to investigate floral induction and initiation, find the means to maintain plants in the vegetative phase, enhance leaf initiation and to gain knowledge of growth and development with an emphasis on light intensity, temperature and daylength. Early experiments aimed to increase the understanding of growth and development. The main cardinal events were identified including the onset of branching (axillary stem growth), visible bud stage, first floret opening and maturity of the inflorescence. To aid future cultivation schedules, four growing degree days (GDD) and one chronological model, using days after sowing (DAS), were established and validated. All GDD models were accurate in predicting first floret opening and maturity, but not in predicting visible bud stage. Best prediction was achieved by using an upper temperature threshold of 18.2 ºC and an own base temperature of 5.0 ºC. Days after sowing were an accurate means of prediction, indicating that temperature and other variables such as light intensity regulate development. A series of defoliation treatments investigated the maintenance of P. nobilis plants in the vegetative phase. Within all treatments, reproductive structures were observed and all plants with more than two true leaves entered the reproductive phase. Plants with less than two true leaves showed a delayed floral bud appearance by up to 20 days. Floral development was affected by most defoliation treatments resulting in vegetative growth within the inflorescences. It is hypothesized that plants have a very short juvenile phase and that a constant floral stimulus may be needed for floral evocation. Using scanning electron and light microscopy a template for the transition from vegetative to reproductive phase was developed. Bract initiation was accompanied by a significant increase in meristem area and diameter, and was defined as the onset of flowering. The established template was used in a subsequent glasshouse trial, which revealed that floral initiation occurred very early and at 25 DAS all plants had entered the reproductive phase. Branching and leaf area expansion were identified as post- initiation processes. Plants were exposed to different light intensities (229.3, 398.6 and 909.3 µmol m-2 s-1) in a glasshouse. Under low light, final leaf number increased by up to three leaves, indicating that the vegetative phase was prolonged. Cardinal events were delayed but all plants reached maturity. It was concluded that a light intensity of 229.3 µmol m-2 s-1 was not low enough to prevent floral initiation and that a further decrease of light intensity in combination with temperature could be more effective. Interactions of light intensity, temperature and daylength were investigated. Plants were grown under high light and low light (< 1.2 MJ m-2) intensities, 25/10 ºC and 35/20 ºC and daylengths of 11 h and 16 h. During the trial period (42 days), low light intensity suppressed floral initiation and high temperatures maintained more plants (70%) in the vegetative phase. However daylength treatments had no effect on the time of bract initiation or the percentage of vegetative plants. After 83 days floral buds and axillary stems were observed on some plants under low light intensity, indicating the onset of the reproductive phase and showing that P. nobilis could not be maintained in the vegetative phase indefinitely. Plants under high light were harvested at maturity and effects of temperature and daylength were analysed. Final leaf number increased under 35/20 ºC supporting the previously established results that floral initiation was delayed and leaf production enhanced by high temperature. Morphological data was collected to classify the photoperiodic response of P. nobilis. Plants under 25/10 ºC and 11 h had the longest inflorescences and greatest number of buds and flowers, which was also reflected in the buds and flowers dry weight, with an increase of up to 3.4 fold under these conditions. The difficulty of classifying some plants accordingly to their photoperiodic response and the proposal that P. nobilis may be a facultative short day plant under 25/10 ºC is discussed. In summary, this study presents the first evidence that P. nobilis has a very short juvenile phase and that growth and development are mainly driven by light intensity and temperature. It was possible to enhance vegetative growth by defoliation, low light intensity and high temperatures, however this did not fully prevent flowering, which indicates that P. nobilis has a very strong flowering response or signal.
| Identifer | oai:union.ndltd.org:ADTP/288372 |
| Creators | Sybille Orzek |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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