Morphological and physiological aspects of flower initiation and development in Tanacetum cinerariaefolium L.

This study investigated the morphological and physiological changes associated with flower initiation and development in pyrethrum, Tanacetum cinerariaefolium L. Detailed morphological descriptions of vegetative and floral apices have been given and a scale of reproductive developmental stages based on these descriptions was proposed. It was shown that each stage of apical development was associated with a narrow range of apical diameters. The irreversible commitment to floral development was observed to occur when the first involucral bract was initiated on the apical dome and this point was characterised by a critical apical size. The apical diameter at this stage was always approximately 220 pm. A juvenile-like condition was described for pyrethrum seedlings, tissue culture explants and vegetatively divided splits. During the period of juvenile-like growth the plants were not competent to respond to normally inductive treatments. The juvenile-like phase lasted until the plants had reached a minimum size or stage of development, but did not depend on chronological age. The attainment of meristem competence was associated with the release of lateral buds from apical dominance. Terminal meristems were never observed to initiate flowers. Axillary meristems became competent to flower a short time after being released from apical dominance, while older axillary meristems were observed to lose their competence. It was noted that the loss of competence to flower of lateral shoot meristems occurred after the release from apical dominance of new axillary buds on each lateral shoot. The effects of the following environmental conditions on flowering were examined in a series of experiments; vernalisation, daylength, day temperature and photon flux. The major environmental requirement for flower initiation in pyrethrum was found to be a period of low night temperature or vernalisation. While flowering occurred eventually under 'non-inductive' conditions through an autonomous induction process, vemalising conditions were required to stimulate rapid flower initiation and development. Plants displayed a quantitative response to vernalisation as longer periods under vernalising conditions resulted in larger numbers of flowers, longer flower stems and more rapid flower initiation and development. Night temperatures of less than 18 °C were required to provide the vernalisation stimulus, with two weeks at 6°C or three weeks at 12°C demonstrated to be the minimum vernalisation requirement under short days and day temperatures of 20- 30°C. Both day temperature and photon flux density conditions were shown to modify the response to vemalisation. Low photon flux density conditions (350 gmol.m -2.s1 or less) retarded flower initiation regardless of day temperature. High day temperatures combined with low photon flux resulted in a devemalisation-like effect where the plants were incapable of responding to otherwise inductive vemalising conditions. A true devernalisation effect was also demonstrated under these conditions with the vemalisation stimulus being reversed by a later high temperature / low photon flux treatment Daylength had a quantitative effect on both flower initiation and development, with both processes promoted by long days. The inhibitory effect of short days was thought to be mediated through reduced assimilate supply and not via the phyochrome reactions. It was concluded that pyrethrum is a day-neutral species as its daylength reaction was due to the daily light integral and not to photoperiod. Autoradiography was used to follow the distribution of 14C photosynthate during flower initiation and development under 'inductive' and 'non-inductive' conditions. This method was also used to study the effect of devemalising conditions on assimilate partitioning. The terminal shoot apex and young developing leaves were the main sinks for assimilates under 'non-inductive' conditions. The sink strength of the axillary shoots in 'inductive' conditions was observed to increase prior to the end of evocation and they became the dominant sink for radiolabelled assimilates as floral development progressed. 'Devemalising' conditions reduced the sink strength of the axillary buds, or prevented the translocation of assimilates to them. Radioimmunoassays were performed to quantify the changes in plant growth regulator concentrations in mature leaf samples under 'inductive', 'non-inductive' and 'devernalising' conditions. Vernalising conditions stimulated an increase in the concentration of gibberellins while 'devernalising' conditions resulted in a reduction in the concentration of gibberellins to levels below that of unvemalised plants. The concentration of the auxin indolylacetic acid declined under vernalising conditions while sdevernalising' conditions prevented this decline. No evidence was found during this study of a role for the cytokinins or abscisic acid in the flowering of pyrethrum. However a possible role for these hormones could not be ruled out as all the assays were performed on mature leaf samples and as such would not have detected any localised fluxes of hormones in other plant organs. Two cultivation techniques for manipulating the flowering behaviour of pyrethrum in the field were examined. The first involved the application of growth retardants to reduce flower stem height and the degree of lodging at harvest. 'EL-500' and 'Cultar' at rates equivalent to 5 Kg active ingredient per hectare or above were shown to significantly reduce flower stem height and lodging. The growth retardants also reduced flower yield if applied during the period of flower initiation. The most effective control of lodging, without reducing flower yield, was found to result from application of the growth retardant immediately prior to the period of maximum stem extension, which under Tasmanian field conditions occurs in October. The growth retardant 'Culiar' reduced the gibberellin concentration in treated plants, and the effects of the growth retardant were reversible by application of gibberellin A3. The effect of trimming on the flowering of plants in the field was also examined. Multiple trimming treatments were shown to be the best strategy for promoting vegetative growth during the normal flowering period. Slashing plants to a height of approximately three centimetres above ground level during October and November, when the inductive vernalising conditions no longer prevail, significantly reduced the flower yield and increased the yield of vegetatively divided splits. This is of economic value in field nurseries where multiplication of planting stock is achieved through vegetative division of nursery stock. The process(es) of flower initiation and development were discussed in light of the experiments detailed above.