Morphological, physiological and biochemical aspects of flower initiation and development in Boronia megastigma Nees.

This study investigated flower initiation and development in Boronia megastigma Nees. Detailed morphological descriptions of flower, vegetative, and reverted buds have been given. Flowers were found to initiate from uncommitted axillary buds in the axils of mature leaves on current seasons laterals. Flower buds usually initiated in autumn and continued to differentiate slowly over winter, reaching anthesis in early spring. Photosynthesis and respiration rates were measured at a range of temperatures (using infra red gas analysis) on both fully expanded and immature leaves of current seasons laterals. Apparent photosynthesis was highest in the mature tissue and peaked at 20°C for both types of leaves. The dark respiration rate was greater in the immature leaves and continued to increase with temperature. True photosynthesis was highest at 25°C. A light saturation curve was carried out on fully expanded leaves (at 20°C) and occurred between 400-600µmol m-2 s-1 . This data was used to estimate photosynthetic rates in the glasshouse and also to calculate uptake rates when labelling with 14CO2. The effects of a night break, PFD, temperature, and daylength on flower initiation and development were studied. As a consequence, low night temperatures (<10°C) in combination with short days (10hrs) and 50 to 100 percent full sunlight was termed "inductive" conditions, whereas moderate night temperatures (>l5°C) and long days (16hrs) was called "noninductive" conditions. Boronia does not appear to be a photoperiodically sensitive plant, and providing PFD is not limiting it has little effect on flowering under "inductive" conditions. Night temperature had a dominant effect on the initiation and development of the flowers. Flower development rather than flower initiation seemed to be more affected by the environmental parameters. The same environments that favoured flower bud initiation also resulted in the highest percentage of flowers that reached anthesis. Some degree of antagonism appeared to exist between apical activity and the initiation of generative buds and their development. Correlative inhibition by the vegetative apex was investigated by removal of the most apical three pairs of leaves in "inductive" and "noninductive" conditions before and after floral initiation. On previously non-flowering laterals the rapid re-establishment of apical dominance prevented floral bud initiation in the most apical remaining pair of leaf axils. On pre-induced laterals the removal of the apex enhanced flower differentiation particularly in the most apically remaining leaf axils, and the extent of this was modified by the environment . Autoradiography was used to follow the distribution of 14C photosynthate during flower initiation and development under "inductive" and "noninductive" conditions. This method was used also to study the reversion of floral differentiation, and the effect of apex removal. The partitioning of labelled 14CO2 into a chloroform soluble fraction, proteins, sugars, starch, free amino acids and amides, organic acids and phosphates, and structural residue was measured (using liquid scintillation counting). Mature leaves and immature leaves were analyzed separately during the first 70 days of "inductive" conditions. Quite distinct differences in the partitioning patterns of some of the fractions were noted between the two types of tissue, and the differences were reduced after 40-50 days. The level of partitioning into the amino acid fraction in both types during the first 20 days of "inductive" conditions. Consequently the fluctuation of free amino acids and amides in the mature leaves was measured by HPLC during the first 30 days of inductive conditions. The major amino acids and amides were found to be alanine, asparagine, methionine, and proline. The concentration of many amino acids was affected by the change in environment and there were several changes of concentration over a period of time. In vitro techniques were used to investigate apical dominance, tissue maturity, and the influence of daylength and temperature on flower initiation and development. The highest percentage of flowers was found on explants that had fully expanded leaves and no vegetative apex. Fewer flowers on these explants reverted or aborted before reaching anthesis. No floral initiation occurred on explants that retained the vegetative apex or that did not have fully expanded leaves. The environmental influence on initiation and subsequent differentiation and development was similar to the results found in the in vivo experiments. Some competition between flowering and vegetative vigour was demonstrated in vitro and on whole plants. The effects of cytokinin and auxin were also studied in vivo and in vitro. The effects of cytokinin and auxin were modified by the environmental parameters used. Increased cytokinin concentration enhanced the initiation and rate of differentiation of floral and vegetative buds, however, many flower buds reverted with higher rates of cytokinin (4-10 mg BAP/l) and it was thought that normal differentiation and development to anthesis possibly requires less BAP than does initiation. The presence of IAA appeared to be unnecessary for flower initiation, differentiation, and development. In "noninductive" conditions higher levels of IAA (3 mg/l) were promotive to rooting. The process(es) of flower initiation and development were discussed in light of the experiments detailed above.