Characterisation and management of flower abortion in inbred male sterile onion parental line ON019A
Creators: name: Padmanaban, Sriram
Publisher: name: University Of Tasmania
Year: 2023

description: Hybrid onion seed production is popular throughout the world. Production in Australia is small by world standards but with significant potential for expansion due to availability of suitable production sites and the counterseasonal production to Northern Hemisphere producers. Variable and often very low seed yield from inbred parent lines has been identified as a significant problem limiting expansion of hybrid onion seed production. This thesis investigated the issue using the weak inbred male sterile onion parental line ON019A used in hybrid seed production by Enza Zaden Australia Pty Ltd as a model system. ON019A is prone to poor seed yield, typically less than 0.5 g/plant but has produced yields up to 8 g I plant at some sites. The typical low yields of this line coupled with potential for much higher yields under certain circumstances make this a useful line for identifying and studying the processes contributing to low yield in inbred male sterile onion parent lines. The project had two broad objectives i) to identify the cause of the variability in seed yield between onion crops and ii) to improve the seed yield by developing commercially applicable management practices. The research initially focussed on pollination as seed industry representatives had suggested that insufficient pollination may be the primary cause of low yields; failure of hand pollinated flowers to set seeds confirmed that pollination was not a significant contributor to low seed yield. Observation of flowers during the supplemental hand pollination experiments identified two classes of flowers; a) flowers with fully extended stigma (&gt;4 mm long) and open petals; and b) flowers in which the stigma extended less ( &lt;2 mm long) and petals did not fully open. Whilst flowers with fully extended stigmas set seed after hand pollination, flowers in which the stigma only extended to 2 mm did not. This observation led to the hypothesis that abortion of the flower before anthesis was the primary cause of poor seed set in ONO 19 A. Evidence supporting the flower _abortion hypothesis was gained in experiments comparing flower development in ON019A with other higher yielding inbred lines ON013A and ON0138A together with Kingswood an open pollinated pollinator line. Experiments then focussed on crop management practices to manipulate flower development of ONO 19 A; these included mother bulb characteristics (timing of lifting, bulb size, bulb storage temperature), planting date, crop nutrient status, irrigation management, application of plant hormone and anti-transpirant sprays. The results suggested that the size of bulb had a significant effect on flower development with large sized bulbs having a reduced flower abortion percentage and increased seed yield. A single late application (at the 60% piping stage) of gibberellic acid (GA3) at 450 ppm also improved the seed yield and reduced the flower abortion percentage. While improvements in seed yield through reduced rates of flower abortion occurred in the large bulb size and GA3 application treatments in most trials, significant variability existed between production locations in the size of the response. It was concluded that site specific environmental conditions could interact with bulb size and GA3 application treatments and affect the seed yield. Subsequent experiments on irrigation and protection of plants from conditions that favoured high transpiration rates under glasshouse conditions revealed a reduction in flower abortion and an increase in the seed yield following the application of an anti-transpirant together with frequent irrigation. The percentage of flower abortion was low and seed yield per plant high in all treatments under glasshouse conditions, again suggesting an environmental influence on flower abortion. Under field conditions protecting umbels with wind breaks resulted in a low percentage of flower abortion and comparatively high seed yields (up to 5g/ plant) in the low vigour inbred ON019A line. Improvement in seed yield from the inbred male sterile line ON019A can be made through careful production site selection, use of wind breaks during flowering, use of large mother bulbs, strategic application of gibberellic acid and anti-transpirants. There is a good opportunity to increase the onion seed production in Australia particularly with further research focussing on understanding the effects of environmental conditions at different locations and refinement of management techniques to overcome the flower abortion problem and to improve reliability of seed yields.