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Abstract

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 (>4 mm long) and
open petals; and b) flowers in which the stigma extended less ( <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.

Item Type:	Thesis - Research Master
Authors/Creators:	Padmanaban, Sriram
Keywords:	Onions, Onion industry
Copyright Holders:	The Author
Copyright Information:	Copyright 2007 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (MAgrSc)--University of Tasmania, 2007. Includes bibliographical references. Ch. 1. General introduction -- Ch. 2. General materials and methods -- Ch. 3. Characterisation of flower abortion in inbred male sterile onion parental line ON019A -- Ch. 4. Agronomic approaches to better understand flower abortion and improve seed yield in ON019A -- Ch. 5. Role of water relations in flower abortion in inbred male sterile onion parental line ON019A -- Ch. 6. General discussion
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