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Identification and management of factors limiting hybrid carrot seed production in Australia

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Spurr, C (2003) Identification and management of factors limiting hybrid carrot seed production in Australia. PhD thesis, University of Tasmania.

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Abstract

Worldwide, carrot seed production is a highly dynamic industry, with vegetable breeders
contracting the production of proprietary varieties to areas where technology and climate
combine to give the most reliable yields of high quality seed. Australia has had a
significant share of the world market for carrot seed production since 1984, servicing
customers in Asia, Europe and the United States. Since the mid 1990s, requirements for
increased quality and reliability of production, coupled with improvements in the
production standards of other carrot seed exporters, have threatened Australia's market
share. The failure of a significant number of Australian crops to meet the current
minimum production standard of 85% seed germination required by export markets has
been the major issue faced by the industry during this period. In addition, producers have
experienced difficulty in consistently achieving satisfactory yields of seed of some hybrid
varieties.
The present study was undertaken to address both of the issues facing Australian
producers, with an emphasis on achieving improvements in the production of seed of
European hybrid carrot varieties. Preliminary work established that the problems of low
germination and unreliable yields were largely unrelated, leading to two research themes
based on cause and management of low germination and unreliable seed yield.
Low germination of Australian seed lots was closely correlated to the occurrence of seeds
without embryos or with embryos exhibiting extensive physical damage. Surveys of
carrot fields and caging trials in South Australia and Tasmania established that both
conditions resulted from feeding of the endemic insect, Nysius vinitor Bergroth
(Hemiptera: Lygaeidae) on developing carrot seeds. Adult N. vinitor were found to infest
carrot seed crops in a series of migratory flights from shortly before flowering until
harvest. In Tasmania evidence was collected that suggested that a significant proportion
of migrants originate from nearby areas of weedy host species. During peak periods of
migration populations of up to 17 insects per carrot plant were observed. Field based caging trials showed that loss of germination due to N. vinitor feeding could occur from
flowering through to harvest. For individual male sterile plants, daily reductions in seed
germination of 0.04 to 0.11% per insect were observed during this period.
Because of the need to respond quickly to N. vinitor infestation to minimise germination
loss, work was undertaken to develop protocols for monitoring the insect in carrot seed
crops. In trial plots, sticky traps detected migrations of N. vinitor into carrot fields, with
the number of insects caught closely correlated to average population densities on the
plants. In commercial crops N. vinitor population densities were non-randomly
distributed across the fields, with gradients associated with the prevailing wind directions
during the periods of migration. Thus, the positioning of the sticky traps was shown to be
important for reliable detection of N. vinitor as well as accurate estimation of population
size.
The issue of unreliable seed yield was examined in two hybrid crosses, No. 22, a 3-way
(Fl male sterile line) Nantes hybrid with brown anther cytoplasmic male sterility (CMS)
and W0030, an Fl Nantes hybrid with petaloid CMS. Two alternative hypotheses; source
limitation (assimilate) and pollination limitation, were tested to explain the occurrence of
low seed yields. Shading, umbel removal and leaf trimming treatments applied to both
hybrids over two seasons did not significantly affect seed yield, seed size or seed quality.
Supplemental hand pollination treatments increased seed yield by up to 284%, providing
evidence that seed yield was strongly limited by inadequate pollination in No. 22, and to
a lesser extent in W0030. In both hybrids, pollination was restricted by variable pollen
viability at anthesis, low pollen longevity under field conditions and low rates of pollen
transfer to the male sterile line.
Commercial hybrid carrot seed production is based on the strip method of hybrid seed
production in which rows of the pollinator line are alternated with the male sterile line.
Within the strip method, varying the ratio of pollinator and male sterile lines between 1 :4
and 4:4 and distance between adjacent male beds between 2.4 to 7 .2m had no effect on
pollination or seed yields of No. 22. Surveys of insect pollinator visitation, pollen loads and foraging patterns explained these results. Honeybees, Apis mellifera ligustica
Linnaeus (Hymenoptera: Apidae) and two Dipteran species Calliphora ruficornis
Macquart (Diptera: Calliphoridae) and Eristalis tenax Linnaeus (Diptera: Syrphidae)
effected most pollination in carrot seed plots. All three species showed a strong tendency
for directional foraging within a single row of plants, thereby restricting the opportunities
for pollination of the male sterile line. These findings suggest that the strip method of
production may be incompatible with the foraging behaviour of some important
pollinators of hybrid carrots and hence the attainment of optimum seed yields in some
varieties.
This thesis identifies significant opportunities for improvement of the quality and
reliability of hybrid carrot seed production in Australia. Management protocols for N.
vinitor derived from the research have contributed to an increase in the percentage of
commercial carrot seed crops achieving the germination standard required for export
from below 70% prior to 2000 to I 00% in 2003. Studies of yield variability identify
inadequate pollination as a contributing factor. Efforts to improve pollen viability and
pollen transfer in commercial production may contribute to greater reliability of hybrid
seed yields.

Item Type: Thesis (PhD)
Keywords: Spurr, cameron
Additional Information:

Copyright 2003 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)

Date Deposited: 24 Mar 2014 01:21
Last Modified: 15 Sep 2017 01:06
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