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The effect of the fungicide phosphite on some aspects of plant reproduction

Fairbanks, Meredith (2001) The effect of the fungicide phosphite on some aspects of plant reproduction. PhD thesis, Murdoch University.

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Phytophthora cinnamomi Rands is a plant pathogen capable of infecting about 20% of native plant species in the south-west of Western Australia. The fungicide phosphite (phosphonate) provides an inexpensive and effective control of the pathogen. Phosphite can be applied by injection, soil drench, spray to run-off and misting. In the Mediterranean climate of the south-west of Western Australia operational sprays to native plant communities are applied in autumn or spring.

The up-take of phosphite was compared using spraying foliage to run-off with 0, 2.5, 5 and 10 gL-1 and misting with 100, 200 and 400 gL-1 phosphite. High Performance Ion Chromatography showed that the phosphite concentration in shoot apices of Corymbia (Eucalyptus) calophylla (marri) sprayed to run-off with 5 gL-1 phosphite, was comparable to the concentration in shoot apices of plants misted with 100 gL-1 phosphite. In plants sprayed with 10 gL-1 phosphite the concentration in shoots was comparable to that in plants misted with 200 or 400 gL-1 phosphite. In root apices, spray to run-off at 5 and 10 gL-1 phosphite gave comparable concentrations to a 100 and 200 gL-1 phosphite mist treatment. Operational treatments of native vegetation are carried out at 5 gL-1 phosphite for spray and 400 gL-1 phosphite for mist. The results on marri suggest that these treatments are not comparable in terms of phosphite uptake. Analysis of additional species is required to determine whether the concentrations used should be reviewed to obtain comparable results from spraying and misting.

Phosphite has in general, low phytotoxicity to vegetative plant parts, but its effect on sexual reproduction had not previously been evaluated. The effect on pollen fertility was studied in detail and it was shown that in some species pollen fertility was significantly reduced for many months after spraying. Phosphite had a varying effect on sexual reproduction of some annuals in the Eucalyptus marginata (jarrah) forest. It reduced pollen fertility of the annual Pterocheata paniculata when plants were sprayed in the vegetative stage and of Pt. paniculata, Podotheca gnaphalioides and Hyalosperma cotula when sprayed at anthesis. Seed germination was reduced by phosphite, in Pt. paniculata and H. cotula when plants were sprayed in the vegetative stage, and in H. cotula when sprayed at anthesis. Phosphite at concentrations of 5 and 10 gL-1 killed a proportion of plants from all3 species with up to 90% of Po. gnaphalioides plants dying.

Phosphite also affected sexual reproduction of some perennial species of the jarrah forest. In Dryandra sessilis, which flowers from autumn to early spring, pollen fertility was reduced by phosphite for up to 35 and 60 weeks after spraying in spring and autumn, respectively. Seed germination was not affected by phosphite. Pollen fertility of Trymalium ledifolium, a winter to early spring flowering species, was reduced by phosphite for up to 38 and 61 weeks after spraying in spring and autumn, respectively. Seed germination was reduced by phosphite 11 weeks after a spring spray. Phosphite was still detected in T. ledifolium shoots 62 weeks after spraying. In Lasiopetalum floribundum a species that flowers in spring, pollen fertility was reduced for 3 weeks when sprayed in spring. There was no effect on seed germination. Phosphite was also found to reduce pollen fertility of perennial species from the northern sandplains of Western Australia for up to 8 months after spraying.

The deleterious effect of phosphite was shown to be partly a result of its effect on cell division. Model plants Vicia faba, Petunia hybrida and Tradescantia virginiana, which are used for cytological research and are readily available and fast growing, were used to study this interaction. Phosphite significantly increased the number of chromosome laggards and bridges/stickiness in mitotic cells in the root tips of V.faba and P. hybrida sprayed with phosphite. Phosphite also increased the number of univalents and micronuclei in T. virginiana pollen microspore cells. The levels of abnormality, although significant, were not large enough to account for the level of pollen sterility observed.

Another mechanism for the effect of phosphite was the disruption of the normal timing of tapetum breakdown. Phosphite at 20 gL-1 brought about the premature breakdown of the tapetum in P. hybrida, and a small alteration in protein synthesis in the tapetum of treated plants, 7 days after spraying.

Phosphite has mutagenic properties. The Ames in vitro spot test showed that 400 gL-1 phosphite increased the number of DH5a Escherichia coli mutants by 60%. Tests of 24 hour cultures of this E. coli strain in solutions of 0.4, 4, and 40 gL-1 phosphite resulted in an increase in mutation rate by a factor of 10 to 100. Phosphite reduced E. coli growth after 24 hours exposure at concentrations as low as 0.4 gL-1.

From the findings of this thesis, it is recommended that the current management and application of phosphite be reviewed. The concentrations of phosphite that are currently applied as spray to run-off or mist are not comparable and measures should be taken to ensure that the application methods are comparable if equivalent effects are expected. Phosphite was found to have deleterious effects on plant sexual reproduction of both annual and perennial native Australian species. When species are in flower at the time
of phosphite application, their pollen fertility is significantly reduced. It would be beneficial therefore to spray when plants are not in flower. However, due to the limited period of ideal climatic conditions for aerial application, in autumn and spring, it is not feasible to apply phosphite at other times. However, when the treatment is being applied in order to protect rare species, it would be advisable to spot spray when the species is not flowering.

Publication Type: Thesis (PhD)
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
Supervisor: McComb, Jen and Hardy, Giles
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