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Title: Effects of the bipyridylium herbicides paraquat and diquat on the ultrastructure and physiology of a duckweed and a blue-green alga
Author: McGinlay, D. C.
ISNI:       0000 0001 3624 2587
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 1976
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Electron microscopical techniques were used to investigate the effects of treatment with paraquat and diquat on cell structure in the duckweed Spirodela oligorrhiza and the blue-green alga Anabaena cylindrica. Treatment of fronds of S. oligorrhiza with both herbicides in the light resulted in ultrastructural damage which clearly preceded any outwardly visible changes in their appearance. Ultrastructural changes developed rapidly, occurring first in the presence of diquat. Paraquat and diquat also caused ultrastructural damage when applied in complete darkness but did so at a much slower rate. The absence of appreciable amounts of catalase or peroxidase activity within the chloroplasts and mitochondria of frond cells was demonstrated by a histochemical technique. The growth of S. oligorrhiza in the light was inhibited by low concentrations of paraquat and diquat. Diquat had the greater inhibitory influence in terms of the reduction of frond number and fresh and dry weights compared with controls. Reduction of the growth rate was detected with both herbicides before the appearance of outwardly visible symptoms of toxicity. Paraquat and diquat caused a rapid destruction of chlorophyll in light-treated fronds, the loss being greater in the presence of diquat. The continued loss of chlorophyll from diquat-treated fronds was shown to be light-dependent. The ability of diquat to cause the loss of chlorophyll from treated fronds could be lessened by the simultaneous application of a tetrazolium salt. Both paraquat and diquat significantly increased the rate of respiration in the dark and caused a rapid inhibition of apparent photosynthesis in the light. The stimulation of respiration and the inhibition of photosynthesis was greater with diquat. Low concentrations of paraquat and diquat caused rapid ultrastructural damage in vegetative cells of A. cylindrica in the light. Under the conditions employed, both herbicides appeared to produce identical effects at about the same rate. The first observable symptom was the gradual disappearance of the polyglucoside granules. This occurred prior to any visible membrane damage. Subsequently, the plasma membrane disintegrated and cell structure deteriorated rapidly. Damage to the heterocysts also occurred in the light but at a slower rate than in vegetative cells. In the akinetes no effects were noted after periods sufficiently long to allow the total destruction of the other cell types. In darkness, diquat caused no visible effects in vegetative cells after durations causing their complete dastruction in the light. Electron microscopical evidence was obtained pointing to the inhibition of carbon fixation in diquat-treated vegetative cells in the light prior to ultrastructural damage. Manometric investigations with A. cylindrica also revealed the rapid complete inhibition of apparent photosynthesis in the resence of paraquat or diquat. There was no significant difference between the effects of paraquat and diquat on the rate of apparent photosynthesis. The time required to completely inhibit apparent photosynthesis was far shorter than the period required to cause visible ultrastructural damage to the cells. The results of the investigations carried out with S. oligorrhiza and A. cylindrica are discussed in relation to existing information concerning the effects and mode of action of the bipyridylium herbicides.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral