An investigation of herbicide resistance in black-grass using safeners and synergists.

Sharples, C.R., 1996. An investigation of herbicide resistance in black-grass using safeners and synergists. PhD, Nottingham Trent University.

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With the expanding world population and the decreasing amount of land available for agriculture it is not possible to grow sufficient food without the use of agrochemicals. However, the advent of herbicide resistant weeds may jeopardise our ability to feed the world in the future. Black-grass (Alopecurus myosuroides) resistant to the herbicide chlorotoluron was first reported in the UK in 1982 and has since become wide spread in Autumn-sown crops. The aim of this study was to characterise chlorotoluron susceptible and resistant black-grass populations in terms of whole plant growth and photosynthesis, and to investigate the activity of glutathione S-transferases (GST) using herbicide safeners and synergists. The possible role for GSTs in chlorotoluron resistance was also considered.

Susceptible and resistant black-grass plants were grown in compost under glasshouse conditions and chlorotoluron was applied to the foliage at a range of concentrations. The ED50 values were calculated to be 0.93 and 39.3 kg ai ha-1 for susceptible and resistant black-grass, respectively, giving a Resistance Factor of 42 under defined growth conditions. The morphology and development of the plants over 14 days was further investigated by assessing several growth parameters. Susceptible plants treated with field-rate chlorotoluron ceased to grow 5-8 days after treatment, whereas treated resistant plants were unaffected. The number of tillers (6), leaf area (3000 mm2) and mean dry weight (160 mg) were the same in both biotypes (untreated) 5 weeks after sowing. Infra-red gas analysis was used to measure gaseous exchange by whole plants in specially designed chambers. Susceptible and resistant plants were equally able to perform photosynthesis at the flux densities examined, and the estimated quantum yield was 0.028-0.030 mol CO2 mol-1 absorbed photons. Resistant plants treated with field-rate chlorotoluron showed no reduction in photosynthesis over this period, whereas photosynthesis by susceptible plants had ceased after 10 hours and did not recover after a dark period.

Crude cell-free extracts were made from susceptible and resistant black-grass shoots and GST activity was measured in vitro using CDNB as substrate. Endogenous GST activity in resistant plants was twice that in susceptible plants, 83 and 154 nmol/min/mg protein, respectively. The GST activity could be further elevated in both biotypes by pre-treating plants grown in a hydroponic system with various herbicide safeners. Benoxacor was the most effective safener, elevating GST activity by 1.5 and 3 times in resistant and susceptible plants respectively. Plants grown in hydroponics were pre-treated with 100 ?M benoxacor for 24 hours and then supplied with 2.4 μM chlorotoluron for 24 hours, after which the photosynthetic rate was measured. Chlorotoluron alone reduced photosynthesis in susceptible and resistant plants by 65% and 25% respectively, but pre-treatment with benoxacor reduced those values to 47% and 0%. Thus, the increase in GST activity due to benoxacor could be correlated with a reduction in chlorotoluron injury to the plants. However, the nature of this relationship remains to be proved. It was hypothesised that elevated GST activity in resistant black- grass could protect plants from chlorotoluron by conjugating it with glutathione or by acting as a peroxidase to protect plants from oxidative stress. Alternatively, it may play no part in chlorotoluron resistance, but provide a mechanism whereby plants could be cross resistant to the herbicide fenoxaprop.

Item Type: Thesis
Creators: Sharples, C.R.
Date: 1996
ISBN: 9781369313482
Rights: This copy o f the thesis has been supplied on the condition that anyone who consults it is understood to recognise that its copyright rests with the author and that no quotation from the thesis and no information derived from it may be published without the author’s prior consent.
Divisions: Schools > School of Animal, Rural and Environmental Sciences
Record created by: Jeremy Silvester
Date Added: 04 Sep 2020 09:25
Last Modified: 28 Jun 2023 09:09

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