Improving Metribuzin Tolerance in Lentil (Lens culinaris)
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(Thesis)
Date
2019
Authors
McMurray, Larn Sharman
Editors
Advisors
Paull, Jeffrey
Preston, Christopher
Vandenburg, Albert
Preston, Christopher
Vandenburg, Albert
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Abstract
Weeds are a major limitation to lentil (Lens culinaris Medik.) production worldwide with grain yield losses of up to 87% from weed competition. In broad-acre mechanized lentil production systems, weed control relies on herbicide application; however, limited options exist. This study identified, characterised and validated novel tolerance in lentil to the photosystem II (PSII) inhibitor herbicide, metribuzin. Field research involving variable sowing dates, induced shade treatments and metribuzin rate were conducted to understand soil and weather factors responsible for herbicide phytotoxicity in lentil. Analysis of soil and weather factors around the time of herbicide application to the cultivar PBA Flash suggested a combination of factors were involved. Heavy rainfall within 10 days of application, particularly on light textured soils or where soil moisture was low, was most strongly linked to plant damage. A higher level of selective tolerance to metribuzin than that currently present in commercial lentil cultivars is required. Two methods, germplasm screening using a hydroponic sand assay and field screening of a large mutated population of PBA Flash, were used to identify lines with improved tolerance to metribuzin compared to current cultivars. Dose response experiments found germplasm line SP1333 had GR50 (the rate required to reduce dry weight (DW) 50%) values up to four-fold that of PBA Flash. However, GR50 values were greater than 25-fold that of PBA Flash in mutant selections M009 and M043. A field study in Canada with 20 Canadian and Australian genotypes confirmed the improved tolerance level of the mutants. Dose response analysis of five PSII inhibiting herbicides and DNA sequencing of the psbA chloroplast gene was undertaken to quantify the spectrum and mechanism of herbicide tolerance in M009 and M043. Compared to PBA Flash, metribuzin tolerance was increased 33-fold in M043 and 10-fold in M009, but no additional tolerance to other herbicides. Nucleotide sequencing of the psbA gene of both mutants identified a substitution at position 751 compared to PBA Flash. The resulting deduced amino acid sequence indicated an Ala251Thr substitution as responsible for the metribuzin tolerance. The substitution is unique in mutagenised higher plants and is the first report of an induced psbA target site mutation in higher plants. Reciprocal F1, F2 and F3 populations developed from M009 and M043 with PBA Flash identified a maternal inheritance pattern, but with paternal leakage in approximately 20% of F1 phenotypes. Reciprocal BC1F2 and BC1F3 populations were developed to identify any fitness cost associated with the tolerance. Field experiments identified reductions in net assimilation rate, DW and grain yield (GY) in tolerant lines with a fitness cost of 20 to 40%. This finding is comparable with the fitness cost measured in triazine tolerant (TT) canola due to tolerance to the PSII inhibiting triazine herbicides. Agronomic field experiments over two years at contrasting sites in South Australia compared the plant growth and GY of M009 and M043 with PBA Flash and SP1333 to post-emergent metribuzin. Clear differences existed in the responses of M009 and M043 compared with PBA Flash and SP1333 to metribuzin rate across sites. This finding confirmed that the mutant genotypes have an agronomically useful level of tolerance to metribuzin in southern Australia. However, DW was generally reduced linearly with metribuzin rate in both M043 and M009 suggesting a level of herbicide sensitivity at higher rates on some soil types. All three lentil genotypes with improved metribuzin tolerance are in use as parents in Australian breeding programs. The higher level of tolerance and superior agronomic performance of M043 makes it the genotype of choice. Knowledge of the genetic controls of inheritance and associated fitness cost of the target site provided by this study will aid plant breeders in rapid and effective incorporation of the tolerance into agronomically accepted plant types. The potential of developing a metribuzin tolerant lentil industry in Australia, similar to that which has occurred in TT canola, now exists.
School/Discipline
School of Agriculture, Food and Wine
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2019
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