Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, Saskatchewan S7N 0X2 Canada.
Many researchers have investigated the potential benefits of apetalous plants of Brassica which are thought to yield more seed due to greater light penetration into the plant canopy and the avoidance of sclerotinia stem rot disease. A new apetalous B. napus mutant was identified in a breeding nursery at AAFC-Saskatoon in 1996. Best apetalous plants were selected and selfed; doubled haploid plants were also produced. In 1998, 250 selfed plant progeny and 68 doubled haploid lines were field tested in a replicated nursery at Saskatoon. The rows were rated for the apetalous trait and best lines identified for further selection. The apetalous characteristic was strongly expressed at the early and full bloom stages, but late developing flowers had petals that were reduced in size and number. No information on the genetic control of this trait is available for this source of apetalous material. Future research will focus on understanding the factors that contribute to lower levels of sclerotinia stem rot in apetalous plants, and the introgression of this trait into elite breeding populations of B. napus.
KEYWORDS: Sclerotinia stem rot avoidance, yield increase.
The development of apetalous varieties of Brassica napus has been of interest to canola breeders for many years. There are two reasons given in the literature for developing such varieties. Plant pathologists have observed that petals falling off rapeseed plants land in leaf axils where they senesce. Senescing petals provide a saprophytic food base for the germination of ascospores of the sclerotinia stem rot fungus and the growth of the mycelium into leaves and stems of plants. Without the food source provided by the petals, as is the case with apetalous plants, the fungus’s ability to infect plants is greatly reduced (Kapoor et al., 1983).
The development of apetalous B. napus varieties could therefore serve as a mechanism to substantially reduce infection of canola by the stem rot fungus. This is particularly important since efforts by canola breeders to develop genuine stem rot resistant B. napus have proven largely unsuccessful. The only disease control mechanism available to canola growers to date is fungicidal control (Dueck et al., 1983); however, the use of fungicides to control stem rot is often not economically feasible, since it is difficult to predict if a field is at risk of infection (Morrall et al., 1991).
The potential effect of the apetalous character on significantly increasing seed yield has been reviewed by Mendham et al. (1991). The yellow petals in a canola field reflect or absorb up to 50% of the incoming solar radiation, which limits photosynthetic efficiency of the canopy during flowering. Sunlight would penetrate the canopy of an apetalous crop more effectively, resulting in higher photosynthetic rates and higher seed yield. In a comparative study of petalled and apetalous B. napus lines with similar vegetative growth and silique number conducted in Tasmania, the apetalous line produced more seeds per silique, larger seeds, a better harvest index, and about 40% higher seed yield (Rao and Mendham, 1987). If a similar increase in seed yield with apetalous B. napus could be achieved on the Canadian prairies, the apetalous character would be well worth exploring. The first apetalous B. napus genotype was identified in Australia (Buzza, 1983) and this material has been developed into a variety in Canada. In this material, the apetalous character is controlled by recessive alleles of two genes. This report describes a new source of apetalous B. napus discovered in breeding material at AAFC-Saskatoon and the work that has been performed on this material over the last three years.
MATERIAL AND METHODS
The new apetalous mutant of B. napus was identified in the F5 line YN94-673, derived from a 3-way cross [N89-53 x (YN90-1018 x Shiralee)] in a field nursery at Saskatoon in 1996. This cross was performed to develop yellow-seeded, high oil content, blackleg resistant B. napus. The yellow-seeded breeding line YN90-1018, a parent in this cross, was derived from interspecific crosses of B. napus with B. alboglabra and B. rapa. This line has abnormal petal development (small petals) and could have been the source of the apetalous mutant allele in YN94-673. Seventeen completely apetalous plants were found that had normal plant vigour and developed normal sized leaves and self seed was produced on eight of these plants in the field. The selfed seed of the eight plants was planted in pots in the greenhouse during the winter of 1996/97 and seed produced. The production of doubled haploid apetalous lines was also initiated via microspore culture. In 1997, a field plot of the inbred apetalous material was grown and self seed of fully apetalous plants individually harvested. In 1998, 250 selfed plant progeny and 69 doubled haploid lines were field tested in a replicated nursery at Saskatoon. The rows were rated for the apetalous character during the flowering period.
RESULTS AND DISCUSSION
It was found that the first flowers that opened on the main raceme and on side branches were completely apetalous. Flowers that opened towards the end of the flowering period developed very small petals, and often had only one or two petals instead of the normal four petals. The reduction in the total number of petals formed on plants, however, was greater than 95%, which is a significant reduction. This reduction is of a magnitude that would drastically reduce the number of petals falling off flowers and landing in leaf axils where they would senesce and serve as the basis of stem rot infection. In fact, we have shown with other apetalous material in 1995 that the incidence and severity of stem rot disease was much lower in plots of apetalous B. napus (9% incidence, 6.6% severity) compared to plots of B. napus canola AC Excel (32.5% incidence, 27.5% severity). The apetalous character in B. napus would therefore be an effective strategy to significantly reduce stem rot disease in canola.
The apetalous character of our new mutant is not associated with abnormal leaf development as was the case with apetalous germplasm developed by Dr. Glen Hughes, Plant Breeding Institute, Cambridge, UK. Our mutant is therefore of much greater value for use in a plant breeding programme than the Cambridge mutant. Work in the future will focus on the introgression of the apetalous trait into elite breeding material and the study of its inheritance.
The project was financially supported in part by the Canola Germplasm Development Program of the Canola Council of Canada, Winnipeg, Canada, and by the Matching Investment Initiative of the Government of Canada. The technical assistance of D. Rode, J. Relf-Eckstein, and J. Helston is gratefully acknowledged.
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