ABSTRACT

The feasibility of using apetalous Ogura cytoplasmic male sterile lines to produce hybrid seed was investigated. Initial trials revealed that apetalous male sterile lines with varied genetic backgrounds produced significantly lower yields than conventionally petalled, male sterile lines. Subsequent field trials indicated that the lower yields were the result of the apetalous male sterile lines setting a lower percentage of productive pods from potential pod sites and fewer seeds/pod than the petalled line. The cause of the reduced productivity was inadequate pollination of the apetalous flowers by honey bees, which are the main pollinators of hybrid canola seed crops in Tasmania.

INTRODUCTION

There have been a number of studies which have investigated the potential benefits of using the apetalous flowering characteristic to produce a more efficient crop canopy during flowering (Rao et al., 1991, Fray et al., 1996). This research project investigated the feasibility of producing apetalous hybrid seed. Potentially this could lead to higher crop yields by capitalising on both the heterosis exhibited in the F₁ generation and a more efficient flower canopy.

With this objective in mind, male sterile apetalous lines containing Ogura type cytoplasm were used to investigate yield, pod set, seed set and pollen transfer under both field and glasshouse conditions.

EXPERIMENTAL

Field trials were conducted over three seasons from 1994/95 to 1996/97 at the University Farm, Cambridge, Tasmania. Three male sterile apetalous A lines and a petalled male sterile A line were grown in randomised blocks, which were replicated twice. Up to 20 rows of A lines were planted on either side of four rows of pollen producing B line plants. The B lines are necessary to produce A line seed and are male fertile plants, identical to the corresponding A line, except for the male sterile cytoplasm in the A line.

Other treatments included hand pollination of petalled and apetalous flowers and a glasshouse experiment which investigated the amount of pollen deposited on petalled and apetalous flowers after exposure to honey bees in the presence of pollen producing B lines.

RESULTS

At least one of the apetalous B lines used in this study was shown to have similar yield potential to the petalled B line, however the corresponding A lines yielded significantly less than the petalled A lines over all seasons (table 1).

Despite the apetalous A lines having different genetic backgrounds they exhibited similar yield component patterns over all seasons (table 1). This was characterised by high numbers of aborted pods, a large proportion of productive pods produced on tertiary branches (data not presented), and low numbers of seeds/pod.

Flowers on the mainstem of apetalous and petalled A lines were pollinated by hand to determine if pollination by natural means was limiting. The percentage of productive pods set from potential pods were compared between hand pollinated and control plants, as was the number of seeds/pod. Hand pollination of apetalous A line flowers significantly increased the percentage of productive pods and the number of seeds/pod on plants located one row from a viable pollen source and in the presence of honey bees (table 2). In contrast hand pollination of the flowers of a petalled A line (20894) did not increase either of these yield components.

The results suggested that insufficient pollen was deposited on stigmas of the apetalous A line flowers by pollinators present in the field. In Tasmania the primary pollinators are honey bees Hives were in close proximity to experimental plots for all trials conducted.

Observations made of bee behaviour in the field indicated that similar numbers of bees visited both petalled and apetalous A line flowers (table 3). However, significantly fewer bee visits were recorded as being “successful” on apetalous A line flowers in comparison with the B line (table 3). A flower visit was scored as successful if during the time on the flower the bee made direct contact with the stigma which would allow pollen transfer to occur. During the majority of visits to apetalous flowers honey bees would “side-work” the flower to reach the nectaries located at the base of the flowers, rather than move over the top of the flower and make contact with the stigma.

The number of pollen grains deposited on the stigmas of both petalled and apetalous A line flowers after exposure to a pollen source and honey bees (table 4), indicated that significantly fewer pollen grains were deposited on apetalous flowers. Even the presence of one petal (as occurred frequently with line 1806), resulted in significantly more pollen grains being deposited on the stigma.

DISCUSSION AND CONCLUSION

The lower seed yields produced by apetalous A lines in comparison with apetalous B lines or petalled A lines was the result of reduced pod and seed set. This was not the result of poor agronomic characteristics, but rather lack of pollination. The absence of petals reduced the likelihood of bees making contact with the stigmatic surface of the male sterile flowers. This resulted in fewer pollen grains being deposited on the stigma, despite the apetalous flowers being equally as attractive to honey bees as conventional flowers.

In effect, the apetalous A line flowers required a greater number of bee visits to individual flowers if all fertile ovules were to be successfully pollinated. Practically this could be achieved by increasing the numbers of pollinators in the crop, or reducing the number of A lines between the pollen source rows.

If an economically viable yield of hybrid seed is set at 500 kg/ha (Pinnisch and McVetty, 1990), then based on the findings in this investigation up to 32 A line rows could be planted between pollen donors (data not presented). This indicates that seed production should not be a limiting factor in the development of apetalous hybrids. The overall viability of the concept will depend on the yields obtained from the F₁ hybrids. At this point in time Ogura restorer lines are close to being available, but have not yet been incorporated into apetalous material.

Table 1. Combined yield and yield component analysis for hybrid seed production trials, 94/95, 95/96 and 96/97 seasons. Productive pods were pods which contained seed at harvest, aborted pods were pods which were not fertilised. * Petalled line.

Table 2. The numbers of seeds/pod and the percentage of productive pods set from potential pod sites on the main stem of hand pollinated and control plants. Significant differences between lines within treatments are denoted by superscripts, where means with the same superscript are not significantly different. Underscoring indicates means not significantly different for treatments and sampling times within lines and line means.

Table 3. The number of bees/m2 recorded on the respective lines and the percentage of successful bee visits. Significant differences (P<0.05) are denoted by superscripts.

Table 4. The number of pollen grains on the stigmas of A line flowers from plants grown under the low temperature treatment.

REFERENCES

1. Fray, M. J., Evans, E. J., Lydiate, D. J. & Arthur, A. E. (1996). Physiological assessment of apetalous flowers and erectophile pods in oilseed rape (Brassica napus). Journal of Agricultural Science, Cambridge 127: 193-200.

2. Pinnisch, R. & McVetty, P. B. E. (1990). Seed production of hybrid summer rape in the field using the pol cytoplasmic male sterility system: a first attempt. Canadian Journal of Plant Science 70: 611-618.

3. Rao, M. S. S., Mendham, N. J. & Buzza, G. C. (1991). Effect of the apetalous flower characteristic on radiation distribution in the crop canopy, yield and its components in oilseed rape (Brassica napus). Journal of Agricultural Science, Cambridge. 117: 189-196.