Studies of the effects of genotype and pre-flowering environmental conditions on the production of multicellular microspores were undertaken th four highly inbred lines of Brassica napus sap. oleifera. These lines were first grown in shaded and unshaded environments at 20/15°C arid unshaded at 30/25°C in a daylight phytotron. Buds were harvested from half the plants when first visible in the rosette and later from the remaining plants at the time when the first flower opened. The frequency of microspores at a specific stage of development varied widely within a relatively narrow range of bud lengths. Uninucleate microspores were not detected in anthers from buds less than 1·5 m or greater than 3·0mm long, but were generally present in frequencies of greater than 50 per cent in anthers from buds which were between 2·0 and 2·5 mm in length. However, the bud length at which the highest frequency of uninucleate microspores was detected varied significantly between genotypes and between the environments in which they were grown. Examination of the remaining anthers from each bud after a period in culture revealed that the proportion of microspores developing into multicellular units varied greatly with bud length, an increase in frequency of multicellular microspores being associated with an increase in the frequency of uninucleate microspores in the uncultured anther. Genotypes differed, however, in respect of the relationship between uninucleate microspore frequency and production of multicellular units. Although the frequency of multicellular units was as high as 57 percent, further development was limited and the number of embryolds formed was low in all cases (<10 per cent). The frequency of multicellular units in pollen samples from buds of a length in which uninucleate microspore frequency was high varied significantly with genotype, temperature and light conditions under which donor plants were grown, and the stage of inflorescence development at which buds were removed. Under conditions most conducive to multicellular unit formation (20/15°C, unshaded), the maximum frequency of multicellular units for each genotype in buds from young inflorescences ranged from 11·5 to 56·5 per cent. Shading or exposure to the higher temperature was associated with a marked reduction in production of multicellular units. Higher frequencies of multicellular units were generally detected in microspore samples from younger inflorescences irrespective of genotype or environment. Two of the four inbred lines were selected for a second experiment in which responses to vernalization and photoperiod duration were monitored. There was a significant reduction in the number of leaf nodes formed prior to floral initiation in both genotypes following exposure to vernalization and/or a longer photoperiod, the response to photoperiod being more pronounced. Exposure to 4 weeks vernalization was accompanied by a significant increase in the frequency of multicellular units in both genotypes, the frequency being double that in unvernalized plants under the longer photoperiod. By contrast, genotypes differed sharply in their response to photoperiod. In TB 20, the frequency of multicellular units was unaffected by an increase in day length irrespective of whether seed had been vernalized or not. On the other hand, in TB 42 the frequency of multicellular units was substantially greater in the 24 h day than in the 12 h day, being 27·3 per cent vs 13·0 per cent in the case of unvernalized plants and 66·7 per cent vs 18·2 per cent in the case of vernalized plants.
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Thurling et al. (1984) studied this question.