| Unlike many parts of Europe, very little has been done to assess the causes and extent of emergence of volunteer oilseed rape in China. Once dormancy has been induced, rapeseed can persist in the soil seed bank for five or more years. This situation poses problems not only for growers of broad-leaved crops that may follow rapeseed in the rotation, but also for breeders and for growers of rapeseed itself. The associated problems with this weed include its competitiveness, difficulty of control, and the reduction of quality through genetic contamination of pure lines of rapeseed and the like.Volunteer rape, originating from seeds left in the field has been associated with premature dehiscence of pods causing seed losses between 0.1 - 1.6 t/ha or even more, depending on the weather condition before and during the operation. Although there is little or no primary dormancy exhibited by oilseed rape, the absence of certain environmental cues that promote germination of imbibed seeds induces secondary dormancy. It is believed to be forced into dormancy (secondary/induced dormancy) by abiotic stress conditions such as light, moisture, anoxia or hypoxia and temperature.Given the above situation, the question that needs the attention of breeders and farmers alike is how to minimize the occurrence of volunteer rape. Breeding for pod shattering resistance from synthetic Brassica napus was suggested, but this has not so far led to improved cultivars. It was also suggested that the safest strategy to prevent future problems of volunteers is to leave the stubble untouched for an appreciable length of time before the next crop. However, the best opportunity for preventing or reducing the incidence of volunteers lie in exploiting breeding opportunities in oilseed rape associated with genotypic variation in the development of secondary dormancy.It is against this background that this work was conducted. It had the following objectives: 1) to determine the level of dormancy induction ability of the following environmental stresses: water deficit, overabundance of water (flooding/waterlogging), oxygen deficiency and temperature. All imposed in darkness, mirnicking actual field conditions. Such environmental stress conditions are also detrimental to the growth and development of oilseed rape. 2) to assess the differences in the development of secondary dormancy between and within genotypes, including differences between storage periods and seed lots of the same genotype. 3) to exploreopportunities for selection and breeding genotypes for zero development of secondary dormancy. 4) to assess the agronomic impact of volunteer rape on the growth and development of other crops that follow in the rotation.Of the environmental factors tested, osmotic stress was most effective in inducing dormancy. Anaerobic treatment produced very few dormant seeds, as did an atmosphere low in oxygen and high in nitrogen. Marked variations were observed in the development of secondary dormancy tinder osmotic stress, between and within genotypes, but variation between breeding methods remains unclear. Percent dormancy ranged from almost zero to about 60% for winter genotypes and about 85% for the spring types. Within genotypes, variations occurred between seed lots and years of harvest. Temperature variations affected the percent dormant seeds produced. More dormant seeds were likely to be produced with incubation under osmotic stress at 20℃ than at 12℃. hi winter genotypes fewer dormant seeds were produced when incubation temperature and germination test temperatures differed. Thus, incubating at 20℃ and 12℃, followed by germination tests at 20℃ and 12℃ respectively produced more dormant seeds. Also in the winter genotypes, the potential development of secondary dormancy was positively correlated with the pattern and speed of germination of untreated seeds.Results from this work indicate the possibility of selecting genotypes that have a low potential in the development of secondary dormancy. Among the Chinese genotypes used, Shu...
|
PDF Full Text Request