Determination and manipulation of the ploidy level of purple Asparagus officinalis L.
This thesis reports on the examination of purple asparagus (Asparagus officinalis L.). Purple asparagus is a novel asparagus genotype which originated in countryside around the Northern Italian town of Albenga, and was thus named Violetto d'Albenga. Selections from this population were termed Violet of Albenga. The original population segregated for colour from a normal green through to a deep purple/burgundy colour. Selections for deeper colour tones has led to the selection from and breeding of two cultivars, Purple Passion and Pacific Purple. Observations of purple asparagus had indicated that it was larger than green asparagus. Quantitative measurements of purple asparagus and green asparagus were made and compared. From this investigation it was found that purple asparagus was significantly larger than green asparagus. Previously published observations of the size of the nuclei of purple asparagus indicate that they are larger than green asparagus nuclei, indicating that an increase in the amount of DNA in purple asparagus cells had occurred. This hypothesis was firstly tested by the examination of asparagus nuclei by flow cytometry. The results form this investigation showed that purple asparagus nuclei contained a DNA content similar to that of a standard tetraploid green asparagus. Further investigation via conventional chromosome counting confirmed that purple asparagus was a tetraploid with 2n=40 chromosomes. Purple asparagus contains a number of traits that would be highly desirable to be transferred to diploid populations. The most desirable of these traits is the increased sweetness. Attempts to cross purple with regular green diploid asparagus have been unsuccessful, although the reasons for this are not clear but it may be due to the differing ploidy levels. In theory the production of a purple asparagus haploid could allow green and purple asparagus to be crossed. In this thesis haploid plants were produced via the screening of 113130 purple asparagus seedlings for the production of spontaneous haploid in the form of twin seedlings. The separation of these twin seedlings provided 15 putative haploid of which 4 were found to be haploids (2n=20). Unfortunately none of the haploids produced flowered during the course of this research and it was not possible to attempt crosses with green asparagus plants. To further understand the nature of tetraploid purple asparagus and the origin of its polyploid level an attempt was made to examine the segregation of molecular markers. It was hoped that an examination of the PCR products of RAPD and satellite sequences of parental tissue and progeny would provide sufficient information to determine whether purple asparagus is an allotetraploid or an autotetraploid. Unfortunately time constraints meant that this objective was not achieved. The results and findings from this research has conclusively proved that purple asparagus is a tetraploid and that it is significantly larger than green asparagus. Furthermore it is possible to produce a haploid of purple asparagus.... [Show full abstract]