β Scribed by G. Hekstra; C. Broertjes
No coin nor oath required. For personal study only.
To make the sterile iris cultivar "Wedgwood" accessible to selection, bulbs were irradiated with X-rays. The aim is to induce flower colour mutations in this early flowering cultivar which is specially suitable for early forcing.
The optimal dose turned out to be 1000 rad X-rays ; the best moment for irradiation is immediately after lifting the bulbs in August when the growing points, which will form the new bulbs, are in the youngest possible ontogenetic stage. The size of the bulb does not seem to be an important factor.
Since 1964 almost 6000 bulbs, representing approximately 25,000 growing points. have been irradiated. So far over 160 flower colour mutants, ranging from paler to darker blue with or without purple, have been obtained. Moreover, a number of other mutants -more flowers per stalk, other form of the flower or different flowering dates -have been observed.
These results are discussed as well as other possible ways to reach the goal.
Mutation induction is one of the methods for obtaining variability. This method offers great possibilities in vegetatively propagated plants, because most of such plants are (highly) heterozygous so that mutations, in most cases from dominant to recessive, can be detected in the irradiated material itself. A mutated shoot or other plant part can usually be propagated by a suitable method and the subsequent clones can finally be compared with each other and with existing cultivars. This especially holds true for ornamentals when a visible character, such as flower colour, is concerned. Starting from a good variety, it is thus possible to alter just one character or a few only, without interfering with the others. This is practically impossible by conventional breeding methods. In sterile plant varieties, such as triploids, species hybrids etc. the only way of inducing variability and making them accessible to selection, is mutation induction.
A recent example of the successful application of these principles has been shown in the garden dahlia, in which four commercial cultivars could be released in no more than 3-4 years after the mutagenic treatment (BROERTJES and BALLEGO, 1967).
Although mutation breeding has been practised in bulb crops since the early thirties by W. E. de Mol in The Netherlands (MOL VAN OUD LOOSDRECHT, 1953) it is only from 1960 that a program was started with the bulbous iris cv. 'Wedgwood"
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