PROSPECTS OF INTERSPECIFIC HYBRIDIZATION IN DUTCH IRIS
The assortment of Dutch iris consists on the one hand of cultivars with a maximum flowerable bulb size of 6–8 cm circumference ('small sized irises') and on the other hand of cultivars with a maximum flowerable bulb size of 8–10 cm ('large sized irises').
Large sized cultivars are much more important than small sized ones, because they can be brought into flower almost year-around; cv 'Ideal' and its sports even all yearround. However, there are only a few large sized cultivars and their variation in flower colour is limited.
Large sized cultivars are interspecific hybrids (2n=31) which originated from crosses between small sized cultivars (2n=34) and the large sized species Iris tingitana (2n=28). Because they are sterile, they cannot be used for further breeding.
The sterility of these hybrids can be broken by doubling the number of chromosomes. Besides, it is expected that by doubling the chromosome number of small sized irises (2n=34) and that of I. tingitana (2n=28), followed by crosses between the tetraploids obtained, also fertile large sized hybrids (2n=62) can be bred.
By scaling the hybrid 'Ideal' (2n=31), and treating the small scales with colchicin, fertile amphidiploids (2n=62) were obtained. In this way sterility was broken. However, applying this in vivo method to other hybrids was less effective because of the frequent occurrence of infection in the colchicin treated scales.
Therefore, present research in cooperation with the Central Research Laboratory for Tissue Culture of Horticultural Crops (COWT) in Lisse is aimed at the development of a reliable method to duplicate the number of chromosomes by treating bulb scales with colchicin in vitro.
In order to obtain more fertile amphidiploids, other sterile hybrids, comparable with 'Ideal', will be treated with colchicin. These hybrids could be crossed with one another and with the above mentioned amphidiploid hybrids (2n=62) in order to obtain large sized irises with a wider colour variation, which can be brought into flower year-around.