Examples

We present two examples that illustrate mutation breeding in citrus. The ‘Star Ruby’ grapefruit resulted from a mutation breeding programme in Texas (Hensz, 1971, 1977). Development of ‘Star Ruby’ was initiated in 1959 by irradiation of seed of ‘Hudson’ grapefruit, a red-fl eshed seedy cultivar with thermal neutrons. The exact dosage of ther- mal neutrons used was not reported. Seeds were planted in 1959. In 1960, buds from the seedlings were grafted on to sour orange rootstock and the resulting trees were planted in the field in the winter of 1961–62. Trees first set fruit in 1966. Budwood was released in March 1970, at which time only the original tree had pro- duced fruit. Thus no replicated yield testing was done prior to release. ‘Star Ruby’ has more intense fl esh colour than ‘Hudson’ as well as few (1–2) seeds per fruit. The deeper red rind and fl esh colour of ‘Star Ruby’ is not attributed to mutation because all nucel- lar seedlings of ‘Hudson’ have a colour sim- ilar to that of ‘Star Ruby’ (Hensz, 1981). Evidently the ‘Hudson’ is chimeric, carrying genes for deep red colour in the L-II layer which are expressed in the non-chimeric nucellar seedlings. Despite its deep colour and few seeds, ‘Star Ruby’ shows a number of horticultural problems including poor production in some areas, low tree vigour and increased susceptibility to some herbi- cides (Gmitter, 1995). It is interesting that most of these problems are reported in the 1971 paper, and thus were apparent in the original tree. Possibly some of these traits are due to deleterious mutations induced by irradiation. ‘Star Ruby’ does not appear to be chimeric – its characteristics have remained quite stable.

The ‘Mor’ mandarin is a low-seeded selection of ‘Murcott’ produced by bud- wood irradiation (Vardi et al., 1993). About 400 buds of ‘Murcott’ were irradiated with

Fig. 16.2. Mutation breeding programme at the University of California, Riverside.

35 Gy of g-rays from a 60Co source. Individual buds were grafted on sour orange rootstocks. Buds from these trees were regrafted about 6–9 months later and the trees were fi eld planted in 1987. Fruits were observed in 1989 and 1990. One tree, named selection 13/24, was distinct from ‘Murcott’ in having 5–7 seeds per fruit

whereas ‘Murcott’ had about 20–25 seeds per fruit. This tree had a more compact growth (one-third to one-half the size of ‘Murcott’) and fl owers with low pollen fer- tility. Productivity is reported to be similar to that of ‘Murcott’. Details on fi eld trial evaluation of this variety have not been reported.