Studies on induction of flower color mutants in gladiolus (Gladiolus x grandiflora Hort.) by gamma irradiation and tissue culture

Most of the commercial varieties of gladiolus have been raised by cross breeding during the last 10 decades. Although the mutation breeding is known to be an effective means for obtaining novel varieties in vegetatively propagated ornamental plants in these days, mutant isolation has been restricted by the phenomenon of diplontic selection and subsequent chimera formation that was due to the multicellular origin of the plants. The purpose of this study was to establish the method for obtaining solid mutants of gladiolus having novel flower color by using gamma irradiation and / or tissue culture. 1) Comparative studies on the capability of various explant sources for regenerating plants revealed that the cormel shoot apex was superior to the cormel pieces and leaf blades for its ability of embryogenesis, cost of preparing materials and easiness to be handled. Histological observation confirmed that the induced regenerants were derived from adventitious embryos. There was varietal difference in the ability of somatic embryogenesis. The plants derived from cormel apices were almost identical with the original variety Traveler' in the flower characteristics compared. Thus, a simple and efficient culture system in gladiolus was established for obtaining regenerants by using cormel shoot apices as explants. 2) To isolate solid mutants in flower color from mutation sectors in the perianth by tissue culture, the cultural responses of various parts of flowers were compared at different ages. Ovaries obtained from 2 to 3 days before flowering were superior to other flower parts in regenerating adventitious shoots through callusing. The highest efficiency in callus formation was obtained using MS medium supplemented with 5mg/l NAA + 5mg/l BAP, while that containing 2mg/l BAP was most effective in inducing shoots. Varietal differences were observed in the ability of shoot differentiation from ovary-derived calli but not in the ability of callus differentiation. 3) In many of the regenerated plants from cormel shoot apices, flower color mutation was found in smaller parts than half of the tepal. In some of the regenerants, however, whole tepals exhibited mutant color. The flower color mutation was observed in 'Traveler' and 'Hector', but not in 'Topaz'. The mutation with both lighter and deeper color as compared with the original plant was obtained. Flower color mutation was also found in conventional vegetatively propagated plants as smaller sectors than half of the tepals. Here again, flower color mutation was not found in 'Topaz'. 4) Flower color mutation was investigated in the M1 plants grown from gamma-irradiated cormels and corms as well as in the plants chronically irradiated in the gamma-field. The chronically irradiated plants exhibited smaller mutation sectors only than half of the tepal. The frequency of mutation and the growth retardation increased in the M1 plants with the exposed dosages of gamma-ray. The optimum dosage for inducing flower color mutation was estimated to range from 100 to 200 Gy at the rate of 10 Gy/h. Although no conspicuous differences were found in the frequency and the spectrum of flower color mutation between the M1 plants grown from cormels and corms, cormels were considered to be more suitable than corms as the materials for gamma-irradiation considering the size of mutation sectors observed, the cost of growing and irradiation and the easiness to handle. 5) Ovaries taken from the flower buds in which tepals mutation sectors were observed were divided into pieces of various size and cultured to obtain regenerants through callusing. It was proved that the ovary pieces divided up to 1/8 of the whole had the capability to form calli and to regenerate plants. Solid mutants of flower color were successfully obtained by culturing ovary pieces adjacent to the tepals in which mutation sectors were observed. 6) Gamma-irradiation was applied either to cormels prior to culture or to cormel-derived callus. Although severer depression of growth and embryogenesis was brought about by the irradiation to the cormels than to the callus, the irradiation to the cormels was recommended as the flower color mutants were obtained at higher rate. 7) Six flower color mutant lines were obtained by the methods mentioned above. These lines showed variations in other traits than flower color such as reduced plant height and decreased floret numbers as compared with the original varieties 'Hector' and 'Traveler'. It was indicated that we should consider not only flower color but also other agronomic traits in order to select elite lines in mutation breeding for flower color of gladiolus. It is also noticeable that the reversion of the flower color to that of the original variety was observed although at very low frequency in some lines. 8) In conclusion, through the experiments stated here, the method of mutation breeding was established for obtaining novel flower color mutants in gladiolus. This method would be applicable to wide range of monocotyledonous ornamental plants which are vegetatively propagated.