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Physical mutagenesis is an effective mutational breeding method for improving various growth and floral characteristics of ornamental plants. The experiment was undertaken with an objective to investigate the effect of various dosages of gamma irradiation on survivability, growth and floral characteristics of Jasminum officinale plants raised by rooted cuttings. The 60 CO Gamma irradiation source at Horticultural Crop Research and Developmental Institute, Gannoruwa was used to expose rooted plants to different dosages such as 0Gy, 17Gy, 21Gy, 25Gy, 29Gy and 33Gy. A Complete Randomized Design with four replicates having 10 plants for each replicate was practiced. The results revealed that gamma irradiation dosage from 25Gy to 33Gy significantly decreased the survivability of J. officinale plants (P<0.05). LD50 of gamma radiation on J. officinale was occurred around 33Gy at maturity stage. Plant height and shoot length were significantly reduced by gamma irradiation dosages of 17Gy to 33Gy compared to the control and significant reduction of shoot number was observed at 33Gy. Gamma irradiation with 17Gy dosage, it showed a significant reduction in petal width (0.82 cm) of flowers. None of the other dosages were able to develop a change in the floral characteristics of J. officinale. The study showed that 33Gy gamma irradiated plant, it is having significant reduction of plant height, number of shoots and shoot length hence it could be domesticated as a dwarf and potted plant.

Changes in the genetic structure through gamma radiation of the plant can produce physiologically and bio-chemically efficient plant type with respect to increase production and attractiveness. For mutation breeding, scientists use gamma radiation to produce effective plant types. Beforehand, an effective range of radiation should be identified for possible suitable mutational effect. In this context, the present study was conducted to determine the effective doses of gamma radiation for mutational breeding of sunflower (H. annuus L.) on vegetative growth performances. Sunflower seeds were exposed to different doses of gamma radiation (0 Gy, 100 Gy, 200 Gy, 300 Gy, 400 Gy and 500 Gy) as treatments. The experiment was laid out in the Complete Randomized Design (CRD) with six treatments in four replications. Each replication comprised ten plants in field. Treating the sunflower seeds with gamma radiation has a significant (p<0.05) effect on the tested parameters except on germination percentage. Untreated plants with gamma radiation and plants treated with lower doses of gamma radiation (100 and 200 Gy) showed comparable values in percentage survival as 100%, 94.5% and 92.5%, respectively. LD50 value was recorded as 440 Gy. In the first two weeks of growth, untreated plants and plants treated with 100 Gy with gamma radiation showed similar values in a number of leaves and plant height. Later in the 4 th and 6 th week highest values were found where plants were not exposed to gamma radiation. A reducing trend in values in the measured variables has been observed with the increase in gamma radiation doses. The lowest values in all observations were recorded by the seeds exposed to 500 Gy, which is the highest dosage used in this experiment. The plants treated with the dosages higher than 100 Gy exhibited misshapen, deformed and abnormalities in leaves compared to the control, but the changes were not persisted. Hence, it could be suggested that exposing the sunflower seeds to gamma radiation dose has caused the negative effects on vegetative growth performances. Further, exposing sunflower seeds close to 440 Gy may have the potential to produce the sunflower variations especially dwarf plant stature with desired characteristics and optimum survival for domestication in future studies.

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