Characteristics of the ion beam exposure for overcoming the cross incompatibility

In wild species, there are valuable genes controlling resistance to disease, insect and environmental stress. The introduction of such genes from wild species to cultivar is extremely useful in plant breeding. However, it is very difficult or impossible to obtain an interspecific hybrid in a wide cross, because of the strong cross-incompatibility. We have been searching for an effective procedure of interspecific hybridization in Nicotiana. Because N. gossei Domin (wild species) has been reported to be resistant to more disease and insects than other Nicotiana species, many attempts to hybridize it with N. tabacum L. (cultivar) have been made. In N. gossei x N. tabacum, it is easy to get hybrid seeds with conventional cross, but hybrid plants can not survive, On the other hand, in N. tabacum x N. gossei, few seeds with viability can be obtained. In this species combination, we had obtained the viable hybrid plant by means of the cross with the gamma-rays-irradiated pollen at the rate of 3.7 x 10-5. Ion beams have higher LET (Linear Energy Transfer), in comparison with that of gamma-rays, and can be controlled the range in the target material. Accordingly, a lot of energy can be deposited on the focused point of material exposed to ion beams. The cross (N. gossei x N. tabacum) with pollen exposed to ion beams, e. g., he, C and Ne ions, produced the viable hybrid plants at the rate 1 x 10-2, which was much higher than that of gamma-rays. Morphological characters in some of hybrid plants were intermediate between parents and uniform, suggesting that they had no mutation induced by the ion beam exposure. Conclusively, it is said that ion beams are more effective for overcoming the cross incompatibility than radiations with low LET. (author)