Verbena, a genus of the Verbenaceae family, consists of approximately 250 species worldwide. Most of these species are native to temperate, tropical or subtropical regions of South, Central and North America. Numerous horticultural cultivars of verbenas (i.e. Verbenaϫ hybrida) have been bred by artificial crossing among several species and were popular for hanging basket plants and garden uses because of their wealth of flower colors and broad range of growth habits. During the last decade, Suntory Flowers Ltd. (Tokyo, Japan) has developed new cultivars of the verbena Temari ® series that have characteristics such as resistance to powdery mildew and heat tolerance that are superior to those of common garden verbena cultivars. Besides, most of the Temari ® cultivars have a continuous flowering habit that is characterized by a large number of inflorescences from spring until autumn in the temperate regions. However, some cultivars sometimes show a decrease in the number of inflorescences. It was observed that they produce self-seeds at a high frequency, and the number of inflorescences tends to decrease after seed setting. Usually, in monocarpic plants, the production of fruits and seeds causes plant senescence (Noodén 1980), and it has been reported in soybean that the removal of fertilized flowers or young pods leads to an increased number of flowers compared with untreated plants (Noodén 1984). Although perennial verbenas are not monocarpic plants, we believe that the number of inflorescences in verbena cultivars might be also increased by the repression of seed setting.The induction of sterile mutants seems to be one of the most effective methods to improve the flowering habits of self-compatible verbenas. Radiations such as gamma rays and X-rays have been widely used for mutation induction in numerous plant species (van Harten 1998). Because of their lower biological effects compared with low linear energy transfer (LET) radiations such as gamma rays, X-rays and electrons (Kraft et al. 1992; Heavy-ion beam-induced sterile mutants of verbena (Verbena؋hybrida) with an improved flowering habit Abstract Sterile mutants of Verbenaϫhybrida were isolated at high frequency from nodal cultures of developed plants irradiated with heavy-ion beams. Sixty four in vitro-cultured nodes of fertile cultivars 'Temari Sakura' (FS), 'Temari Coral Pink' (FC) and 'Temari White' (FW) were irradiated with 1 to 10Gy of 14 N-ion beam (1890MeV). Lateral shoot development of FS, FC and FW was not affected by irradiation with up to 10Gy. After open-pollination, shoots with inflorescence forming unenlarged ovaries were selected and propagated several times by cutting. Shoots were grown to flowering and the selection process for isolating stable sterile mutants was carried out by the same method. Finally, one mutant out of 104 FS lateral shoots (5Gy), one mutant out of 115 FC shoots (5Gy) and 3 mutants out of 108 FC shoots (10Gy) were successfully isolated. With the exception of sterility all these mutants showed normal morphology. Two ...

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