The presence of 2,4-dichIorophenoxyacetic acid (2,4-D) or naphthaleneacetic acid (NAA) and 6-benzyladenine (6-BA) in the medium is required to induce an acceptable yield of gynogenic embryos from unfertilized ovary and flower cultures in onion. Four different exposure times of ovary and flower cultures to exogenous growth regulators (15, 30, and 45 days, and the entire culture period) were assayed. The objective was to ascertain the effect of these substances and of their period of application on the formation of gynogenic embryos and on the yield of haploids. An exposure of 15 days was sufficient for ovaries and flowers to be stimulated towards the gynogenic response, whereas, for the remaining period of 30-80 days, the pro-embryos could easily grow on a growth-regulator-free medium. In the gynogenic material obtained, phenotypic differences were visible between genetically independent lines but not between plants within each Une, even when they had a different ploidy level {n or In). Almost all lines obtained by gynogenesis were sterile. Only a small percentage (1%) became fertile through spontaneous chromosome doubling, and these produced 2-20 seeds each, giving normal plants. The recovery of fertility occurred more often during the generations of bulbils. To exploit this natural propensity towards diploidization in this phase, different amounts and numbers of applications of colchicine were evaluated in two experiments. The treatments corresponding to 10 and 100 mg/1 of colchicine applied for 24h gave the highest number of diploid cells in root tips but no diploidization occurred in the shoot apices. Three days of colchicine treatment at 10 mg/1 produced 46% of plants completely diploid in the shoot apex. The flower fertility of these doubled haploid plants is being evaluated.The induction of haploid plants of onion via gynogenesis in vitro has recently been achieved (Campion and Azzimonti 1988, Muren 1989, Campion and Alloni 1990, Keller 1990, Smith et al. 1991, Campion et al. 1992, and the technique can be used successfully for the production of haploid plants. Dore and Marie (1993) recently demonstrated the induction in planta of gynogenic plants of onion after crossing with irradiated pollen. Even though the hybrids, varieties or onion populations tested all responded to the in vitro production of haploids, the yield of gynogenic plants produced was often less than 1 %.In onion, gynogenic embryos often take more than 50-60 days to develop inside flowers and ovaries before sprouting (Campion et al. 1992); moreover, it is known that the presence of exogenous growth regulators in the medium can induce genetic changes (D'Amato 1985, Ziauddin andKasha 1990). These considerations prompted a study of the effect of different exposure times of explants to growth regulators present in the media in order to ascertain the duration of treatment with exogenous growth regulators necessary for the formation of gynogenic embryos, and to reveal the effects of these substances on the yield of gynogenic embryos and plants.I...
