Regeneration and frequency of tetraploid variants of Cucumis metuliferus are affected by explant induction on semi-solid medium versus the liquid/ membrane system

Ninety-eight percent of Cucumis metuliferus (PI 482439) cotyledons regenerated shoot buds after 5 weeks on basal Murashige and Skoog medium amended with 10 μm benzyladenine. Regeneration rates of explants maintained only on agar-gelled medium versus explants induced first on a liquid/membrane system were compared after weekly transfers of tissue from liquid/membrane to agar during the first 5 weeks of regeneration. The number of regenerant buds increased from six per cotyledon (on agar-gelled medium) to nine per cotyledon when explants induced on the liquid/membrane system for 3 or 4 weeks were transferred to agar-gelled medium. Shoot development, rooting and survival in the greenhouse were adequate, regardless of whether regeneration was initiated on the agar or liquid/membrane system. Tetraploid regenerants accounted for 9% of the 391 regenerants screened. Pollen morphology was a reliable screening technique to identify tetraploid plants. The frequency of tetraploid plants varied from 13.5% to 6.9% after 5 weeks of induction on the agar or liquid/membrane system, respectively. This frequency decreased to 1.5% if the explants were transferred from the liquid/membrane system to agar after the first week of induction. Transfers during this period play a critical role in eliminating tetraploid variants. Received: 17 June 1996 / Revision received: 27 August 1996 / Accepted: 10 March 1997