Regeneration In Vitro From the Hypocotyl of Cucumis Species Produces Almost Exclusively Diploid Shoots, and Does Not Require Light

Hypocotyl explants of three cul ti vars of melon (Cucumis melo L.) (cvs. Revi- gal, Topmark and Kirkagac), and a cu cum ber (C. sativus L. cv. Taoz) rapidly di rect ly regenerated multiple shoots on Murashige and Skoog medium aug ment ed with 4.4 µM benzyladenine. Re gen er a tion from the hypocotyl resulted in near ly 100% diploid shoots, whereas regeneration from the cotyledons resulted in 40% to 70% poly p loid regener- ants. Regeneration from cot y le don explants of melon cv. Revigal required light, whereas regeneration from hypocotyl explants of melon cv. Revigal occurred in both light and darkness. Direct regeneration also occurred from the hy po cot yl of cucumber cv. Taoz in both light and darkness, even though cotyledonary ex plants did not regenerate buds or shoots under the same conditions. This is the fi rst report of regeneration from the Cucumis genus producing a fully diploid plant pop u la tion. tech nique may alter the rate of pro duc tion of tetraploids in tissue culture of melon and other Cucumis species (Adelberg, 1998; Adelberg et al., 1994; Guis et al., 2000). Con se quent ly, in our studies of a new re gen er a tion pathway from the melon hy po cot yl, we tested our new regen- erant pop u la tions for ploidy, and we report for the fi rst time regeneration of near 100% diploid pop u la tions from the genus Cucumis. Additionally, we determined the light re quire ment for regeneration from hypocotyl and cotyledon explants, as light is known to affect in vitro morphogenesis (Lercari et al., 1986), and show that photocontrol of re gen - er a tion responses differ between cotyledon and hypocotyl explants. Materials and Methods Seed germination. Melon seeds (Cucumis melo L.) of cultivars Revigal (an Israeli 'Ga- liaʼ type) (Hazera Seeds, Israel) and Topmark (a U.S. Western Shipper can ta loupe type) (Asgrow Seed Co., Kalamazoo, Mich.) with seed coats re moved, were surface ster il ized for 20 min in 1% sodium hypochlorite, with two drops of Tween 20 (polyoxyethylene- sor bitan) per 100 mL. Af ter washing with sterile distilled water, seeds germinated on Murashige and Skoog (1962) (MS) medium with 3% sucrose, MS vitamins and 8 g·L -1