The effects of light, auxin and cytokinin on vessel element formation in phloem slices of carrot root were examined.When slices of carrot cultivars,‘Nakamura-senko-futo’and‘Yamada-hyakunichisenk6- naga’, precultured in the dark on modified Murashige and Skoog's medium for two days were cultured on a medium containing 5×10−6M 2,4-D in the dark, no vessel element formation occurred. When precultured slices were cultured in the light with 5×10−6M 2,4-D, vessel element formation was remarkable. But when 5×10−7M kinetin, benzyladenine or zeatin was added, vessel elements were readily formed even in the dark.When slices were cultured in the light, a cytokinin-like substance(s) that causes vessel element formation was produced in the slices, then was released to the medium. The substance(s) was fairly stable to heat.In slices of carrot cultivars, kuroda-gosun,‘Kintoki’and‘Kokubu-senk6-6naga’, a different result for vessel element formation was obtained. When slices of these cultivars were cultured on a medium containing 5×10−6M 2,4-D in the dark, vessel element formation was remarkable. It seemed, therefore, that these cultivars contain enough of a cytokinin-like substance(s) to form vessel elements. In fact, vessel element forming activity was found in the alcohol extract of carrot root
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