IN VITRO SELECTION AND CHARACTERIZATION OF LEAD RESISTANT SOMACLONAL VARIANTS FROM DAUCUS CAROTA L.
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Lead was shown to inhibit both callus initiation and callus growth in cultures of Daucus carota L. subsp. sativus (Hoff. Thell.) cv. Nantes ''Tiptop'' and ''Nanthya''. Taproot explants of Daucus carota were stressed with lead. The callus cell lines which initiated under this stress were shown to exhibit resistance to the effects of lead ions. The growth of the selected and nonselected cell lines on non-lead containing media was comparable and the resistance possessed by the selected cell lines did not result in reduced growth rates in the presence of lead. The resistance characteristic was shown to be stable and to be successfully transmitted over mitotic and meiotic barriers. Plants were regenerated from the selected cell lines and ion uptake studies were conducted on isolated cortical tissue from mature taproots. The uptake of lead into the cortical cell tissue from the selected lines was shown to be reduced and a greater proportion of the lead that did enter the tissue was present in the Apparent Free Space and did not enter the cells. The regenerated plants were self-pollenated to produce an F1 generation. F1 plantlets were grown in hydroponic culture containing various concentrations of lead. The selected plants were seen to be resistant to the lead stress. The sites of lead accumulation in these roots were determined using x-ray microanalysis in a scanning electron microscope with a cryo-stage. The lead was found to be associated with the epidermal layer and cell walls. The mechanism of the lead resistance is discussed along with the implications of selection for somaclonal variants from initiating callus cultures.
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