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Studies pertaining to the detection, isolation anid chemical characterization of naturally occurring and synthetic cytokinins have been aided by the utilization of cultured callus tissues derived from soybean cotyledons (6, 9, 10, 11, 12) and tobacco stem pith (7, 8, 13, 14). Both tissues, when exposed to a specific anid defined chemical milieu, require kinetin or another similarly active 6-substituted aminopurine for continual growth and maintenance in culture ( 10). It has also been reported that numerous structural variants of phenylurea stimulate cell division in isolated stem pith of tobacco (2). Although an auxin such as indole-3-acetic acid (IAA) or a-naphthalene acetic acid (NAA), is a necessary constituent of the basal medium for optimum growth, the auxin does not elicit significant callus proliferation in the absence of a cvtokinin (10). In the soybean callus, however, the specific auxin used appears to be quite importalnt with respect to the nature of the cytokinin-auxini mediated growth response. For example, adenine is knowni to exhibit slight cvtokinin activity, no activity or even inhiibitionl of calltus growth wheni supplied in comibination with I,AA (10, 11). 'On the other lhand, Miller (11) has recently demonstrated that adenine will consistently stimulate soybean callus growtlh whein NAA is substituted for IAA. In essence, a highly effective auxin seems to be a requiremenlt for the expression of cytokinin activity by adenine. In view of these results, it seemed reasonable to assume that ani auxin even more effective than NAA might stimulate the calluts proliferation in the absence of any added cvtokinin. 2,4-D meets this requiremenit anld is knowvn to stimnulate growtlh of soybeanl root cells cultutes (4) and has been used without added cvtokinins in the ctulture of other tisstues (3. 5, 15. 16, for examples). The tissues exanminedl were originally (lerived fromii stem pith of tobacco (Nicotiana tobacumnt, var. Wisconsin No. 38) and from cotyledons of sovbean

Effect of 2,4-Dichlorophenoxyacetic Acid on the Cytokinin Requirement of Soybean Cotyledon and Tobacco Stem Pith Callus Tissues