The cotyledons of secondarily dormant cocklebur (Xanthium pennsylvankum Wallr.) seeds lacked not only growing potential but also chlorophyll-forming ability. These properties developed as the seeds established secondary dormancy during a soaking period. The inability to form chlorophyll in cotyledonary segments was not improved in the presence of S-amino levulinic acid. But this ability was greatly restored by benzyladenine or ethylene treatment, which is effective in increasing cotyledonary growth. Similarly, the application of ethylene together with enriched oxygen and carbon dioxide, or subsequent to a KCN treatment, which were the most effective means for breaking secondary dormancy, completely restored both chlorophyll formation and growing ability of the cotyledons. The depression of the greening-ability in cotyledons may be involved in development of secondary dormancy of cocklebur seeds in concert with the decline of their growth pontential.
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