Abstract
The accumulation of betacyanin, in dark-grown Amaranthus tricolor seedlings, in response to cytokinins or red light, occurs mainly in two specific tissues, the lower epidermal cells of the cotyledons (with the exception of guard cells), and the endodermis of the hypocotyl. The possible significance of this 'spatial pattern of competence' is discussed, together with the concept of target cells in relation to plant hormones. The effect of removing exogenously supplied cytokinin at various times during a 24 h induction period is reported. There is no evidence that cytokinins act by a 'triggering' effect with a long half life, the response in the target cells being the same as that expected from the amount of cytokinin and cytokinin metabolite remaining in the tissue at the time of extraction. Either continuous presence of cytokinin is needed or any trigger action is short lived, and continuous 're-triggering' is needed to achieve maximum response.
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