Effects of Growth Retardants, Gibberellic Acid and Indol-3-ylacetic Acid on Stem Extension and Flower Development in the Pot Chrysanthemum (Chrysanthemum morifolium Ramat)

Abstract

The growth retardants chlorphonium chloride, daminozide and a new, quaternary ammonium compound, piproctanyl bromide, all reduced shoot length and delayed the time of flowering of the pot chrysanthemum (Chrysanthemum morifolium Ramat) cv. Bright Golden Anne grown throughout in short days. The retardants delayed flowering by reducing the rate of flower bud development and not by influencing bud initiation. In the case of chlorphonium chloride and daminozide, a single dose of 20 or 40 μg gibberellic acid (GA) completely overcame the effects on both stem length and flowering, whereas when piproctanyl bromide had been applied GA did not always bring about a total reversal. Responses to GA were recorded a few days after its application. Neither the retardants nor Ga altered leaf number. Only when piproctanyl bromide was the retardant did indol-3-ylacetic acid produce a small but significant increase in stem length at flowering. The results are consistent with a theory of retardant action in which gibberellins play the dominant role and strongly suggest that these hormones are a major factor influencing both stem extension and the rate of flower-bud development in the chrysanthemum. They may promote flower development and thus hasten flowering by attracting assimilates to these organs.