Origin and development of secondary capitula in Calendula officinalis L. in response to gibberellic acid

Abstract

Foliar application of gibberellic acid greatly enhanced the formation of secondary capitula. The proportion of primary capitula showing this feature increased from 6% in the controls to 28, 58, and 54% at 100, 250, and 500 parts 10^-6 GA₃, respectively. Secondary capitula were initiated either along the periphery or in the centre of the receptacle or from both; their total numbers for 15 plants per treatment were 53, 215, 660, and 404 in response to 0, 100, 250, and 500 parts 10^-6 GA₃, respectively. Secondary capitula were smaller and had fewer disc florets and fruits, particularly of the 'wingless and small' type. Tertiary capitula, not observed in nature, formed on 2.72 and 3.21% of the secondary capitula on plants sprayed with 250 and 500 parts 10^-6 GA₃.The formation of a secondary inflorescence meristem in treated plants in place of a floret primordium can be detected histologically early in its development, and its pattern of development resembles that of the primary capitulum.