Origin and Development of Secondary Capitula inCalendula officinalisL. 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 106 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. INTRODUCTION Scotch marigold (Calendula officinalis L.) is grown as a winter annual in Delhi. In this plant the shoot apices of the main axis and the axillary branches terminate in heterogamous capitula. The ray florets are unisexual (female) and produce polymorphic fruits of the following types; sickle-shaped spiny; double-winged; single-winged; and wingless and small. The disc florets are bisexual but are functionally male. About 6% of the capitula borne by the 'double yellow' variety have been observed to bear smaller secondary capitula with fewer involucral bracts, florets, and fruits. It was of interest to study possible factors converting a determinate to an indeterminate inflorescence meristem. Bose and Nitsch (1970) reported the formation of secondary capitula in scotch marigold in response to gibberellic acid. The present work was designed to study quantitatively the effect of exogenously applied gibberellic acid on the origin and development of secondary and tertiary capitula. MATERIAL AND METHODS Two to 3-week-old seedlings of Calendula officinalis L. (double yellow) raised in a nursery bed in mid-October were transplanted to soil in 25 cm pots. This content downloaded from 157.55.39.144 on Wed, 07 Sep 2016 06:27:15 UTC All use subject to http://about.jstor.org/terms 654 Mohan Ram andMehta—Secondary Capitula in Calendula Young plants under outdoor conditions having 10-12 leaves (just after floral evocation) were given three foliar sprays of gibberellic acid (100, 250, and 500 parts 10~6) at 10 d intervals. There were 15 replicate plants per treatment. All sprays, including water controls, contained Triton X-l 14 (0-01%) as wetting agent. Inflorescence apices were fixed in 4% (w/v) aqueous formaldehyde at pH 7, dehydrated through ethyl alcohol-xylene, and embedded in a mixture of paraffin wax and Histowax (3:1, by vol.). Serial sections, 10 /xm thick, were stained with haematoxylin and counterstained either with orange G or erythrosin. The pattern of branching of the plant and the location of the capitula are shown in Plate 1. PBCj Tc-m m -