Wound-Induced Initiation of Involucral Bracts and Florets in the Developing Sunflower Inflorescence 1

The production of additional floral organs by the inflorescence of Helianthus annuus as a boron deficiency symptom was examined and found to be related to the occurrence of minute splits in the receptacle of the young capitulum. Wounding the receptacle by puncturing or cutting at an early stage when the receptacle dome was forming (floral stage 3) or later when the receptacle had either become saucer-shaped (floral stage 4) or the first rows of disc floret primordia were appearing on the rim of the receptacle (floral stage 5) resulted in the initiation of involucral bracts, ray and disc florets in the wound area, reproducing the symptoms of boron deficiency. When the receptacle was wounded at later floral stages, when the receptacle was partially or wholly covered by disc floret primordia, involucral bracts and ray florets were not formed in the proximity of the wound, leading to the conclusion that the commitment of floral organ primordia begins at the time of their initiation. The inductive effects of wounding could not be reproduced by indoleacetic acid, naphthaleneacetic acid or benzyladenine applied to the receptacle surface in lanoline paste. The results are related to the normal development of the sunflower inflorescence and it is concluded that the first floral organ primordia that appear in floral stage 3 and develop into involucral bracts during floral stage 4 may determine the positions of subsequent primordia and establish the radial symmetry of the inflorescence.