A CLONAL ANALYSIS IN SUNFLOWER (HELIANTHUS-ANNUUS L) .1. THE CAPITULUM DEVELOPMENT

The development of the capitulum was investigated in sunflower (Helianthus annuus L.) after X-irradiation of dry achenes heterozygous for a fruit epidermis marker. Sectors from cells of the shoot apical meristem were formed as evidenced by chimeric capitula. At maturity, the sectored capitula were analysed for the pattern of colour of the achenes. The size and location of these patterns were correlated with the cell number of the initial shoot apex in order to deduce the number of cells contributing to the capitulum development. Most capitula develop from two to four cells of the embryo; when there are four initials, the cells are arrayed as a quadrant. The number of the initial cells participating in single achene development was estimated from chimeric achenes along the boundary between mutated and non-mutated sectors. To test for the existence in the embryo of dry seed of L1 and L2 as individual layers, a genetic analysis was undertaken. The results suggest the absence, at this stage, of a clonal separation between the two layers. The results from a study of the mutation frequencies of two markers (achene and ray flower colour) induced at different times during plant growth suggest that initial cells of the capitulum are relatively inactive in terms of mitotic activity until the transition to flowering. However, the observation that mutated sectors were always visible from the edge of the capitulum was interpreted to mean that even after the transition to flowering in sunflower there is no compartment of cells destined to form the capitulum.