Effect of environment on percentage female ray florets per capitulum and outcrossing potential in a self-compatible composite (Senecio vulgaris L. var. hibernicus Syme).

Ten lines of radiate groundsel (Senecio vulgaris L. var. hibernicus Syme) were raised in growth cabinets over a wide range of nutrient and temperature conditions to determine the effect of environment on female ray floret number, total floret number, percentage female ray florets and seed number per capitulum. Despite considerable variation in percentage female ray florets per capitulum among capitula within lines, the mean expression of the character remained relatively stable over all environments and lines tested with a mean over all treatments equal to 16–4%. Thus, the marked variation in outcrossing rate that has been reported for radiate groundsel in the wild is unlikely to result from variations in mean percentage of female ray florets per capitulum induced by changes of temperature or nutrient status.

The most frequent number of ray florets per capitulum recorded was 8, a Fibonacci number, found in 39·6 % of 821 capitula sampled. Moreover, the overall mean for total floret number was 56 which approximates very closely to another Fibonacci number, 55. This might suggest that the expected proportion of ray to total florets in each capitulum, based on Fibonacci numbers, would be 8/55 or 14–54%. The actual recorded mean value for percentage ray florets per capitulum exceeded this ‘expected’ value as most capitula surveyed bore more than 8 florets (59–2 % of sample).

Seed set per capitulum was always below 100 % and varied over environments. A consequence of this finding is that future studies of the relationship between percentage female ray florets per capitulum and outcrossing potential in self-compatible Compositae will need to consider the distribution of seed produced by female ray florets and hermaphroditic disc florets within the capitulum.