Implications of geitonogamous pollination for floral longevity in Iris gracilipes

1. We analysed the longevity of individual flowers with respect to the effects of the floral display size of a plant (number of flowers open at one time) and spatial arrangement of flowers in Iris gracilipes (A. Gray). We found that the actual floral longevity of this species (about 2 days) was much shorter than that predicted by the model of Schoen and Ashman (about 7-10 days). 2. A large display size and an overlap of flowering periods within a shoot facilitated successive pollinator probes within a plant, though the frequency of pollinator visits per flower was independent of display size. Consequently, the cost of geitonogamous selfing increased with an increase in display size and an overlap of flowering periods within a shoot. The short floral longevity of this species would be effective in such situations because shorter floral longevity leads to a smaller display size and a smaller overlap of flowering periods within a shoot. 3. A small display size and small overlap of flowering periods within a shoot could also be realized by the strategy that staggers the flowering period of each flower without floral longevity being shortened (extended blooming). However, a mutant plant with an extended blooming schedule would be unlikely to invade the population because of the stabilizing selection pressure on the flowering date. This may be why such a short floral longevity, rather than extended blooming, has evolved in this species.