Floral costs in Nigella sativa (Ranunculaceae): compensatory responses to perianth removal

Because internal resources are finite, it has been assumed that attractive, floral organs represent a significant drain on the energy and nutrient budget of a plant. Despite the broad significance of such trade-offs, in relatively few studies have investigators manipulated floral investments, then evaluated allocation to subsequently produced flowers, fruits, and seeds. In the present study of Nigella sativa, the cost of maturing and/or maintaining perianths was documented after all sepals and nectaries were removed at the bud stage and a significant increase in mean seed mass, the total amount of biomass allocated to seed production, and mean germination rate of the maternal seed crop were measured. The increased biomass, carbon, and nitrogen allocated to seeds were similar in magnitude to the reduction in biomass, carbon, and nitrogen invested in sepals and nectaries after perianth removal. Perianth removal did not significantly affect flower production, maternal fecundity, or progeny seed number. Taken together, these observations indicate the potential for selection--mediated through resource trade-offs with seed mass and time to germination--to cause, or at least facilitate, evolutionary reductions in flower size.