Within and among plant variation in seed mass and pappus size in Tragopogon dubious

Individual seed mass of two seed morphs of Tragopogon dubious (L.) varied among plants and within and among flowering heads within individual plants growing near Salt Lake City, UT. Seeds of the light morph from the center of a receptacle were smaller than outer seeds, suggesting that variation in seed mass within heads was due to position effects. The total seed mass per head, seed number per head, and mean seed mass per head of both morphs declined in most plants over the season. Heads with larger total seed mass contained larger seeds both within and between plants so seasonal patterns in seed mass may be influenced by seasonal variation in the amount of resources available for investment in seeds. Seeds of the light colored morph had a greater dispersal potential than seeds from the dark colored morph because their disk loading was lower. Pappus size (both pappus radius and stalk length) increased with the number of seeds per head which may increase the efficiency of packaging fruits on the receptacle. Heads with a larger total seed mass allocated a smaller proportion of their resources to the dark seed morph which may serve to decrease the level of sibling competition experienced by the less dispersing seed morph. Thus, variation in investment in offspring may arise as a combination of developmental constraints and selection for variable investment.