Floral Phenology Patterns and Breeding Systems in Thaspium and Zizia (Apiaceae)

Abstract

Protogyny, in concert with a well-defined pattern of andromonoecism in umbel inflorescences and strong synchrony of floral and inflorescence develop- ment within individual stalks of Thaspium and Zizia, produces sexual phase alter- nations that promote a strong outcrossing system in plants of these genera despite full self-compatibility. Study of these phenology patterns in conjunction with selfing studies, analysis of stigmatic pollen loads, and determinations of pollen: ovule ratios in 9 populations (including 4 taxa) of Thaspium and Zizia revealed that incomplete protogyny in some individuals allows 5-10% mechanical self-pollination in T. bar- binode and Z. trifoliata and up to 20% mechanical self-pollination in T. trifoliatum. Stalks of one plant do not generally flower synchronously and insect-mediated gei- tonogamy between stalks of a single plant may account for significant amounts of selfing within populations. Strong xenogamy within Thaspium and Zizia is suggested, however, by high pollen: ovule ratios and by stigmatic pollen-load data. A sparse literature suggests that members of Apiaceae (Umbelliferae) have a potential for strong outcrossing systems based on characteristic timing in floral and inflorescence development. Muller (1883) noted that protandrous dichogamy in species of Apiaceae effected phase alterna- tions so that in the first period of flowering, a whole community dusts with pollen the insects that visit it, and in the second, exposes its stigmas to be dusted in return. Similarly, Knuth (1908) commented that sexual patterns in umbellifers, in combination with well-marked protandry, pro-