Variation in Pollen Limitation Among Individuals of Sabatia Angularis (Gentianaceae)

Abstract

Pollen limitation is unambiguously demonstrated when hand-supplementation of outcrossed pollen to all flowers on an individual results in a significant increase in seed and/or fruit set per plant compared to naturally pollinated plants (e.g., Johnston 1991 a, and references therein, Young and Young 1992, and references therein). The degree of pollen limitation can vary among years (e.g., Campbell 1987, Lubbers and Lechowicz 1989), within a season (e.g., Hainsworth et al. 1985, Zimmerman and Aide 1989), among sites within a season (e.g., Campbell 1987, Johnston 1991 a), and among plants flowering synchronously within a site (e.g., Snow 1986). Individual variation in pollen limitation may have direct consequences on an individual's fitness. However, we know little about the factors responsible for variation in pollen limitation among individuals within a population. Plant populations are often size structured, consisting of many small individuals and relatively few large ones (e.g., Harper 1977). If a taxon is semelparous, adult size structuring can have direct effects on an individual's reproductive fitness in terms of potential female (seeds and fruits) and male (pollen production) function (e.g., Dudash 1991). The interactions among plant size, pollinator attraction, and pollen limitation may influence an individual's fitness. Flowering phenology may vary as a function of plant size and differentially influence reproductive output. Selection on floral traits associated with pollinator attraction may vary among individuals in different size classes. Larger inflorescences often have greater probability of maturing fruit (Ackerman and Montalvo 1983, Paton and Turner 1985) and attracting visually cued pollinators (Inoue 1985) than smaller ones. Additionally, significantly greater pollen loads are found on stigmas of large plants compared to small plants (Dudash 1991, but see Weller 1980, Webb and Bawa 1983), differentially in-