The role of fungal pathogens in flower size and seed mass variation in three species of Hydrophyllum (Hydrophyllaceae).

Identifying ecological factors that affect seed number and seed size is key to understanding the persistence of large seed mass variation in some plant species. Pathogens may increase seed mass variation by increasing resource demand over the growing season such that late fruits experience higher resource competition than early fruits. We tested whether Fusarium sp. and Rhizoctonia sp., soil fungi that cause wilt, contributed to seasonal decline in flower size, seed number, or seed mass in Hydrophyllum appendiculatum and H. canadense. A third species not infected by these soil fungi, H. virginianum, was studied to determine how seasonal decline in floral traits and seed mass variation varies within this genus. Flower size declined seasonally for all species, but was greatest for H. appendiculatum, a monocarpic biennial with indeterminate inflorescences. Seed number decreased between first and last inflorescences in H. appendiculatum, but not in H. canadense or H. virginianum, perennials with determinate inflorescences. Seed mass varied most in H. appendiculatum and H. canadense (4-20-fold in 50% of individuals) and least in H. virginianum (4-8-fold in >30% of individuals). Fungal infection increased seed mass variation among diseased plants in H. canadense and H. appendiculatum. However, within plants fungal infection only increased seasonal decline in flower size, seed number, and seed mass in H. appendiculatum when flowers received supplemental pollination.