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Chemical Defense and the Persistence of Pioneer Plant Seeds in the Soil of a Tropical Cloud Forest

We present evidence that differences in soil seedbank persistence among pioneer plants in the cloud forest of Monteverde, Costa Rica, are influenced by differences in seed chemical defense. We used extracted seed chemicals from Bocconia frutescens (Papaveraceae), Guettarda poasana (Rubiaceae), Phytolacca rivinoides (Phytolaccaceae), Urera elata (Urticaceae), Cecropia polyphlebia (Cecropiaceae), and Witheringia meiantha (Solanaceae) to assess seed chemical defense in two ways: (1) a plant pathogen inhibition assay using Pythium irregulare; and (2) a brine shrimp toxicity assay using Artemia salina. The combined performance of each species in the two assays positively correlated with seedbank persistence. In the pathogen assay, mycelium growth was reduced when Pythium was cultured on media containing seed extracts from the three species with the greatest seed longevity in the soil (i.e., Bocconia, Guettarda, and Phytolacca). Bocconia, the most persistent species, was the only species that contained chemicals toxic to brine shrimp, an indication of defense against arthropods. We focused on Bocconia defense by isolating the chemicals toxic to brine shrimp and identified them as dihydrosanguinarine, dihydrochelirubine, and dihydrochelerthrine. We found these alkaloids in Bocconia seeds at much higher concentrations (∼50 mg/g seed material) than in leaves. These chemicals are likely responsible for the exceptional longevity of Bocconia seeds in the soil. Phytolacca and Guettarda seeds also remain viable in the soil for long periods probably due to antipathogen chemicals detected in our analyses. In contrast, the species that do not persist (i.e., Urera, Cecropia, and Witheringia) lacked seed defensive chemicals in our assays.