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

Abstract

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.Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp.

The spatial and temporal distributions of arthropods in forest canopies: uniting disparate patterns with hypotheses for specialisation

“…Some of this variation may be due to the differential herbivore defences employed by host plants in different plant parts (Gould, 1991;Merritt, 1996;Hoy, Head & Hall, 1998;Veldman et al, 2007), and can thus be explained by the physiological-efficiency hypothesis. Under this hypothesis, host specificity should decline with decreasing levels of defence, which should in turn decline with the nutritional quality of the resources (Mattson, 1980), or their importance to the host tree's reproduction and growth.…”

“…Insects that feed on well-defended plant parts must overcome what are usually host-specific chemical and structural defences Grubb et al, 1998;Veldman et al, 2007), thereby reducing their ability to overcome the defences of other potential hosts (Ehrlich & Raven, 1964;Rhoades & Cates, 1976). Variation in host specificity of insects utilising low-and high-quality resources was shown in a community of Lepidoptera, where monophagous and oligophagous species were restricted to feeding on flush foliage, which is often well-defended chemically, while polyphagous species fed predominantly on mature foliage, which is better defended structurally (Cates, 1980).…”

The Distribution of Fruit and Seed Toxicity during Development for Eleven Neotropical Trees and Vines in Central Panama

“…Higher toxicity at the mature fruit developmental stage may reflect higher value of fully mature fruit and/or higher predation pressures at this stage. Chemical defenses in mature seeds have been shown to reduce fungal growth and Artemia survivorship; this may correspond with increased persistence in the soil seed bank [30].…”

“…), I determined whether plant extracts reduced the growth or survival of bioassay organisms compared to controls. Bioassays are a cost-effective method commonly used to screen for toxicity and integrate effects over unknown bioactive compounds [30], [31]. Using these bioassay data, the influence of fruit toxicity on seed survival in the presence of natural enemies was determined using data from a previous study [26].…”

Longevity of Horseweed Seed Bank Depending on the Burial Depth

“…Under these conditions, seeds develop both biochemical and physical protection characteristics (Davis et al, 2008). Thus, biochemically speaking, seeds can concentrate secondary metabolites such as phenols and alkaloids (Veldman et al, 2007), and can physically develop their tegument with greater resistance to mechanical damages (Davis et al, 2008). These developed characteristics protect seeds from attacks by pathogens, and provide greater resistance to unfavorable environmental conditions; this may explain the low results of abnormal and dead seedlings.…”