Exceptional desiccation resistance in recalcitrant seeds of Brosimum alicastrum may confer ecological advantage

The recalcitrant seeds of Brosimum alicastrum, a widespread tropical tree, have an exceptional ability to resist desiccation, which we propose has contributed to the dominance of the species across a broad precipitation gradient in forests of Latin America. Seed desiccation sensitivity is relatively common in tree species of tropical rain forests. For such species, pre-germination survival may be as important as seedling establishment in determining reproductive success, yet the adaptive traits important for survival are poorly understood. We studied seeds of Brosimum alicastrum Sw., a dominant tree species across a very broad precipitation gradient in Central America. This ecological success seems counterintuitive to the putative presence of seed desiccation sensitivity, that potentially severely limits propagule survival. We evaluated the anatomical, chemical and physiological traits for pre-germination survival in Brosimum alicastrum. Seeds were subjected to a series of desiccation experiments to determine the role of the seed coat and cuticular layers in controlling the rate of water loss. The structural properties were characterised using light and electron microscopy and complemented by biochemical and biophysical characterization of the seed cuticle. We confirm that Brosimum alicastrum seeds are highly desiccation sensitive but exhibit an exceptional resistance to desiccation. We show that the mechanisms for this trait of exceptional control of water loss are multifaceted and relate to the structural, biochemical and biophysical properties of the cuticle surrounding the embryo. When the cuticle is punctured, seed resistance to drying is lost and the seeds die rapidly. We propose that, combined with dispersal by winged fauna, this unique feature of seed desiccation resistance enables this species to colonise and occupy a broad range of edaphic and precipitation niches and so contribute to its prevalence in the forests of Latin America.