Differential Germination of Two Closely Related Species of Solanum in Response to Bird Ingestion

Abstract

This study evaluated the effect which ingestion of seeds of Solanum nigrum and S. luteum by bulbuls (Pycnonotus xanthopygos) and blackbirds (Turdus merula) had on their germination, and tried to reveal the ecological implications of such endozoochory. Those closely related plant species are often sympatrical but whereas S. nigrum is a noxious weed invariably associated with human disturbed habitats, S. luteum may also occur in natural plant associations. Our data indicated that germination of seeds of S. nigrum was not affected by ingestion by either bulbuls or blackbirds. On the contrary, germination of the seeds of S. luteum was significantly (p < 0.01) improved by their ingestion by either bird species. Comparative scanning electron micrographs of the seeds showed that seed coat sculpture of S. nigrum was not affected by ingestion. In contrast, in ingested seeds of S. luteum, the sculpture of the seed coat was abraded compared with the uningested seeds; here, also, in the seeds ingested by the bulbuls, the intracellular contents disappeared, and in most of those ingested by blackbirds, there was destruction of the cell wall as well. Assumedly abrasion of the seed coat sculpture of S. luteum increases the seed coat permeability and consequently germination. It would seem that germination and dispersal of this more arid species which grows in unstable habitats is mediated largely by birds, whereas the germination of S. nigrum seeds is not affected by avian ingestion; for this weed, birds serve only as dispersal agents.