Chromosome duplication and ploidy level determination in African nightshade Solanum villosum Miller.
Octoploids were induced in vivo from wild-type tetraploid Solanum nigrum ssp. villosum plants using colchicine sprays. Seedling survival rates and numbers of induced octoploids from 72 seedlings were 95.8%, 73.6% and 48.6%, and 4, 2 and 1, from 0.01%, 0.05% and 0.25% colchicine treatments, respectively. The applicability of pollen area and stomatal length, as indirect methods to determine ploidy level, was investigated. Further confirmatory tests involving direct chromosome counts (in root tip cells) and flow cytometric analysis revealed that pollen and stomatal cell size may not correlate accurately with ploidy level. Although octoploids generally had larger pollen and larger stomata, plants that were identified in the first generation (G1) progeny on a large-pollen and large-stomata basis were not necessarily octoploids. In addition, a number of tetraploids also had "large" stomata or "large" pollen. Flow cytometric analysis revealed that this species exhibits polysomaty in leaf tissues (i.e., the tissues consist of cells with different ploidy levels), which could affect the size and morphology of both pollen grains and stomatal (guard) cells, thus explaining the inconsistency observed. We conclude that plant pollen and stomatal size can provide a good general guide to ploidy level determination in this species; but confirmatory tests, including direct chromosome scoring in root tip cells and flow cytometry in young leaves, are indispensable.