Polyploid Distribution and Seed Dormancy in Relation to Provenance Rainfall in the Digitaria milanjiana Complex

Abstract

Chromosome numbers were counted in 72 accessions in the Digitaria milanjiana complex and 2 of D. macroblephara from contrasting climates in Africa; 18 were diploids, 38 tetraploids, and 18 hexaploids. Higher polyploids were mostly from lower rainfall provenances. Diploids were mostly accessions which had been attributed to other species in pre-1982 classifications but which are now considered to be synonyms of D. milanjiana. Some 29 accessions, including diploids, tetraploids and hexaploids from low-rainfall and high-rainfall provenances at equatorial or near-tropical latitudes were grown and seed was harvested on 2 occasions (Jan. and Mar. 1987). Seed was treated at alternating temp. of 30/25°C or constant 60°C for 0, 2, 6, 10 or 16 weeks and then germinated. Percentage germination increased over time for all accessions, from 1% at harvest to 26% at 16 weeks. Germination was markedly lower in accessions from low-rainfall (14%) than high-rainfall (33% at 16 weeks) provenances. Similarly, percentage germination was lower in hexaploids (16%) than tetraploids (26%) and lower in tetraploids than diploids (38% at 16 weeks). High temp. treatment increased germination percentage but the effect was only marked in accessions with comparatively high levels of germination at low temperatures. Percentage dormancy (estimations based on live but ungerminated seed after germination treatment following 16 weeks at 60°C) was significantly and negatively correlated with rainfall of provenance. It was concluded that seed dormancy in D. milanjiana is related both to polyploidy and rainfall of provenance and that genetic adaptation to arid regions has been associated with polyploidisation and increasing dormancy.