Onset of Dormancy, Dormancy Levels, and Appropriate Seed Production Environment for Two Subspecies ofVernonia galamensis(Cass.) Less

Detailed germination studies were conducted on seeds of two subspecies of Vernonia. galamensis (nairobiensis and afromontana var. gibbosa), a potential industrial oil crop. Plants of both subspecies were grown at two experimental sites in Kenya and sequential harvesting carried out during seed development to determine the best production environment and optimum harvest time for low-dormant, high-quality seed-lots. Seeds harvested at different maturity levels were germinated under different temperature regimes before and after desiccation. The studies revealed that the onset of germinability and desiccation tolerance was achieved during the dry matter accumulation phase of seed development and that seeds developed dormancy 3 to 6 d after acquiring germinability, with high levels of germination only reaching under an alternating temperature regime. The seeds of the subsp. nairobiensis were shown to be more dormant than those of the subsp. afromontana var. gibbosa. Similarly, seeds harvested from a warmer environment were less dormant than those from a cooler environment.