Seed Dormancy and Comparative Longevity in two Sub-Species of Vernonia Galamensis(Nairobiensis& Afromontana Var. Gibbosa) (Cass.) Less

Abstract

Vernofliagalamensis (Cass.) Less. is a potential industrial oil crop whose seeds contain 35-45% of triglyceride oil rich in vernolic acid, a naturally epoxidized fatt~ acid with low viscosity. Naturally epoxidized, vernolic acid is a potential useful raw material for manufacturing paints and coatings. Exploitation of V galamensis as a crop is however hampered by poor seed retention and ill understood seed dormancy characteristics. To address the issues of seed retention, seed maturation studies were conducted with a view to establishing morphological markers of seed maturity that shall enable determination of the optimum seed harvesting point. Changes in seed colour were monitored during seed development. Seeds harvested at different maturity levels were subjected to desiccation and germination evaluationto tease the onset of germinability dormancy and storability. In addition, changes in seed water status and seed mass during seed development were monitored to establish the point at which seeds acquired mass maturity. It was found that a "scorched earth" (dark brown) achene colour largely denoted a common indicator of mass maturity to the extent that no seed lot registered any appreciable increase in dry weight subsequent to achievement of this colour. It thus appears reasonable to recommend that harvesting of seeds be undertaken once the achene colour turns scorched earth in order to minimise seed losses due to natural dispersal. In regard to dormancy, seeds harvested at the point of natural dispersal and dried to equilibrium water content at 20°C and 18 - 20% relative humidity were subjected to a variety of dormancy release experiments namely: soaking, de-coating, chilling, alternating temperature after-ripening and gibberellic acid & potassium nitrate treatments, to determine the nature of dormancy and develop appropriate germination testing protocol. The percent germination data were arcsine transformed and subjected to analysis of variance. The results showed that seeds of v MASENO UNIVERSITY S.G. S. LIBRARY the subsp nairobiensis were more dormant than those of the subsp afro montana var. gibbosa. Similarly, seeds harvested from Muguga (with a cooler climate) were shown to be more dormant than those harvested from Maseno (warmer climate). The observed significant positive \ germination response due to nipping, decoating and soaking implicate the role of the seed pericarp in regulating dormancy and/or germination. However, the observed positive response due to 2-weeks of cold chilling at SOC, alternating temperatures, after-ripening, light exposure, potassium nitrate and gibberellic acid (GA3) treatment, point to the nature of dormancy as being of the non-deep physiological type. Moreover, the results of the thermo-gradient plate experiment confirm this non-deep physiological dormancy as being of type II, according to the classification by Baskin and Baskin. Seed soaking for at least 24 hours seems to be the most pragmatic protocol for use by farmers to enhance seed germination in their fields. The same procedure may also be used in laboratory seed germination testing. Alternatively, chilling for at least two weeks at SOC can be used to overcome seed dormancy prior to incubating at 30/2S or 2SI17°C.