Desterio Nyamongo

Maternal environment effects on seed germination and dormancy in Vernonia galamensis

Nyamongo next to a stand of his study species, Vernonia galamensis (Image: D. Nyamongo)

“My rural home, where I spent the larger part of my youth, is in the Gusii highlands of western Kenya, a region that rarely experienced droughts in those days. Rivers and streams flowed all the year round, which made my comprehension of seasonal rivers as taught by my primary teachers very difficult. On completion of my first degree in agriculture, I was posted to North Eastern Kenya, an arid and semi arid region where seasonal rivers and streams are a common feature. This triggered my interest in plant conservation biology, an understanding of which I believed was critical if I was to be able to make a contribution towards reversing the trends of drought and desertification.

I thus proceeded for an MSc course in Conservation and Utilisation of Plant Genetic Resources at the University of Birmingham. The programme included an introduction to seed conservation at Kew’s Millennium Seed Bank partnership. After completing the MSc course I was deployed to the National Genebank of Kenya to head the Seed Technology and Conservation section. Our collaboration with MSBP through the Seeds for Life Project offered me the opportunity to pursue a PhD at Maseno University that I completed in November 2007. My thesis was on seed dormancy and comparative longevity in two subspecies of Vernonia galamensis (Cass.) Less.

Vernonia galamensis is a potential industrial oil crop whose seeds contain 35-45% of triglyceride oil rich in vernolic acid, a naturally epoxidized fatty acid with low viscosity. 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 would 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 evaluation to determine the onset of germinability, dormancy and desiccation tolerance. In addition, changes in seed water status and seed mass during development were monitored to establish the point at which seeds acquired mass maturity. It was found that a ‘scorched earth’ (dark brown) achene colour was a good 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 poor seed retention.

In regard to dormancy, seeds harvested at the point of natural dispersal and dried to equilibrium water content at 20ºC and 18–20% relative humidity were subjected to a variety of dormancy-release experiments, namely: soaking, de-coating, chilling, alternating temperature, after-ripening, gibberellic acid and potassium nitrate treatments, to determine the nature of dormancy and develop an appropriate germination testing protocol. The results showed that seeds of the subspecies nairobiensis were more dormant than those of the subspecies afromontana 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 to 2 weeks of cold chilling at 5°C, alternating temperatures, after-ripening, light exposure, potassium nitrate and gibberellic acid treatment, point to the nature of dormancy as being of the non-deep physiological type. Moreover, the results of a thermo-gradient plate experiment confirm this non-deep physiological dormancy as being of type II, according to the recent 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 5ºC can be used to overcome seed dormancy prior to incubating at 30/25 or 25/17ºC.

This work has to date been presented at various fora including the KARI biennial conference in Nairobi. A number of papers are lined up for publication in refereed journals. I greatly enjoyed working at the state of the art seed science lab at the MSBP and interacting with so many seed specialists. Having sorted out the germination and seed retention issues, attention now needs to be directed towards identifying and/or developing elite genotypes for oil extraction and commercialization. With the eminent threats of global warming, perhaps the solution lies partly in the exploitation of this naturally epoxidised vernolic acid.”