Desiccation tolerance of yeheb (Cordeauxia edulis Hemsl.) seeds :

Abstract: In this master's thesis, the desiccation tolerance of seeds from yeheb, Cordeauxia edulis Hemsley, a multipurpose, evergreen shrub, native to the semi-desert areas in the horn of Africa, is studied. Due to overexploitation and bad regeneration, yeheb is now threatened with extinction. Seeds of different species can be classified into three storage behaviour categories, depending on their desiccation tolerance, longevity and tolerance to low temperatures during storage. When determining the storage behaviour of seeds, the first step is to study desiccation tolerance. Seeds that maintain their viability at <5% moisture content (wwb=wet weight basis), are probably orthodox. If they tolerate 10-12.5% moisture content (MC), but not lower, they may be intermediate, and if most seeds die when desiccated to moisture contents >15-20%, then they are recalcitrant. Desiccation tolerance of seeds is controlled by several physiological processes, and depends on factors such as drying rate, tissue, degree of development at harvest and environmental influence of the mother plant prior to seed maturation/harvest. To achieve the desiccation of seeds, silica gel was used. The seeds were dried to six different moisture contents: 9.6, 12.3, 24.4, 27.4, 33.4 and 39.3% (wwb). Moisture content of the fresh seeds was 41.6%. Desiccation tolerance was assessed by germination tests followed by a tetrazolium test of viability on ungerminated seeds that were not estimated to be clearly dead at the end of the germination test. Germination tests were done on desiccated and fresh seeds, and on seeds from 'control of time factor during drying' -replications, in which vermiculite were used instead of silica gel during the drying treatment. Germination percentage was dependent on seed moisture content (p-value 0.0001), seen as a reduction of germinability when seed moisture content dropped from >24.4 to 12.3% (p-value 0.0062). Further drying to 9.6% moisture content, reduced germination percentage even more (70-83.8% at MC >24.4%, 57.5% at 12.3% MC and 41.3% at 9.6% MC). The reduction of germination capacity at the latter MC was highly significant compared to the control replications (p-value 0.0001). Yeheb seeds may therefore be classified as having seeds of intermediate storage behaviour. However, further studies on viability in storage and tolerance to low temperatures are necessary before any certain conclusions on the classification of seed storage behaviour of yeheb can be drawn.