Desiccation tolerance of yeheb (Cordeauxia edulis Hemsl.) seeds

In this master's thesis, the desiccation tolerance of seeds from yeheb, Cordeauxia \nedulis Hemsley, a multipurpose, evergreen shrub, native to the semi-desert areas in \nthe horn of Africa, is studied. Due to overexploitation and bad regeneration, yeheb is \nnow threatened with extinction. \n \nSeeds of different species can be classified into three storage behaviour categories, \ndepending on their desiccation tolerance, longevity and tolerance to low temperatures \nduring storage. When determining the storage behaviour of seeds, the first step is to \nstudy desiccation tolerance. Seeds that maintain their viability at <5% moisture \ncontent (wwb=wet weight basis), are probably orthodox. If they tolerate 10-12.5% \nmoisture content (MC), but not lower, they may be intermediate, and if most seeds die \nwhen desiccated to moisture contents >15-20%, then they are recalcitrant. Desiccation \ntolerance of seeds is controlled by several physiological processes, and depends on \nfactors such as drying rate, tissue, degree of development at harvest and \nenvironmental influence of the mother plant prior to seed maturation/harvest. \n \nTo achieve the desiccation of seeds, silica gel was used. The seeds were dried to six \ndifferent moisture contents: 9.6, 12.3, 24.4, 27.4, 33.4 and 39.3% (wwb). Moisture \ncontent of the fresh seeds was 41.6%. Desiccation tolerance was assessed by \ngermination tests followed by a tetrazolium test of viability on ungerminated seeds \nthat were not estimated to be clearly dead at the end of the germination test. \nGermination tests were done on desiccated and fresh seeds, and on seeds from 'control \nof time factor during drying' -replications, in which vermiculite were used instead of \nsilica gel during the drying treatment. Germination percentage was dependent on seed \nmoisture content (p-value 0.0001), seen as a reduction of germinability when seed \nmoisture content dropped from >24.4 to 12.3% (p-value 0.0062). Further drying to \n9.6% moisture content, reduced germination percentage even more (70-83.8% at MC \n>24.4%, 57.5% at 12.3% MC and 41.3% at 9.6% MC). The reduction of germination \ncapacity at the latter MC was highly significant compared to the control replications \n(p-value 0.0001). Yeheb seeds may therefore be classified as having seeds of \nintermediate storage behaviour. However, further studies on viability in storage and \ntolerance to low temperatures are necessary before any certain conclusions on the \nclassification of seed storage behaviour of yeheb can be drawn.