Comparative seed longevity studies in selected Succullent genera: euphorbia, kalanchoe and edithcolia

Abstract

The future utility value of seeds of Euphorbia, Kalanchoe and Edithcolia pose challenges as none has been investigated for storage longevity under seed bank conditions yet their wild populations are threatened by over-exploitation During storage, seed moisture content is one of the factors known to affect seed longevity in general, but no study has quantified the effect of moisture content and other factors implied in literature. Therefore in this study, these challenges are addressed through the objectives; I) to determine comparative longevity in seeds from single populations of 24 species from three succulent genera, 2) to determine seed -longevity differences between three populations of four species within the genus Euphorbia, 3) to quantify moisture relations of longevity _in six Euphorbia species, and 4) to determine how various factors such as moisture sorption properties, oil content, climate and seed structure affect longevity in at least five Euphorbia species. Seed-lots collected from diverse field sites in Kenya and representative for species were dried in a Gene bank dry room at 20 degrees celcius -(0C) and 15% relative humidity (rh) prior to subsamples being tested for comparative longevity at 45°C and 60% rh using a defined protocol. A viability model was used to determine moisture effects on longevity following controlled ageing tests for seed sub-samples held at six moisture contents (range 4 - 9%) at 45°C. Water sorption properties were assessed in an automated IGAsorp at 45°C and oil content determined using supercritical carbon dioxide extraction performed by a fat analyser. Seed structure was assessed after longitudinal sections of seeds were photographed using a colour Carl Zeiss camera and area occupied by the embryo and endosperm traced thus enabling calculation of the respective seed area. Dry weights were assessed gravimetrically after seeds were dried for 17 hours at 103°C according to the International Seed Testing Association (1STA) procedure for oily seeds. Using.geographical information system, climate data for six months post seed dispersal were derived from Wordclim against latitude and longitude data taken at the point of seed collection. Results revealed greater intragenus differences for species. from the genus Euphorbia compared with species from the genus Kalanchoe. Edithcolia grandis indicated longevity between these two other genera. Contrary to the supposition of the viability model, the standard deviation of the frequency distribution of seed deaths in time (0) varied for some seed-lots collected from different years and / or populations. Determined species viability constants Cw, KEand Crh did not show sensitivity to species life-forms. Further, this study revealed that the number of water binding sites, seed size, embryo size, seed dry weight, oil content and climate do not correlate with longevity for these species. However, the ratio of seed embryo to seed area was significantly related to seed longevity. It is concluded that longevity differences in these species are attributed to the initial quality of seed-lots (Ki) and the frequency distribution of seed deaths in time (0). In view of reasonable seed longevity predicted in four species for which viability constants were derived, re-testing in seed banks should be relaxed from the typical 10 years to 15-20 years for seed-lots with high initial viability and probably every five years for accessions with lower initial viability. This study recommend that further research towards developing viability constants of economically important species is vital to enable seed longevity prediction for species found in different agroecological zones in Kenya Nevertheless, ambient seed storage is not recommended for seeds stored under field conditions similar to those for Kisumu or Nairobi investigated in this study.