Intraspecies Variation in Seed Traits

A range of accessions of species from the UK, Europe, Australia and the USA were characterized for seed functional traits as influenced by seasonal variation in environment.

Colour heteromorphism in Capsella bursa-pastoris seeds is associated with varying degrees of myxospermy and small but significant differences in seed dimensions. The X-ray images show three seed morphology types.

Flowering time and seed dormancy are key functional traits in the plant life cycle that display diversified bet hedging, which is defined as reduced plant fitness offset against enhanced variance for a trait (Childs et al., 2010). Although progeny is reduced (due to a delay in the start of the life cycle) and even mortality can be increased under any environmental condition; sufficient individuals in the cohort will survive, thus ascertaining survival of the germline. These benefits are particularly clear for species that are highly prolific in producing seeds.

The aim of the MSB project is to have at least one copy of 95% of the alleles occurring in the population at frequencies greater than 0.05 represented in all collections. This goal is said to be achieved by randomly sampling 30 individuals in a completely outbreeding sexual species or 59 individuals in a species with self-fertilisation. Seeds are usually collected at the height of seed maturity on a single collecting trip, thus maximising the seed numbers harvested before dispersal. For obvious reasons this collecting protocol does not cover sub-populations with early or late seed maturation.

Genetics forms the basis for variation in functional traits, including flowering time and seed dormancy. Bet-hedging for flowering time, depending on the variation in maturation time, may results in a smaller coverage of the genetic diversity than desired in a collection. Synergism between flowering time and seed dormancy would maximise the bet-hedging strategy, and would further reduce this coverage, resulting in accessions with less genetic variation for both traits than anticipated.

In the current project we studied variation in seed dormancy related to flowering time in Capsella bursa-pastoris from arable fields (Farm Scale Evaluation). Whilst primary dormancy was marginal and no differences were observed, secondary dormancy was more important and weaker among the seeds of early flowering genotypes. This synergism of two bet-hedging strategies enhances the functional diversity between accessions of a single species. This finding has implications for the representation of alleles in banked accession of this and other species.

Myxospermy was observed in all genotypes, but was not displayed by all seeds. This mucilage heteromorphism was taken into consideration in the current study. The osmotic pressure of the seed mucilage is considerable (-0.54 MPa) and may influence the seed’s response to osmotic stress. This will be investigated in more detail in planned studies for the next 5 years.

In a related study, a prairie species (Echinacea angustifolia) under threat by rangeland development was studied. Based on microsatellites, seeds from early flowering / harvest time diverged genetically from peak and late cohorts. Late harvested seeds from late flowering plants had lower quality than those earlier in the season. Nonetheless, remnant populations in a fragmented biome have not yet suffered inbreeding or loss of genetic diversity. Results form the major part of a PhD thesis, being converted into peer-reviewed publications.

Research in Kenya, which formed a PhD project and subsequent peer-reviewed publications, explored the effect of growing conditions (particularly temperature) on seed traits of the potential oil crop Vernonia galamensis (Asteraceae). Studies on Vernonia demonstrated a key role for maternal environmental conditions in determining seed quality; seeds, from the same original seed lot, were grown in sites with contrasting climatic conditions resulting in substantial differences in dormancy level in the ensuing seed crops.

We also observed that seeds of an Australian native forb (Goodenia fascicularis) from plants grown ex situ are likely to exhibit physiological dormancy regardless of the maternal environment. However, seeds collected from warm, dry environments are likely to have lower relative dormancy and be more responsive to warm stratification than seeds from cooler, wetter environments (Hoyle et al., 2008).

Finally, spatial sampling of seeds of Acer pseudoplatanus across 19° latitude in Europe displayed phenotypic variation in seed functional traits (dormancy, desiccation tolerance, mass, etc) with developmental heat sum, which was 50% higher in southern compared to northern Europe (Daws et al., 2006). Only southern populations had sufficient desiccation tolerance for low temperature storage (banking) and lower dormancy.

In conclusion, flowering time, harvesting time and maternal environment have profound implications for seed functional traits in herbaceous and woody species at the point of collecting with possible implications for coverage of alleles in populations and downstream handling in agricultural production systems and seed banks alike.