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Background and Aims: The duration of the plant life cycle is an important attribute that determines fitness and coexistence of weeds in arable fields. It depends on the timing of two key life-history traits: time from seed dispersal to germination and time from germination to flowering. These traits are components of the time to reproduction. Dormancy results in reduced and delayed germination, thus increasing time to reproduction. Genotypes in the arable seedbank predominantly have short time to flowering. Synergy between reduced seed dormancy and reduced flowering time would create stronger contrasts between genotypes, offering greater adaptation in-field. Therefore, we studied differences in seed dormancy between in-field flowering time genotypes of shepherd's purse., Methods: Genotypes with early, intermediate or late flowering time were grown in a glasshouse to provide seed stock for germination tests. Secondary dormancy was assessed by comparing germination before and after dark-incubation. Dormancy was characterized separately for seed myxospermy heteromorphs, observed in each genotype. Seed carbon and nitrogen content and seed mass were determined as indicators of seed filling and resource partitioning associated with dormancy., Key Results: Although no differences were observed in primary dormancy, secondary dormancy was weaker among the seeds of early-flowering genotypes. On average, myxospermous seeds showed stronger secondary dormancy than non-myxospermous seeds in all genotypes. Seed filling was similar between the genotypes, but nitrogen partitioning was higher in early-flowering genotypes and in non-myxospermous seeds., Conclusions: In shepherd's purse, early flowering and reduced seed dormancy coincide and appear to be linked. The seed heteromorphism contributes to variation in dormancy. Three functional groups of seed dormancy were identified, varying in dormancy depth and nitrate response. One of these groups (FG-III) was distinct for early-flowering genotypes. The weaker secondary dormancy of early-flowering genotypes confers a selective advantage in arable fields.

Wild arable plants can be an economic burden but they also support diverse arable food webs and contribute to valuable ecosystem functions. These benefits may have been compromised over recent decades by declining weed diversity. The decline in wild arable plant diversity has been viewed predominantly in terms of species shifts a view that ignores the genetic and functional variation existing within species and the impact on ecological and evolutionary processes which this may have. To examine within-species diversity, ISSR markers were used in parallel with environmental and phenotypic characterisation, to investigate the population structure and diversity of Capsella bursa-pastoris (shepherd's purse) from arable fields in the UK. Analysis of 338 ISSR products for 109 individuals from 51 accessions obtained from the seed banks of 33 arable fields showed that in-field populations of shepherd's purse were genetically differentiated between individuals, and among accessions and fields. In addition, cluster analysis identified three genetically distinct regional-scale populations. Phenotypic variation was present at all scales of genetic differentiation, including the regional scale where populations differed in their key life-history traits: flowering time, fecundity and dormancy. Genetic drift is proposed as a contributor to differentiation among genetically isolated but locally co-occurring accessions. In addition, the genetic and phenotypic variation in shepherd's purse exhibited large scale, spatial trends and showed statistically significant associations with cropping intensity and soil-pH. These results suggest that adaptation as a result of selection by cropping practise and soil-pH has played a role in the ability of shepherd's purse to colonise and persist in arable fields. [ABSTRACT FROM AUTHOR]