Hardseededness in annual clovers: variation between populations from wet and dry environments

Abstract

Annual legumes rely on hardseededness, a form of seed dormancy, to spread the risk of mortality associated with germination and to encourage germination at the optimal time in a season. This paper examines seed softening strategies of a number of clover (Trifolium) species collected over a range of environments. Three hypotheses are tested. The first is that within species, long-term hardseededness increases with aridity due to the greater chance of failure to reproduce in dry environments. It is tested by comparing seed softening patterns of populations of a number of clover species collected from a range of environments. The second hypothesis, that annual legumes from dry environments have a smaller subset of seed softening strategies than legumes from wetter environments, is tested by comparing the range of seed softening patterns from all species found at collection sites. The third hypothesis is that within-year patterns of seed softening that prevent germination after summer rainfall may partially substitute for long-term hardseededness. It is tested by examining the relationship between within-year hardseededness and between-year hardseededness. Accessions of the same species from different collection sites differed in both within-year pattern of seed softening and long-term hardseededness but there was little evidence that these differences were between ecotypes. Different species from the same collection site did not have similar seed softening strategies, and seed softening strategies of clovers from dry sites were as variable as those from wetter environments. Within-year pattern of seed softening does not appear to substitute for between-year hardseededness as a dormancy strategy.

Salt tolerance and avoidance mechanisms at germination of annual pasture legumes: importance for adaptation to saline environments

“…Scimitar in this study, although the extent of seed softening was less in their study. The Lake Grace accession of T. tomentosum had much lower seed softening than in the study of Norman et al (2002), although both studies showed the greatest period of seed softening occurred in late autumn. A high residual hard-seed carryover into the seed bank appears important for annual legume persistence on saline land.…”

“…A high residual hard-seed carryover into the seed bank appears important for annual legume persistence on saline land. On non-saline land hardseededness is widely recognised as a means of ensuring persistence of annual pasture legumes between seasons in the absence of seed production, such as can occur during severe cases of drought, flooding, insect attack, disease infection or overgrazing (Norman et al 1998(Norman et al , 2002 or after a cropping phase when grown in rotation with crops (Taylor et al 1991). Theoretical models (Cohen 1967;Venable and Brown 1988) indicate that as the level of environmental uncertainty increases, the requirement for long-term seed dormancy increases.…”

“…Dalkeith and a composite of three Melilotus siculus genotypes, were used in all experiments (Table 1). A naturalised strain of T. tomentosum from Lake Grace, Western Australia (Norman et al 2002), which differed from that used by Nichols et al (2008), was also included in Experiments 1 and 2. Seeds of each species were sourced from well fertilised, irrigated plots grown in 2005 at Medina Research Station (32.2°S, 115.8°E) in a randomised block design with three replicates.…”

“…Sample size in each replicate consisted of 200 seeds in Experiment 1, (Norman et al 2002) 50 seeds in Experiments 2, 3 and 4, apart from T. subterraneum (23 seeds) in Experiment 4, and 20 seeds in Experiment 5. Scarification, to remove the effects of seed coat impermeability, was conducted for each experiment, apart from Experiment 1, by placing seeds between two pieces of sand paper (quartz coated, 240 grit) and rubbing with gentle hand pressure.…”

Seed dormancy-breaking in a cold desert shrub in relation to sand temperature and moisture

“…Variation in temperature and substrate moisture content among different burial periods and depths are considered key factors that influence PY-breaking in natural conditions (Norman et al 2002; Van Klinken et al 2006, 2008; Hu et al 2009). However, substrate moisture does not seem to play an important role in modulating PY- break of E. songoricum seeds under natural conditions.…”