Variation in seed softening patterns and impact of seed production environment on hardseededness in early-maturing genotypes of subterranean clover

Abstract

This paper describes variation in the dynamics of seed softening (loss of impermeability) in 20 early-maturing genotypes, including 6 cultivars, of subterranean clover (Trifolium subterraneum L.). It reports the effect of 3 sites of seed production in south-western Australia on the pattern of softening in the first summer–autumn and on total softening over the subsequent 2 years. Seeds were softened at a single field location and in a diurnally fluctuating cabinet (60°C/15°C). There was significant variation among genotypes in the pattern of seed softening over the first 5 months after senescence. Cultivars Nungarin, Dwalganup, and Geraldton softened most rapidly in late February, whereas cultivars Dalkeith, Urana, and Izmir softened most rapidly in late March. The duration of field exposure required in order for 50% of the first season’s softening to occur ranged from 44 to 108 days among the 20 genotypes. Persistence of hard seeds into the second and third years also varied among genotypes. Of the cultivars, Nungarin and Izmir had the highest levels of residual hard seed after 30 months (5.3% and 3.9%, respectively), whereas Dalkeith had the lowest (0.9%). Site of seed production had a small but significant effect on both the pattern of softening in the first summer–autumn and the persistence of hard seeds in subsequent years. Seeds produced in a relatively high-rainfall site (768 mm of growing-season rainfall plus supplementary irrigation) had a slower rate of hard seed breakdown than those from either of 2 sites located in the wheatbelt (217 and 423 mm growing-season rainfall). Seed softening through exposure in the field and in a 60°C/15°C fluctuating-temperature cabinet was compared for all genotypes. The cabinet treatment was fairly successful in ranking genotypes for relative between-season hardseededness, although it underestimated total softening by an average of 16%. However, the cabinet treatment was a poor predictor of the within-season pattern of seed softening.

Inter- and intra-population variability in physical dormancy along a precipitation gradient

“…In this sense, Jaganathan (2016) suggested that when in the maternal environment the relative humidity and rainfall are high, seeds could develop a seed coat permeable to water. However, although the lack of significant differences found in our study could be due to the extent of our precipitation gradient, previous studies obtaining the expected pattern in seed with PY were performed in narrower or similar precipitations gradients (Norman et al 2006;Gresta et al 2007). In particular, the study performed by Norman et al (2006) on three populations of Trifolium subterraneum in a precipitation gradient similar than our did find differences in initial dormancy but as the authors mention the differences were less marked than might be expected in view of the substantial precipitation differences among sites.…”

“…However, although the lack of significant differences found in our study could be due to the extent of our precipitation gradient, previous studies obtaining the expected pattern in seed with PY were performed in narrower or similar precipitations gradients (Norman et al 2006;Gresta et al 2007). In particular, the study performed by Norman et al (2006) on three populations of Trifolium subterraneum in a precipitation gradient similar than our did find differences in initial dormancy but as the authors mention the differences were less marked than might be expected in view of the substantial precipitation differences among sites. Therefore, this previous study and our results suggested that maintaining high amount of dormant seeds along this type of precipitation gradients might be the most favourable strategy, although more studies are needed to confirm this trend.…”

Urana subterranean clover (Trifolium subterraneum L. var. subterraneum)

“…Over 6 seasons Urana seeds derived from 1-m single rows averaged 67% hard seed after 16 weeks in a 15/60°C cabinet, using the standard laboratory procedures of Quinlivan (1961), whereas York, Dalkeith, Nungarin, Daliak and Seaton Park had 58, 56, 55, 40 and 25% hard seed, respectively. Laboratory results from spaced plants (Nichols 2000) and field results (Norman et al 2006) support these relative differences.…”

Seed softening patterns of forage legumes in a temperate/subtropical environment in Uruguay

“…However, if softening is delayed until autumn, germination will occur when the probability of follow up rains is much greater (Smith et al, 1996). Such a pattern has been found in Medicago polymorpha L. (Taylor, 1996b) and in some cultivars of T. subterraneum (Norman et al, 2006). There is general agreement that seed softening patterns should be considered in forage legume breeding programs, particularly for environments where substantial losses of seed may occur due to false breaks of season (Piano et al, 1996;Smith et al, 1996;Taylor, 1996a;1996b;Zeng et al, 2005a;Norman et al, 2006).…”

“…Many studies have been conducted in Mediterranean environments in annual species such as Trifolium subterraneum L. and Medicago spp. (Blumenthal and Ison, 1994;Piano et al, 1996;Taylor, 1996a;1996b;2005;Norman et al, 2006) to characterize their patterns of seed softening (the process whereby the seed coat becomes permeable).…”