Genetic improvement of subterranean clover (Trifolium subterraneum L.). 1. Germplasm, traits and future prospects

Abstract

Subterranean clover (Trifolium subterraneum L.) is the most widely sown annual pasture legume species in southern Australia, valued in the livestock and grains industries as a source of high-quality forage and for its ability to fix atmospheric nitrogen. From its initial accidental introduction into Australia in the 19th Century and subsequent commercialisation in the early 1900s, 45 cultivars have been registered in Australia. These consist of 32 cultivars of ssp. subterraneum, eight of ssp. yanninicum, and five of ssp. brachycalycinum and range in flowering time from 77 to 163 days from sowing, enabling the species to be grown in a diversity of rainfall environments, soil types, and farming systems. Eleven of these cultivars are introductions from the Mediterranean region, 15 are naturalised strains collected in Australia, 18 are the products of crossbreeding, and one is derived from mutagenesis. Cultivars developed in Italy have been commercialised for the local market, whereas other cultivars developed in Spain, Portugal, and France have not had commercial seed production. Important traits exploited include: (i) selection for low levels of the oestrogenic isoflavone formononetin, which causes reduced ewe fertility; (ii) increased levels of dormancy imposed by seed-coat impermeability (hard seeds) for cultivars aimed at crop rotations or unreliable rainfall environments; (iii) strong burr-burial ability to maximise seed production; (iv) resistance to important disease pathogens for cultivars aimed at medium- and high-rainfall environments, particularly to Kabatiella caulivora and root rot pathogens; (v) resistance to pests, particularly redlegged earth mites; and (vi) selection for unique leaf markings and other morphological traits (where possible) to aid cultivar identification. Cultivar development has been aided by a large genetic resource of ~10 000 accessions, assembled from its centre of origin in the Mediterranean Basin, West Asia, and the Atlantic coast of Western Europe, in addition to naturalised strains collected in Australia. The development of a core collection of 97 accessions, representing almost 80% of the genetic diversity of the species, and a genetic map, provides a platform for development of future cultivars with new traits to benefit the livestock and grains industries. New traits being examined include increased phosphorous-use efficiency and reduced methane emissions from grazing ruminant livestock. Economic analyses indicate that future trait development should focus on traits contributing to increased persistence and autumn–winter productivity, while other potential traits include increased nutritive value (particularly of senesced material), increased N2 fixation ability, and tolerance to cheap herbicides. Beneficial compounds for animal and human health may also be present within the species for exploitation.

Effects of dark or of red, blue or white light on germination of subterranean clover seeds

“…However 'Seaton Park' only partially fits in these characteristics. 'Seaton Park', and subterranean clover in general, is not found in high stress habitats as those mentioned, seeds hardly qualify as very small, and at maturation they show an intermediate level of hardseededness (Taylor, 1981;Smetham, 1999;Nichols et al, 2013). Conversely, like in other Trifolium, the embryo occupies almost all seed, the endosperm being almost completely absent (Martin, 1946).…”

“…Thus, in all likelihood seed germination of 'Seaton Park' will be more responsive to light cues and less to seed coat permeability. Adding to these features, 'Seaton Park' has very low levels of formononetin, an oestrogenic isoflavone that is the main cause of cystic glandular hyperplasia in sheep, commonly named 'clover disease' (Nichols et al, 2013). …”

“…We selected 'Seaton Park' among cultivars of subspecies subterraneum to investigate responses of germination to dark or to red, blue or white light not only because of its high burial ability but also because of the intermediate level of hardseededness at maturation it presents (Taylor, 1981;Smetham, 1999;Nichols et al, 2013). Thus, in all likelihood seed germination of 'Seaton Park' will be more responsive to light cues and less to seed coat permeability.…”

The career and contribution to Australian and international agricultural science of Clive McDonald Francis: an introduction

“…Analysis of these plant collecting efforts, particularly the focus on subterranean clover, is detailed by Berger et al (2013) and Nichols et al (2013) …”

Forage breeding for changing environments and production systems: an overview

“…For instance in Australia, subterranean clover (Trifolium subterraneum) and phalaris (Phalaris aquatica) are two species that have been the beneficiaries of germplasm introduction and characterisation to greatly extend zones of agronomic adaptation and improve seasonal growth patterns (Oram et al 2009;Nichols et al 2013). At ISFB 2013 this concept was exemplified and extended to the successful incorporation of molecular tools for interspecific hybridisation in white clover and the breeding of the perennial ryegrass/endophyte (Neotyphodium lolii) symbiosis in the plenary presentations by Profs Warren Williams (Williams 2014) and German Spangenberg respectively and in the use of Trifolium arvense genes to develop transgenic white clover with increased condensed tannin production (Hancock et al 2014).…”