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Amphicarpy is an adaptation that enhances persistence under various environmental stresses, including heavy grazing, through the production of both aerial and underground seeds on the same plant. The trait occurs in several Australian endemic tropical herbaceous legumes, including Vigna lanceolata. The objective of the current research was to compare aerial and underground reproductive structures in six amphicarpic morphotypes of V. lanceolata (four perennials and two annuals), and in hybrids among these morphotypes and also with a seventh, non-amphicarpic perennial morphotype. The aim was to establish how easily the trait might be manipulated in a breeding program to develop improved amphicarpic cultivars, for ley and forage purposes. In the four amphicarpic perennials, the underground fruiting structures formed on leafless underground stems or rhizomes, and in all four, amphicarpy was habitual, i.e. individual plants routinely developed both aerial and underground fruiting structures. In the two annuals, the underground fruiting structures developed on specialised axillary geotropic stems that arose along the prostrate aerial stems and penetrated the soil or leaf litter. One of the annuals exhibited habitual amphicarpy, whereas the other was opportunistic, in that the underground fruiting structures developed only where aerial stems became covered with soil or leaf litter. F1 hybrids between the two annual morphotypes exhibited habitual amphicarpy, indicating dominance of habitual over opportunistic amphicarpy. F1 hybrids between the four perennial amphicarpic morphotypes were all amphicarpic, reflecting the similar expression of the trait in these forms. By contrast, when these four were crossed with the non-amphicarpic perennial, none of the viable F1 hybrids exhibited amphicarpy, suggesting dominance for non-expression. Many of the perennial F1 hybrids were self-sterile, but in the hybrids that were at least partially fertile, many more geocarpic than aerial seeds were produced, raising the possibility that in the wild, amphicarpy may facilitate the persistence of chance natural outcrosses that might otherwise not survive. The results suggested that it would be feasible to develop amphicarpic annual and amphicarpic perennial cultivars for, respectively, ley and forage purposes.

As well as being part of the wider gene pool of cultivated species such as mungbean and cowpea, Vigna lanceolata Benth. is of agronomic interest as a potentially useful species in its own right. It is widely adapted across northern Australia from the coast to inland desert regions, and possesses attributes that make it potentially suited as a ley, cover crop or pasture legume in seasonally arid tropical environments. The species comprises several morphotypes that variously differ in their broad geographic distribution, life cycle, habit, edaphic adaptation and/or amphicarpy. In order to assess the potential for breeding improved cultivars, eight representative accessions were hybridised in a complete diallel cross and the viability and fertility of the resultant hybrid progeny evaluated. Of the 56 parental combinations in the diallel, 33 resulted in healthy F1 hybrid plants, and of these, nine were at least partly self-fertile. Six of these combinations were the reciprocal crosses between three of the perennial, tuberous-rooted, amphicarpic morphotypes. Another two were the reciprocal crosses between the two annual morphotypes. The patterns of relatedness among accessions indicated by the cross-pollination studies were broadly supported by DArT molecular marker analyses, and suggested that there has been some genetic differentiation within the V. lanceolata complex. Although the process of speciation remains far from complete, the levels of genetic compatibility between some morphotypes would be inadequate to enable a breeding program to draw easily on the full range of genetic diversity within V. lanceolata. Two plausible breeding options are suggested. The first is the development of perennial, tuberous-rooted, amphicarpic ideotypes suited perhaps as pasture or understorey legumes, drawing on selected accessions from the three perennial morphotypes, where there was sufficient genetic compatibility and which collectively have wide geographical spread. The second is an annual, freely seeding, fibrous-rooted, amphicarpic ideotype suited perhaps for use as a self-regenerating ley legume, drawing on selected accessions from the two annual morphotypes.