Expression of amphicarpy in Vigna lanceolata morphotypes and their hybrids and implications for cultivar development

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.