Amphicarpy, an adaptive trait whereby both aerial and underground fruits are formed on the one plant, occurs in several plant taxa, notably the Phaseoleae legumes. Amphicarpic species offer the dual potential benefits of enhanced persistence through their underground seed, combined with ease of harvest of their aerial seed. While amphicarpy has been reported in several endemic Australian tropical legumes, information on the trait is sparse. The objective of the current research was to compare aerial and underground reproductive structures in amphicarpic tropical legumes from four different sub-tribes within the Phaseoleae: three Australian endemic species, Vigna lanceolata (sub-tribe Phaseolinae), Flemingia pauciflora (sub-tribe Cajaninae), and Glycine falcata (sub-tribe Glycininae); and the exotic pasture legume Centrosema rotundifolium (sub-tribe Clitoriinae). As far as we know, this report of amphicarpy in F. pauciflora is the first record of the trait in a member of the Cajaninae. Descriptions, drawings, and photographs of the morphology and anatomy of the aerial and underground fruiting structures were documented. In general, the aerial flowers in all genotypes studied were chasmogamous, allowing at least some opportunity for outcrossing. In contrast, the underground flowers were invariably much reduced, with a small, non-pigmented corolla enclosed in much-reduced, scale-like sepals. Nonetheless, anthers and viable pollen were observed in the underground flowers in all four species. With the exception of C. rotundifolium, the underground fruiting structures formed on rhizomes which initially arose either from the underground cotyledonary nodes or, in the case of G. falcata, which is epigeal, from the junction of the stem and taproot. The rhizomes gave rise to ramets when they emerged at the surface or from holes in pot bases. The V. lanceolata accessions also produced fleshy tubers which gave rise to rhizomes, especially in subsequent years. In C. rotundifolium, the geocarpic structures arose on specialised, fleshy, geotropic stems that grew down from the stoloniferous stems. In all species, the number of seeds per underground pod was fewer than in the aerial pods, and the underground seeds were invariably larger, although the extent differed between legume genotypes. There was no evidence of effects on growth or development depending on whether plants were grown from aerial or underground seeds. Some of the adaptive and agronomic implications of the key findings are discussed. In particular, it is argued that amphicarpy in the Australian species is an adaptation to seed predation, and to spatially heterogeneous inland soils.
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Vol. 63 • No. 6