Structural diversity and its adaptive significance in seeds of Vigna minima (Roxb.) Ohwi § Ohashi and its allies (Leguminosae-Papilionoideae)

Abstract

The surface features of seed coat, hilum and strophiole, histological aspects of hilum and strophiole, and mechanism of seed dormancy in Vigna minima, V. umbellata, V. dalzelliana and V. calcarata auct. pi. have been investigated. Scanning electron microscopy has revealed a substantial variability in seed coat microtopography, hilum and strophiole, which is of taxonomic and evolutionary significance. The histology of the hilum is uniform in all four taxa, but the structure of the strophiole shows differences between wild and cultivated species. All four taxa have hard seeds; those of V. umbellata have no dormancy but those of wild species have water-impermeable dormancy. Seed germination experiments using araldite-coated, and hilum-micropyle- and strophiole-plugged hard seeds of V. umbellata and water-impermeable seeds of V. minima, which were pin-pricked through the strophiolar groove, demonstrate that the zone of weakness to the permeability of water in hard seeds of these species is the strophiolar region, and not the seed coat, hilum or micropyle. The parenchymatous tissue of the strophiolar plug in the seeds of V. umbellatais associated with the lack of dormancy, whereas the sclerenchymatous tissue of the strophiolar plug in the seeds of V. minima is responsible for the water-impermeable seed dormancy. Under natural conditions, the water-impermeable dormancy of wild species breaks down as a result of the degradation of the hard seed coat, by microbial action, at the strophiolar groove.