Seed morphology of Astragalus (Fabaceae, Astragaleae) and its systematic implication: an effort towards a standard terminology

In big plant genera such as Astragalus (Fabaceae), the evaluation of morphological evidence from reliable resources is a prerequisite for understanding the evolutionary history, relationships, and taxonomic placements. The present study is the most comprehensive effort to assess the seed morphological characters in a large sample of species. Seed characteristics including size, color, shape, and testa ornamentation of 494 seed samples representing 348 species and 83 sections and all 11 main clades recognized in Astragalus and five species of related genera were investigated using light and Scanning Electron Microscopy. We provide here a standard terminology for all examined characters to suggest a consensus for future seed morphological studies. Accordingly, seed morphological features show considerable variability, making them useful in some taxonomic aspects, but they may also be heterogeneous, which sometimes limits their application. Based on our results none of the natural groups known as section or clades in Astragalus can be defined based on one character, but rather using a combination of distinctive character states. Considering the currently available phylogenetic trees of Astragalus , the present seed morphological survey allows us to suggest some evolutionary trends. In the course of evolution in most clades, the seed size tends to reduce, reddish seeds tend to be frequent over other colors, black dots or spots appear or distribute more densely on the surface, oblong and square as well as semi-oblong-elliptic shapes become abundant, the pit-like depressions on the seed surface tend to increase, and the multiple reticulate-rugulate testa ornamentation appears more frequently.