Caryopsis micromorphological survey of Sorghum (Poaceae)—Taxonomic implications

• Caryopsis size and stylopodium persistence pattern have limited taxonomic application at the infrageneric level, while the caryopsis shapes show a certain taxonomic significance at the infrageneric level and spermoderm sculpture patterns are important diagnostic characters at the interspecific level in Sorghum.
• Consistent micromorphological characters of stylopodium persistence, dorsoventral compression, embryo proportion (the ratio of embryo to caryopsis length) and hilum proportion (the ratio of hilum to caryopsis length) are firstly found in Sorghum.

Caryopsis micromorphological characters of 25 Sorghum species were studied using stereoscopic microscopy and scanning electron microscopy. There are three caryopsis shapes, including obovate-elliptic, ovate-elliptic and subulate; two patterns of stylopodia, including with or without an apical tuft of unicellular microhairs; and five spermoderm sculpture patterns, that is, reticulate with wavy cell walls, reticulate with straight cell walls, substriate, undulating and rugose in Sorghum. Caryopsis size and stylopodium persistence pattern have limited taxonomic application at the infrageneric level, while the caryopsis shapes show a certain taxonomic significance at the infrageneric level and spermoderm sculpture patterns are important diagnostic characters at the interspecific level in Sorghum. The separation of Sorghum sudanense from S. × drummondii is supported by our result of caryopsis size and spermoderm sculpture patterns. Consistent micromorphological characters of stylopodium persistence, dorsoventral compression, embryo proportion (the ratio of embryo to caryopsis length) and hilum proportion (the ratio of hilum to caryopsis length) are firstly found in Sorghum. Caryopsis micromorphological characters provide evidence that Sorghum is a highly heterogeneous genus at the micromorphological level.