Diversity of seed-coat structure and thickness among six genera representing tribes Sophoreae, Crotalarieae, and Genisteae (Papilionoideae, Fabaceae) in Egypt

The anatomical structure of seed coats of 13 Egyptian taxa belonging to three Fabaceae-Papilionoideae tribes, Sophoreae, Crotalarieae, and Genisteae, were investigated using light microscopy and scanning electron microscopy. Mature seeds show an anatomically complex structure, characteristic of members of Papilionoideae, composed of three main layers: macrosclereids, hypodermis, and parenchyma, in addition to an outermost layer of seed cuticle. However, there were considerable variations among studied taxa in seed-coat characteristics, including the total seed thickness, the shape and thickness of seed cuticle, the shape and length of macrosclereids, and the shape, number of layers, arrangement, and length of both hypodermal and parenchyma layers. The taxonomic importance of these variations is discussed, and a key for differentiation among Egyptian taxa using seed anatomical features is presented. Two-layered hypodermis was characteristic to Anagyris foetida, whereas the remaining taxa contained one-layered hypodermis. Wavy seed cuticles were observed only in the four species of Lupinus (L. angustifolius, L. digitatus, L. palaestinus, and L. albus) and Retama raetam subsp. raetam. The study showed that multivariate (cluster and PCA) analyses of seed anatomical characteristics are helpful in clarifying the phenetic relationships among studied taxa at both tribal and intrageneric levels. The taxonomic value of the classification obtained from the cluster and PCA analyses is compared with recent systematic reviews of the tribal positions of the investigated six genera. Our results suggest placement of Argyrolobium in tribe Crotalarieae, instead of Genisteae.

Automated summary

The anatomical structure of seed coats of 13 Egyptian taxa belonging to three tribes of Fabaceae-Papilionoideae was investigated. Variations in seed-coat characteristics were observed among the studied taxa. The analysis of seed anatomical characteristics helps in understanding the phenetic relationships among taxa at different levels.