Characterization and location of a semipermeable layer in seed coats of leek and onion (Liliaceae), tomato and pepper (Solanaceae)

Abstract

Semipermeability may be defined as the ability of seed coverings to allow water uptake and gas exchange while solute diffusion is restricted or prevented. The presence and location of a semipermeable layer was studied in seed coats of cabbage, leek, onion, tomato and pepper. Seed coat thickness varied by species, although all species readily imbibed water. Cabbage seeds had permeable seed coats and non-germinable seeds readily leaked amino acids and non-aged seeds showed positive staining with tetrazolium chloride (TTC) without pre-treatment. Tomato and pepper seeds had relatively impermeable seeds coats, while leek and onion were intermediate with respect to amino acid leakage and TTC uptake. Seeds were soaked in an aqueous solution of lanthanum nitrate, and then cross-sections of seeds were examined with a microprobe or STEM using energy dispersive X-ray analysis and back-scattered imaging. The penetration of lanthanum crystals was limited by a thin semipermeable layer of the seed coat adjacent to the endosperm in leek, onion, tomato and pepper. Morphological studies did not reveal a semipermeable layer in cabbage.