A histological and ultrastructural study of variegated leaf chimeras of Saintpaulia ionantha Wendl. and their regeneration in vitro

Abstract

Variegated leaves of three African violet cultivars were studied by light and electron microscopy. This analysis combined with inheritance data revealed the following possible LI, LII, LIII periclinal chimera histogenic organizations: (a) cv. Marge Winters, GGW-type, (b) cv. Bold Dance, GWG-type, (c) cv. Calico Kitten, WWG-type. Electron micrographs of the mutated sections showed plastids that had dilated thylakoids typical of PSI mutants. Leaf and petiole explants of these variegated cultivars were cultured using a variety of techniques and growth regulator combinations. From these studies, the growth regulator combination most successful for the uniform regeneration of variegated chimeras was 2.0 mg/l IAA and 2.0 mg/l kinetin. This combination promoted the least amount of callus formation and allowed for more uniform direct plantlet formation from the upper leaf surface. Adventitious shoot formation from callus showed more phenotypic differences in the variegated pattern and was therefore a less predictable method of propagation. Light microscopy was used to study the in vitro formation of adventitious shoots that developed directly from the adaxial leaf surface. The early event in these meristematic regions was xylem vessel formation in the upper spongy mesophyll cells located below two or more periclinally dividing epidermal cells. The majority of the meristematic activity appeared to be in these epidermal cells but subjacent palisade cells also showed cell divisions. The uniform regeneration of three different periclinal chimeras each showing a different LI, LII, LIII shoot apex organization, strongly suggested that the final formation of the shoot required a multicellular and multilayer development.