FLORAL ORGANOGENESIS IN NICOTIANA TABACUM: A COMPARISON OF TWO CYTOPLASMIC MALE‐STERILE CULTIVARS WITH A MALE‐FERTILE CULTIVAR

Flower buds of two cytoplasmically male‐sterile cultivars and one male‐fertile cultivar of Nicotiana tabacum (L.) were studied by light and scanning electron microscopy to compare their floral ontogenies. In [ gla ] tbc , a cultivar with N . tabacum nuclear genes and N. glauca cytoplasm, the five small stamen primordia cease growth soon after they are initiated. They remain unchanged at the flanks of the ovary throughout flower maturation, so the mature flower appears stamenless. In `[ pbg ] tbc ,‘ a N. tabacum cultivar with cytoplasm that was thought to have been derived from N. plumbaginifolia , the divergence from normal stamen development occurs before carpel emergence at which point the primordia are already notably smaller and less cylindrical than primordia of the male‐fertile cultivar. After carpel emergence the primordia continue to develop, forming pink structures called petalodes, which are more similar to petals than stamens. Histological sections show that formation of procambial vascular traces is similar in the abortive stamen primordia of [ gla ] tbc , the petalode primordia of `[ pbg ] tbc ‘ and stamen primordia of male‐fertile N. tabacum . The two cytoplasmic male‐sterile cultivars studied here and one reported elsewhere show a common developmental stage at which the normal ontogenic pattern is altered. We suggest that this stage is the first visible point in N. tabacum androgenesis which requires genetic input from the mitochondrial genome.