Comparative floral ontogeny of single-flowered and double-flowered phenotypes of Alcea rosea (Malvaceae)

Abstract

A comparative study of floral ontogeny in single- and double-flowered Alcea rosea L. was conducted using epi-illumination light microscopy. In both floral types, floral differentiation starts with the appearance of three epicalyx lobes, which subsequently subdivide to produce a 7–10-parted epicalyx. Five sepals appear then in a unidirectional or possibly spiral sequence. In single flowers, a corolla-androecium common primordium is formed and subsequently differentiated into five androecial sectors (= primary androecial primordia). Petals are developed at the base of the androecial sectors and secondary androecial primordia are initiated centrifugally in two rows on each sector. Later, tertiary androecial primordia are formed by the subdivision of secondary androecial primordia, which then differentiate into androecial units. Three types of double flowers were identified regarding androecial development. The first type of double flowers shows a more or less disorganised nature. However, 10 proliferation zones can be indentified in the proximal and distal tips of the androecial sectors. In the second and third types of double flowers, androecial development follows similar developmental pathways to that of single flowers. However, in second-type double flowers, the secondary androecial primordia differentiate into petals and the stamens then develop from the free space between the two rows of secondary androecial primordia. In third-type double flowers, after complete primordial partitioning, some primordia on the marginal parts of each androecial sector develop into petaloids or intermediate appendages. The gynoecium appears similarly in both floral types as numerous congenitally united carpel primordia. The double-flowered phenotypes of Alcea appear to fit the criteria for homoheterotopy with complete or partial replacement of stamens with petals, as well as for neoheterotopy, with the formation of stamens in a new position. Based on mutant phenotypes, it is suggested that different functions possibly contribute to the proliferation and differentiation of common primordia.