Abstract
Stipules are leaf structures common in many groups of plants that can take a variety of forms. In Gentianales, interpetiolar stipules are considered a synapomorphy of Rubiaceae; however, some reports in the literature refer to their presence in other families. The goal of this study was to analyze the development of leaf primordia to investigate the possible presence of reduced or modified stipules in Apocynaceae. Shoot apices of 12 genera were analyzed under light and scanning electron microscopy comparatively with one species of Rubiaceae. Early in their development, leaf primordia form two lateral expansions at the base of the petiole (stipules) that give rise to colleters in 11 of the 12 genera of Apocynaceae studied, similarly to the Rubiaceae species. The basal genera have pairs of stipules modified into colleters positioned laterally to the petiole, while other species belonging to the derived subfamilies have interpetiolar stipules that each project towards the opposite stipule and merge, forming a sheathing stipule and from this arc the interpetiolar colleters originate. The ontogenetic study proved for the first time that Apocynaceae is a stipulate family whose stipules are modified into colleters and their absence might be a secondary loss, changing the interpretation of stipule evolution in Gentianales.
Highlights
Stipules are usually small, inconspicuous projections of the petiole base of leaves with protective function mainly in the bud, and there might be other functions as well
Stipules occur in many families of Eudicots and are especially important in some orders, such as Gentianales, which is composed of the families Apocynaceae, Gelsemiaceae, Gentianaceae, Loganiaceae and Rubiaceae (APG III 2009)
The goal of this study was to verify the presence of reduced or modified stipules in the leaves of 12 genera of Apocynaceae belonging to different subfamilies, focusing on the beginning of leaf development and the ontogeny of interpetiolar colleters occurring on the petiole base to evaluate their possible stipular nature
Summary
Inconspicuous projections of the petiole base of leaves with protective function mainly in the bud, and there might be other functions as well. Often they appear as a pair of leafy appendages located on each side of the leaf base, but in some plants they may be modified into thorns or glands (Sinnott and Bailey 1914; Simpson 2006). Bremer and Struwe (1992) considered the interpetiolar stipules as an ancestral character for Gentianales families (symplesiomorphy), and the absence of such structure is probably due to reversion
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