Flora - Morphology, Distribution, Functional Ecology of Plants
Stigma and style anatomy and ultrastructure in Italian Oxalis pes-caprae L. and their possible connection with self-incompatibility
Introduction
Oxalis pes-caprae L. is a bulbiferous geophyte native of the Cape Region (South Africa), which was introduced as an ornamental plant into Europe and the Mediterranean region in the second half of the eighteenth century. It later became naturalised in several countries of the world with a Mediterranean or subtropical climate and in many of these it is currently a troublesome invasive weed (Baker, 1965, Ducellier, 1913, Hantz, 1985, Lambdon, 2006, Michael, 1964, Ornduff, 1987, Rappa, 1911, Vignoli, 1937). The flowers, grouped in cymose umbrellas, are pentamerous and pentacyclic, with five sepals, five petals fused at the base, ten stamens in two whorls of five and a gynoecium composed of five carpels with connate superior ovaries and distinct styles and stigmas. As in most species of the genus (see e.g. Judd et al., 1999, Stevens, 2012), flowers are heteromorphic, with three different stylar morphotypes: long-styled flowers (L) with a long style and two whorls of stamens below the stigma; short-styled (S), with stamens at two levels above the stigma; and mid-styled (M), with one whorl of anthers above the stigma and the other below it. In its native area, all three morphotypes can be found and both isopletic (1:1:1) and anisopletic populations have been observed (Castro et al., 2013, Ornduff, 1987). In Europe and the Mediterranean region, the S morph is known to be dominant, but in the western part of the area, L and M plants have also been recently detected (Castro et al., 2007, Castro et al., 2013). In naturalised populations growing wild in Italy, only S plants have been reported for certain so far (Signorini et al., 2013). Due to complex self- and morph-incompatibility systems linked to heteromorphic flowers (see Barrett and Cruzan, 1994, for a discussion on different aspects of the so-called ‘heterostylous syndrome’), it is commonly believed that in monomorphic populations outside its native range the plant is hardly able to bear seeds (Castro et al., 2007, Michael, 1964, Ornduff, 1987). However, sporadic seed production has been reported in the past in Italy (Vignoli, 1937) and more recently, the production of viable seeds was observed in the western Mediterranean area (Castro et al., 2013). In any case, the plant has efficiently spread in the invaded areas due to vegetative propagation through bulbils.
As in all heteromorphic species, self-incompatibility in O. pes-caprae is sporophytic: during microsporogenesis, proteins involved in recognition processes are most likely deposited by diploid tapetum cells onto the exine, where they participate in forming the pollen coat (De Nettancourt, 1977, Edlund et al., 2004, Heslop-Harrison et al., 1973, Vasil, 1974). In most heterostylous species, self-incompatibility is considered to be the result of a cascade system formed by a sequence of barriers, none of which alone is necessarily completely effective (Barrett and Cruzan, 1994). In a review on stigma structure and function in pollination, Edlund et al. (2004) have underlined the role of these barriers in self-incompatibility and Cresti et al. (1976) noted that stylar transmitting tissue probably plays a role in the incompatibility reaction; Vasil and Johri (1964) pointed out that a morphological and anatomical investigation of stigma and style features is a prerequisite in studies on sterility and incompatibility.
The aim of this investigation is to clarify anatomical and ultrastructural features of stigma and style in flowers of O. pes-caprae from Italian populations (S morph), in order to point out possible relationships between these features and self-incompatibility mechanisms that prevent incompatible pollen tubes development and/or block their growth towards ovules. For a better understanding of these aspects linked to self-incompatibility in Italian populations, additional observations on flowers of L and M morphs (plants from bulbs collected in the native area) were also carried out. This investigation is part of a global study currently in progress on the biology, ecology, genetics and distribution of O. pes-caprae in Italy and the Euro-Mediterranean area.
Section snippets
Plants
Living material of Italian plants was collected at Riomaggiore (Liguria, NW-Italy; 44°05’52”N; 9°44’23”E) from a wild population susceptible both to self-pollination and to free pollination. According to Signorini et al. (2013), this population is pentaploid (2n = 5 × = 35), as well as all the wild populations in Italy studied in the cited paper. Samples were taken seven times during the year 2012, from January 8 to April 16, at intervals of 9–16 days. During sampling, flowers were collected at
Results
As expected, all the flowers collected at Riomaggiore belonged to the S morph. Fresh flowers in all phenological stages showed no copious fluid secretion on stigmatic papillae.
Due to style bending, the vertical alignment of styles and stigmata changes during anthesis: they are erect at pre-anthesis, markedly curved outward (90° approx.) at anthesis and slightly curved inwards post-anthesis (Figs. 1A–C). Style bending at anthesis allows stigmata of S flowers to be exposed and easily reached by
Discussion
In agreement with the classification proposed by Heslop-Harrison and Shivanna (1977) and Heslop-Harrison (1981), flowers of the studied populations of Oxalis pes-caprae have stigmata of the DPMMs type (i.e. dry surface, distinctly papillate, with multicellular, multiseriate papillae), thus confirming what has been reported for the Oxalidaceae by Heslop-Harrison and Shivanna (1977), Heslop-Harrison (1981) and Rosenfeld and Galati (2009). According to Heslop-Harrison and Shivanna (1977), dry
Acknowledgements
We thank Dr. Enrico Della Giovampaola for his contribution in preliminary observations at LM and in collecting living plants in Italy and Europe. This study was supported by University of Florence (Fondo ateneo 2012, resp. M.A. Signorini). A first draft of the manuscript was highly improved thanks to the suggestions and criticisms of two anonymous reviewers.
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