Stigma structure and receptivity in Bromeliaceae

Abstract

Morphoanatomical characterization of the stigma of different species has provided important data for the taxonomy of Bromeliaceae. Stigma receptivity is fundamental for the effectiveness of reproduction in plants, and the production of hybrids by controlled pollination. This study aimed to characterize stigma morphoanatomy of 18 Bromeliaceae species, from five genera, by means of light and scanning electron microscopy, and to determine the stigma receptivity of these species. The species investigated were: Aechmea bicolor, Aechmea bromeliifolia, Aechmea distichantha, Aechmea fasciata, Aechmea nudicaulis, Ananas sp., Ananas ananassoides, Ananas bracteatus, Ananas lucidus, Ananas parguazensis, Alcantarea nahoumii, Pseudananas sagenarius, Vriesea carinata, Vriesea friburgensis, Vriesea michaelii, Vriesea paraibica, Vriesea simplex and Vriesea unilateralis. Three methods were used comparatively to determine stigma receptivity during floral opening: hydrogen peroxide, benzidine and α-naphthyl acetate. Two stigma types were observed: conduplicate-spiral in Aechmea, Ananas, Alcantarea and Pseudananas and convolute-blade in Vriesea. The stigma is trifid, formed by a unistratified epidermis, parenchyma with numerous idioblasts containing raphides, and three vascular bundles, one for each carpel. In the conduplicate-spiral stigmas type, the inner epidermal cells contain dense cytoplasm and a prominent nucleus, a characteristic not observed in the convolute-blade stigmas type. The stigmatic papillae vary according to species, as well as the presence of an ornamented cuticle. The longest pistil length was observed in A. nahoumii (91.75mm) and shortest in A. bromeliifolia (4.19mm). Higher stigma receptivity is observed at anthesis for the species studied, with the highest receptivity at 8 am and remaining receptive until noon, with the exception of V. unilateralis, for which the greatest receptivity was observed at midnight and extended until dawn. The use of α-naphthyl acetate was the most efficient methodology for the detection of stigma receptivity, and to identify the receptive areas of the stigma.