Floral and reproductive biology of Alpinia purpurata (Vieill.) K. Schum. (Zingiberaceae): An important tropical ornamental plant

Alpinia spp. play an important ecological role as a source of nectar in the understory of tropical and subtropical forests and many are common. The perennial species Alpinia purpurata (Vieill.) K. Schum. is native to the tropical rainforests of Asia and has been used as an ornamental cut plant because of its lush inflorescences and post-harvest durability. The internal and external morphology, floral biology, and reproductive system of four A. purpurata cultivars were investigated to contribute to studies on the genetic improvement of this species. The inflorescence of A. purpurata is terminal and globose, and has flowers protected by colorful and showy bracts. The white flowers are tubular and bisexual. The floral anthesis of these four cultivars lasted 9 to 12 hours, and the pollen viability was above 80% throughout the pre-flowering period in all evaluated cultivars. During anthesis, the stigma and anther crest were receptive because of the presence of stigmatic fluid. Fruiting was observed after hand cross-pollination, whereas no fruiting was recorded in the treatments with spontaneous self-pollination, hand self-pollination or geitonogamy. Natural pollination induced low fruiting, suggesting low pollinator availability in the studied area. The obligatory xenogamy of A. purpurata facilitates the development of new hybrids for the ornamental plant market. Additional keywords: breeding system; floral structure; stigmatic fluid; stigmatic receptivity. 1 Universidade do Estado de Mato Grosso, Tangará da Serra (Brazil). ORCID França, R.P.A. : 0000-0001-8504-2928; ORCID Botini, A.F. : 0000-0003-4060-7942; ORCID Silva, I.V. : 0000-0003-0281-0608; ORCID Krause, W. : 0000-0002-5308-7715; ORCID Silva, C.A. : 0000-0001-7898-7931 2 Universidade Federal de Mato Grosso, Cuiabá (Brazil). ORCID Koch, A.K. : 0000-0002-9662-6459; ORCID Caetano, A.P.S. : 0000-0002-8256-0380 3 Corresponding author. celice@unemat.br Zingiberaceae Martinov is the largest family of the order Zingiberales, with 52 genera and about 1,600 species (WFO, 2019). The family has a pantropical distribution, with only one genus (Renealmia L.f.) distributed in the Neotropics and Africa, while four others (Alpinia Roxb., Curcuma L., Hedychium J. Koenig and Zingiber Mill.) were introduced into and naturalized in the Neotropics (Kress, 1990; Stevenson and Stevenson, 2004; Hamidou et al., 2012). In Brazil, the family is represented by eight genera, 32 species and one subspecies (BFG, 2015, JBRJ, 2020). The floral morphology of the family invariably consists of bisexual and zygomorphic flowers with a normally tubular calyx, formed by two sepals, a corolla with three petals and two fused petaloid staminodes, forming an abaxial labellum, and a functional stamen with two thecae (Kirchoff, 1988; Endress, 1994). Additionally, more than 25 genera in this family have a crest at the anther apex, with broad variation in shape and size (Fan and Li, 2012; 2016). The anther crest is a specialized anther appendage that extends up from the top of the anther to form a tail-like structure, with broad variation in shape and size between the different genera, and is widely characterized in the Zingiberaceae family. The function of the anther crest is to boost the male and female functions of plants, directing pollinators to an ideal position for removing and receiving pollen (Fan and Li, 2016). The style is thin, and the anther fillet is inside a ventral groove between the thecae. The fusion of the lateral staminodes into a labellum, the presence of a pair of nectariferous glands at the style base, and cells containing essential or ethereal oils are autapomorphies in this family (Kress, 1990). The peculiar floral morphology of the family Zingiberaceae may be related to a specialized floral system. Flexistyly is a sexual dimorphism consisting of two forms that differ in the temporal expression of sexual function and that involve the reciprocal movement of the stigmatic surface through a vertical axis during the flowering period, which combines reciprocal herkogamy and dichogamy and promotes outcrossing (Li et al., 2001; Cardoso et al., 2018). This strategy has been documented in 24 Zingiberaceae species, including those within the genus Alpinia (Kress et al., 2001). Alpinia is the largest genus of the family Zingiberaceae, with 346 widely distributed species in tropical Asia (Smith, 1990; Kress et al., 2001). The following three species are frequently cultivated in Brazil: A. RESUMEN Alpinia spp. desempeñan un importante papel ecológico, como fuente de néctar, en el sotobosque de los bosques tropicales y subtropicales, donde muchas de estas especies son comunes. La especie perenne Alpinia purpurata (Vieill.) K. Schum. es nativa de las selvas tropicales de Asia y se ha utilizado como planta de corte ornamental debido a las exuberantes inflorescencias y la durabilidad posterior a la cosecha. Se investigó la morfología interna y externa, la biología floral y el sistema reproductivo de cuatro cultivares de A. purpurata, para apoyar los estudios de mejora genética de la especie. La inflorescencia de A. purpurata es terminal, globosa y tiene flores protegidas por brácteas coloridas y vistosas. Las flores blancas son tubulares bisexuales. La antesis floral de los cuatro cultivares duró entre 9 y 12 horas. La viabilidad del polen fue superior al 80% durante todo el período de pre-floración en todos los cultivares evaluados. Durante la antesis, el estigma y la cresta de la antera fueron receptivos. La fructificación de los cultivares se observó después de la polinización cruzada manual. No se registró fructificación en los tratamientos de autopolinización espontánea, autopolinización manual y geitonogamia. La polinización natural indujo baja fructificación, lo que sugiere una baja disponibilidad de polinizadores en el área estudiada. La xenogamia obligatoria de A. purpurata es un facilitador para el desarrollo de nuevos híbridos para el mercado de plantas ornamentales. Palabras clave adicionales: sistema de reproducción; estructura floral; fluido estigmático; receptividad estigmática. Received for publication: 14-03-2020 Accepted for publication: 11-06-2020 INTRODUCTION 258 FRANÇA / BOTINI / KOCH / CAETANO / SILVA / KRAUSE / SILVA Rev. Colomb. Cienc. Hortic. purpurata (Vieill.) K. Schum. , A. vittata W. Bull, and A. zerumbet (Pers.) B.L. Burtt & R.M. Sm. (JBRJ, 2020). Studies based on molecular data have suggested that this genus is highly polyphyletic (Kress et al., 2001), and most authors consider it to be taxonomically difficult and complex (Smith, 1990; Kress et al., 2001). Apart from an important ecological role in the understory of tropical and subtropical forests, Alpinia species can be used as medicinal plants (e.g., A. officinarum Hance), as food (A. galanda (L.) Willd. ) or for landscaping purposes (A. zerumbet), mainly as cut flowers, as is A. purpurata (Althaus-Ottmann et al., 2011), which has varied color bracts and great ornamental value for decorative purposes in gardens and landscapes because of the durability and exuberance of its inflorescences with year-round flowering. Reproductive biological studies are essential for the cultivation, conservation, and genetic development of plants (Baskorowati et al., 2010). Knowledge on the reproductive barriers of Alpinia helps the development of research on this promising ornamental plant. The main goal of the present study was to examine the floral morphology and morphometry of four A. purpurata cultivars and to analyze their reproductive system, focusing on pollen and stigma viability. MATERIALS AND METHODS