Flora 242 (2018) 89–101
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Research Article
Seed development and germination of Strombocactus species (Cactaceae): A comparative morphological and anatomical study
T
Aldebaran Camacho-Velázqueza, Salvador Ariasb, Florencia García-Campusanoc, ⁎ Emiliano Sánchez-Martínezd, Sonia Vázquez-Santanaa, a
Laboratorio de Desarrollo en Plantas, Departamento de Biología Comparada, Facultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad de México, 04510, México b Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, Ciudad de México, 04510, México c Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias, CENID-COMEF, Progreso 5, Coyoacán, Ciudad de México, 04010, México d Jardín Botánico Regional de Cadereyta Ing, Manuel González de Cosío, CONCYTEQ, 76500 Cadereyta de Montes, Querétaro, México
A R T I C LE I N FO
A B S T R A C T
Edited by Alessio Papini
Seed development, germination and seedling establishment were compared between Strombocactus species, S. corregidorae, S. disciformis ssp. disciformis and S. disciformis ssp. esperanzae, emphasizing the origin and role of the seed appendage, which is present only in both S. disciformis subspecies. The development and morpho-anatomy of both seeds and seedlings were evaluated using standard light microscopy and scanning electron microscopy. Embryo and seed development proceeded similarly in all three taxa. However, we unequivocally demonstrate the funicular origin and aerenchymatic structure of the seed appendage, the aril in S. disciformis ssp. disciformis and S. disciformis ssp. esperanzae, which differentiates soon after fertilization. Other distinguishing features include the delayed growth of the testa in the micropyle in S. corregidorae, and testa micromorphology in mature seeds. Seed potential and production were similar between species, although seed efficiency was higher in both subspecies of S. disciformis, which may reflect pollination problems in S. corregidorae natural populations. Seed flotation occurs in the three taxa, due to the presence of air spaces in the lumen of the seed coat cells and the aril aerenchymatic, when present. Neither physiological nor structural dormancy were detected in Strombocactus, and a high percentage of seeds germinated in the different taxa, as no scarification or disinfection treatments were necessary. The absence of the aril in S. corregidorae shows that further genetic, phylogenetic and comparative ontogenic studies that include the three taxa of Strombocactus and the nearest genera, such as Ariocarpus, Turbinicarpus and Epithelantha, are required in order to determine if this structure was gained or lost during evolution of Strombocactus. This research provides a greater understanding of the reproductive biology of this endangered genus endemic to Mexico, necessary for the establishment of future restoration and conservation programs.
Keywords: Aril Cactaceae Germination Seed appendage Seedling
1. Introduction Cactaceae species have evolved a diversity of life forms and various adaptations to aridity, which are reflected in their morphological and physiological characteristics, as well as in their reproductive versatility (Mandujano et al., 2010; Arias and Flores, 2013). The genus Strombocactus are shrubs that belong to the tribe Cacteae in the Cactoideae subfamily (Anderson, 2001; Arias and Sánchez-Martínez, 2010). Strombocactus was originally described as Mammillaria by De Candolle (1828), and was later proposed as a monotypic genus by Britton and Rose (1922) (Strombocactus disciformis (DC.) Britton & Rose). Currently, two species are recognized, S. disciformis (DC.) Britton & Rose (with the
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Corresponding author. E-mail address: svs@ciencias.unam.mx (S. Vázquez-Santana).
https://doi.org/10.1016/j.flora.2018.03.006 Received 15 July 2017; Received in revised form 8 March 2018; Accepted 8 March 2018 Available online 11 March 2018 0367-2530/ © 2018 Elsevier GmbH. All rights reserved.
subspecies S. disciformis ssp. disciformis and S. disciformis ssp. esperanzae Glass & S. Arias), and S. corregidorae S. Arias & E. Sánchez (Arias and Sánchez-Martínez, 2010). To date, cacti phylogenies that include the three taxa are still lacking. These species are associated with dry environments in the Chihuahuan-Queretaroan desert, have a very restricted distribution and are experiencing a decline in the number of mature individuals (Álvarez et al., 2004; Arias and Sánchez-Martínez, 2010). Although many cacti have efficient reproductive strategies and produce numerous seeds, commonly the survival and establishment of the seedlings is low, as in Neobuxbaumia tetetzo (Valiente-Banuet and Ezcurra, 1991). Strombocactus exhibits increased vulnerability, due to low seedling recruitment (personal observation), as well as to the