Bicarpellate gynoecium in two species of Senna (Fabaceae, Caesalpinioidae, Cassiinae)

ABSTRACT

Flowers of species of Senna are very morphologically diverse, however, nothing has been reported regarding variation in the presence of a unicarpellate gynoecium, which is characteristic of the group. This study reports the occurrence of a bicarpellate gynoecium in two enantiostylous species of Senna in an area of dry forest (Caatinga) in NE Brazil. Observations of floral morphology and estimates of the proportions of floral morphs in the populations were performed. The species produce three floral types: left (L), right (R) and bicarpellate (B). The proportion of these floral morphs were similar in the populations of Senna macranthera var. micans, but the number of B flowers in populations of S. trachypus was lower than that recorded for L and R flowers. The occurrence of this morphological variation may be related to enhancing pollen capture in both species; in S. trachypus, this variation may also be related to reducing florivory by caterpillars that were observed eating sexual elements of the flowers.

Keywords
Leguminosae flower; pluricarpellate Fabaceae; reproductive fitness; Senna macranthera var. micans; Senna trachypus

Enantiostylous flowers (Carvalho & Oliveira 2003; Laporta 2005) with a unicarpellate gynoecium (Queiroz 2009) are very common among species of the genus Senna Mill. (Caesalpinioidae - Cassiinae). Patterns of enantiostyly vary, and seem to enhance reproductive ability (Almeida et al. 2013). Although enantiostylous flowers with a pluricarpellate gynoecium are not common within Fabaceae, their occurrence is interpreted as a strategy to maximize pollen capture by the stigma, thereby increasing reproductive success (Prenner 2004; Endress & Doyle 2009; Paulino et al. 2013).

Pluricarpellate gynoecia have been recorded in Bauhinia L., Ceratonia L., Caesalpinia L. and Cassia L. (Tucker 1988; 1992; Stergios & Aymard 2008), but not in Senna. This study reports the occurrence of bicarpellate gynoecia in two enantiostylous species of Senna in an area of dry forest (Caatinga) in NE Brazil.

Flowers of Senna macranthera var. micans (Nees) H.S. Irwin & Barneby (five individuals, two populations) and of S. trachypus (Mart. ex Benth.) H.S. Irwin & Barneby (eleven individuals, one population) were collected in the municipalities of Mauriti and Brejo Santo (Ceará State, NE Brazil) and preserved in 70% alcohol. The number of left, right and bicarpellate flowers was recorded for each species, and their proportions compared using the Chi-square test (software BioEstat 5.0, Ayres et al. 2007). Samples of both species were deposited in the HVASF (Vale do São Francisco Herbarium; 21488 for S. macranthera var. micans and 16792 for S. trachypus).

The three floral morphs of S. macranthera var. micans occurred in similar proportions (Tab. 1; Fig. 1A-C), as did those of right and left flowers of Senna trachypus; the proportion of bicarpellate flowers in this latter species was much smaller (Tab. 1; Fig. 1D-F). Similarities in the proportions of floral morphs had already been reported for Chamaecrista flexuosa (L.) Greene, which is considered an atypical enantiostylous species (Almeida et al. 2013).

Figure 1
Flowers of Senna macranthera var. micans and S. tracchypus bearing bicarpellate gynoecium in an area of Caatinga, NE Brazil. A-C: Left, right and bicarpellate flowers of Senna macranthera var. micans, respectively; D-F: Left, right and bicarpellate flowers of S. trachypus, respectively.

Table 1
Number of right, left and bicarpellate flowers of Senna trachypus and S. macranthera var. micans in an area of Caatinga, NE Brazil. Different letters in the same line indicate significantly different values (Qui-square test).

Species	Floral morph
	Left	Right	Bicarpellate
Senna trachypus (n=11)	72a	58a	15b
S. macranthera var. micans (n=5)	32a	33a	28a

The presence of two fruits in S. macranthera var. micans reinforces the hypothesis that variation in plant reproduction may be the result of selective pressures favoring reproductive success. These data corroborate the study of Paulino et al. (2013), which recorded double fruit in the bicarpellate Swartzia dipetala Willd. ex Vogel.

The reproductive structures of Senna trachypus were consumed by caterpillars, and so the observed production of more pistils by this plant may be interpreted as a strategy to compensate for the losses caused by these herbivores. Additionally, the formation of more pistils may result in satiation of the caterpillars, thereby reducing the chances of attack on other flowers in the population (Coley & Kursar 1996). However, studies that measure the impact of florivory upon the reproductive biology of this species are needed to test this hypothesis.

Acknowledgements

The authors thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brazil) for grants awarded to N. M. de Almeida, and the Programa de Pós-graduação em Botânica, Universidade Federal Rural de Pernambuco and the Centro de Referência para Recuperação de Áreas Degradadas da Caatinga (CRAD-UNIVASF) for logistic support.

REFERENCES

• Almeida NM, Castro CC, Novo RR, Leite AV, Machado IC. 2013. Floral polymorphism in Chamaecrista flexuosa (Fabaceae-Caesalpinioideae): a possible case of atypical enantiostyly? Annals of Botany 112: 1117-1123
• Ayres M, Ayres Jr M, Ayres DL, Santos ASS. 2007. BioEstat: Aplicações estatísticas nas áreas das ciências biomédicas. Belém, Sociedade Civil Mamirauá.
• Carvalho DA, Oliveira PE. 2003. Biologia reprodutiva e polinização de Senna sylvestris (Vell.) H.S. Irwin & Barneby (Leguminosae, Caesalpinioideae). Revista Brasileira de Botânica 26: 319-328.
• Coley PD, Kursar TA. 1996. Antiherbivore defenses of young tropical leaves: Physiological constraints and ecological trade-offs In: Mulkey SS, Chazon RL, Smith AP. (eds.) Tropical forest ecophysiology. London, Chapman and Hall. p. 305-336.
• Endress PK, Doyle JA. 2009. Reconstructing the ancestral angiosperm flower and its initial specializations. American Journal of Botany 96: 22-66.
• Laporta C. 2005. Floral biology and reproductive system of enantiostylous Senna corymbosa (Caesalpiniaceae). Revista de Biologia Tropical 53: 49-61.
• Paulino JV, Mansano VF, Teixeira SP. 2013. Elucidating the unusual floral features of Swartzia dipetala (Fabaceae). Botanical Journal of the Linnean Society 173: 303-320.
• Prenner G. 2004. Floral development in Polygala myrtifolia (Polygalaceae) and its similarities with Leguminosae. Plant Systematics and Evolution 249: 67-76.
• Queiroz LP. 2009. Leguminosas da Caatinga. Feira de Santana, Universidade Estadual de Feira de Santana.
• Stergios BD, Aymard GA. 2008. A Striking New Species of Aldina Fabaceae-Swartzieae-Aldininae) from the Venezuelan Guayana Highlands. Harvard Papers in Botany 13: 29-33.
• Tucker SC. 1988. Dioecy in Bauhinia Resulting from Organ Suppression. American Journal of Botany 75: 1584-1597.
• Tucker SC. 1992. The Developmental Basis for Sexual Expression in Ceratonia siliqua (Leguminosae: Caesalpinioideae: Cassieae). American Journal of Botany79: 318-327

Publication Dates

• Publication in this collection
  Apr-Jun 2016

History

• Received
  30 Mar 2016
• Accepted
  28 Apr 2016