Nodal structure and leaf anatomy as an aid to Paullinieae (Sapindaceae) taxonomy and phylogenyAbstract
Paullinieae is a tribe of Sapindaceae that includes several genera of climbing plants widely distributed in Brazil. Despite their ecological and taxonomic relevance, the anatomical features of nodes, petioles, and midveins remain poorly explored as diagnostic characters. In this study, we analyzed the nodal structure and leaf anatomy of representative species of Cardiospermum, Paullinia, Serjania, and Urvillea, and compared them with Allophylus edulis, a former member of Paullinieae with an arboreal habit. Samples were collected from branches and processed for light microscopy. The species of Paullinieae investigated showed lobed stems with epidermis, cortex, and a sclerenchymatous sheath surrounding the vascular cylinder. Three species exhibited cambial variants, whereas Allophylus edulis lacked both sclerenchyma sheath and cambial variant. Nodal structure varied among taxa: Paullinia elegans had a unilacunar node, while most other species exhibited trilacunar nodes with three leaf traces. Petiole shape and vascular organization differed among genera, providing useful taxonomic characters. Midvein anatomy also showed diagnostic features, such as the presence and composition of the sclerenchymatous sheath and the number of vascular bundles. These findings demonstrate that nodal and leaf anatomical traits are valuable tools for distinguishing species within Paullinieae and contribute to a better understanding of their taxonomy and phylogeny.
Keywords:
Paullinieae; Sapindaceae; nodal anatomy; petiole; taxonomy
Resumo
Paullinieae é uma tribo de Sapindaceae que inclui diversos gêneros de plantas trepadeiras amplamente distribuídas no Brasil. Apesar de sua relevância ecológica e taxonômica, os caracteres anatômicos de nós, pecíolos e nervuras centrais ainda são pouco explorados como ferramentas diagnósticas. Neste estudo, analisamos a estrutura nodal e a anatomia foliar de espécies representativas de Cardiospermum, Paullinia, Serjania e Urvillea, comparando-as com Allophylus edulis, ex-integrante da tribo com hábito arbóreo. Amostras foram coletadas de ramos e processadas para microscopia de luz. As espécies de Paullinieae investigadas apresentaram caules lobados, com epiderme, córtex e bainha esclerenquimática envolvendo o cilindro vascular. Três espécies exibiram variantes cambiais, enquanto Allophylus edulis não apresentou bainha esclerenquimática nem variante cambial. A estrutura nodal variou entre os táxons: Paullinia elegans possui nó unilacunar, enquanto a maioria das demais espécies apresentou nós trilacunados com três traços foliares. A forma do pecíolo e a organização vascular também diferiram entre os gêneros, fornecendo caracteres úteis para a taxonomia. A anatomia da nervura central revelou ainda características diagnósticas, como a presença e a composição da bainha esclerenquimática e o número de feixes vasculares. Os resultados demonstram que caracteres anatômicos nodais e foliares são ferramentas valiosas para distinguir espécies dentro de Paullinieae e contribuem para um melhor entendimento de sua taxonomia e filogenia.
Palavras-chave:
Paullinieae; Sapindaceae; anatomia nodal; pecíolo; taxonomia
1. Introduction
Paullinieae is a tribe of the supertribe Paulliniodae (Sapindaceae) that includes genera such as Serjania Mill., Paullinia L., Urvillea Kunth, Cardiospermum L., Lophostigma Radlk., and Thinouia Triana & Planch. (Acevedo-Rodríguez et al., 2011). Species of Paullinieae are mostly lianas, vines, or climbing shrubs, rarely erect or hemicryptophytic shrubs. They are characterized by axillary shoots with distal pairs of coiled tendrils, minute to foliaceous stipules, and alternate leaves with a well-developed distal leaflet that may be pinnate, trifoliolate, biternate, bipinnate, or rarely unifoliolate. Paullinieae is represented in Brazil by 258 species, including 102 of Paullinia, 121 of Serjania, 7 of Cardiospermum, 11 of Thinouia, and 16 of Urvillea (JBRJ, 2022).
Nodal anatomy of angiosperms has long been studied, although the adaptive significance of nodal variation remains uncertain (Soltis et al., 2005). Nodal traits are useful for distinguishing species and genera (Sinnott and Bailey, 1914). While most angiosperm families exhibit relatively uniform nodal anatomy (Paliwal and Anand, 1978), some families and even individual species may display considerable variation. For instance, seedlings of some Serjania species show juvenile variation: Serjania communis Cambess. has unilacunar nodes (Lopes and Souza, 2015), whereas S. caracasana (Jacq.) Willd., S. fuscifolia Radlk., and S. laruotteana Cambess. have trilacunar nodes (Hernandes et al., 2023).
The vascular system of the node, petiole, and midvein, along with stipules, buds, and branches, still provides a wide field for investigation (Howard, 1979). Previous studies highlighted the taxonomic relevance of petiole vascularization in eudicots (Metcalfe and Chalk, 1957; Solís and Ferrucci, 2006). In Sapindaceae, features such as midvein structure and sclerenchymatous tissues have been recognized as diagnostic.
In search of taxonomically informative characters in Paullinieae, the present study investigates the nodal structure, petiole anatomy, and midvein of species belonging to Cardiospermum, Paullinia, Serjania, and Urvillea (Table 1). Allophylus edulis (A.St.-Hil. et al.) Hieron. ex Niederl., a former member of Paullinieae with arboreal habit, was also included for comparative purposes.
Species of Paullinieae and Allophylus edulis that were selected for study of nodal structure and leaf anatomy.
2. Material and Methods
2.1. Plant material
Samples were collected from remnants of Semideciduous Seasonal Forest in northwestern Paraná, Brazil. Branch fragments were obtained between the second and eighth nodes, including nodal regions, mid-petiole segments, and midveins of the leaf blade. We analyzed two species each of Cardiospermum, Paullinia, Serjania, and Urvillea, in addition to Allophylus edulis (Table 1). Voucher specimens were deposited in the Herbarium of the State University of Maringá (HUEM).
2.2. Sample preparation and light microscopy
Plant material was fixed in FAA 50 (formaldehyde, acetic acid, and ethanol), then dehydrated through a graded ethanol series (80%, 90%, and 100%). Samples were embedded in Leica Historesin following the manufacturer’s protocol. Sections 6-8 µm thick were obtained using a rotary microtome and stained with Toluidine Blue in phosphate buffer (pH 6.8) (O’Brien et al., 1964).
Microscopic images were captured with a Leica ICC50 microscope equipped with a digital camera, using Leica Application Suite software (v.1.8). Photomicrographs were prepared according to the magnification scale.
3. Results
3.1. Nodal structure
All species investigated showed lobed stems with a uniseriate epidermis covered by a cuticle of variable thickness. The cortex consisted of collenchyma, either continuous or interrupted by parenchyma, followed by parenchymatous tissue (Figure 1; Table 2). The vascular cylinder was surrounded by a multiseriate sclerenchymatous sheath and composed of primary and secondary phloem and xylem with a vascular cambium. The pith was parenchymatous. In contrast, Allophylus edulis lacked both the sclerenchymatous sheath and cambial variants.
Stems of the selected species: (a) Serjania laruotteana; (b) Serjania fuscifolia; (c) Paullinia elegans; (d) Paullinia meliifolia; (e) Urvillea laevis; (f) Urvillea ulmacea; (g) Cardiospermum halicacabum; (h) Cardiospermum grandiflorum; (i) Allophylus edulis. vc – vascular cylinder; ss – sclerenchymatous sheath; cc – central cylinder. Scale bar = 400 µm.
Anatomical characters of the node and leaf (petioles and midveins) of Paullinieae that have taxonomic value at the specific level.
Most Paullinieae species displayed secondary growth through the formation of a bifacial vascular cambium, typically producing a simple cylinder of primary and secondary xylem and phloem around a parenchymatous pith (Table 2). In Serjania laruotteana, S. fuscifolia, and Paullinia elegans, a cambial variant was observed (Figures 1ac; Table 2), a condition common in lianas of Sapindaceae.
Regarding nodal type, Paullinia elegans exhibited a unilacunar node with one leaf trace (Figures 2ad), while all other Paullinieae species had trilacunar nodes with three leaf traces (Figure 1; Table 2). Allophylus edulis also exhibited a trilacunar node with three leaf traces, similar to most Paullinieae species.
Stem anatomy of (a-d) Paullinia elegans, showing vascular cylinders, stipules, and leaf traces. tf – leaf trace; cs – secondary cylinder; vb – vascular branches; svb – stipule vascular branches. Scale bar = 400 µm.
3.2. Petiole structure
Petioles of Serjania species were pentagonal in cross section, with two tapered adaxial expansions in S. fuscifolia (Figure 3b) and two rounded adaxial lobes in S. laruotteana (Figure 3a). The epidermis was uniseriate with a thick cuticle and sparse nonglandular trichomes. The cortex consisted of collenchyma beneath the epidermis, followed by parenchyma, and the vascular cylinder was enclosed by a sclerenchymatous sheath. Vascular bundles were arranged in a ring, seven in S. laruotteana and ten in S. fuscifolia.
Petioles of the selected species: (a) Serjania laruotteana; (b) Serjania fuscifolia; (c) Paullinia elegans; (d) Paullinia meliifolia; (e) Urvillea laevis; (f) Urvillea ulmacea; (g) Cardiospermum halicacabum; (h) Cardiospermum grandiflorum; (i) Allophylus edulis. fv – vascular bundle; be – sclerenchymatous sheath; pe – periderm; bei – interrupted sclerenchymatic sheath; sbe – without sclerenchymatous sheath; cvc – continuous vascular cylinder. Scale bar = 400 µm.
Petiole shape varied substantially among the remaining Paullinieae species.: pentagonal (P. elegans and Cardiospermum spp.), quadrangular (P. meliifolia), heart-shaped (Urvillea laevis), and rounded (U. ulmacea). Allophylus edulis had a rounded petiole with a flat adaxial surface. Vascular cylinder organization and the number of vascular bundles varied among taxa (Table 2).
3.3. Midvein structure
In all species, the midvein was prominent on both leaf surfaces, rounded abaxially and tapered adaxially (Figure 4). The epidermis was uniseriate, with collenchyma and parenchyma in the cortex. In some taxa, palisade and spongy parenchyma extended into the adaxial region of the midvein. Variation in the presence and histological nature of the sclerenchymatous sheath, as well as in the number and size of vascular bundles, provided taxonomically relevant characters (Figure 4; Table 2).
Midribs of the selected species: (a) Serjania laruotteana; (b) Serjania fuscifolia; (c) Paullinia elegans; (d) Paullinia meliifolia; (e) Urvillea laevis; (f) Urvillea ulmacea; (g) Cardiospermum halicacabum; (h) Cardiospermum grandiflorum; (i) Allophylus edulis. fv – vascular bundle; be – sclerenchymatous sheath; bc – colenchymatous sheath; bei – interrupted sclerenchymatic sheath. Scale bar = 400 µm.
4. Discussion
The nodal structure and vascular organization observed in Paullinieae provide important insights into the taxonomy and evolution of the group. The presence of cambial variants in Serjania laruotteana, S. fuscifolia, and Paullinia elegans supports previous studies reporting this condition as common in lianas of Sapindaceae (Metcalfe and Chalk, 1957; Cunha-Neto et al., 2023). Cambial variants are considered derived features in eudicots and may represent adaptive strategies for climbing growth forms (Carlquist, 2001; Marques et al., 2024). The compound vascular cylinder observed here is consistent with the terminology proposed by Tamaio and Angyalossy (2009), and highlights the taxonomic relevance of developmental pathways in vascular ontogeny.
Variation in nodal type was also observed. Most species exhibited trilacunar nodes with three leaf traces, whereas Paullinia elegans showed a unilacunar condition. This feature is consistent with previous studies suggesting that nodal anatomy, although variable, may provide useful characters for distinguishing taxa within Sapindaceae. (Esau, 1974; Soltis et al., 2005). The occurrence of trilacunar nodes in most Paullinieae supports the hypothesis that this state is ancestral in eudicots (Soltis et al., 2005), whereas unilacunar nodes may have arisen secondarily (Takhtajan, 1980, 1997).
Petiole anatomy further contributed useful characters. Differences in shape, sclerenchymatous sheath distribution, and vascular bundle number among genera indicate that petiole traits can help delimit species and genera within Paullinieae. These observations reinforce the role of vegetative anatomical features in complementing reproductive characters, which are traditionally emphasized in Sapindaceae taxonomy (Howard, 1979; Solís and Ferrucci, 2006).
Midvein anatomy also showed diagnostic value. Variations in the sclerenchymatous sheath and vascular bundle organization are consistent with patterns described for other Sapindaceae taxa (Metcalfe and Chalk, 1957; Pace et al., 2022). Such features can be applied in both taxonomic identification and phylogenetic interpretation.
Taken together, nodal, petiole, and midvein anatomy provide a set of stable characters that are informative for distinguishing Paullinieae species. These findings highlight the importance of integrating vegetative anatomy into systematic studies of Sapindaceae, offering a more comprehensive understanding of relationships within the tribe.
Acknowledgements
The authors thank the staff of the Herbarium of the State University of Maringá (HUEM) for support in specimen processing and identification. We are also grateful to colleagues from the Research Center/NUPELIA for providing technical assistance during field and laboratory work.
Data Availability Statement
All data supporting the findings of this study are included in this article.
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