Passiflora L. (Passifloraceae) is the largest genus of lianas in the Neotropics (Acevedo-Rodríguez et al. 2015; Feuillet and Acevedo-Rodríguez 2020), comprising about 600 species, subdivided into five (Feuillet and MacDougal 2003; Krosnick et al. 2009, 2013) or six subgenera (Mezzonato-Pires et al. 2025). Among them, the monophyly of P. subg. Astrophea (DC.) Mast., P. subg. Decaloba (DC.) Rchb., P. subg. Passiflora L., and P. subg. Tetrapathea (DC.) P.S.Green is supported by molecular phylogenetic analyses (Muschner et al. 2003, 2012; Krosnick et al. 2013).

Passiflora subg. Astrophea comprises about 60 species distributed in South and Central America (Ulmer and MacDougal 2004), with the plains of northern South America being the centre of diversity for the subgenus (Ulmer and MacDougal 2004). Brazil is the country with the highest number of recorded species (31 species), followed by Colombia (20 species), predominantly occurring in the Amazon rainforest (Mezzonato-Pires et al. 2018).

Several molecular phylogenies (Muschner et al. 2003, 2012; Krosnick et al. 2009, 2013) support the basal placement of P. subg. Astrophea in Passiflora, which presents the most atypical morphological traits within the genus, together with a distinct chromosome number (n = 12), which is considered the base number for Passiflora (Hansen et al. 2006). Moreover, P. subg. Astrophea is the most differentiated group within Passiflora, with a notable diversity of growth forms, ranging from shrubs and small trees to lianas, distinguishing it from other Passiflora subgenera in which tendril-climbers are prevalent (Feuillet and MacDougal 2003; Muschner et al. 2012; Mezzonato-Pires et al. 2017). Some authors correlate the basal position of this clade with the diversity of growth forms and distinctive morphology in P. subg. Astrophea (Feuillet and MacDougal 2003; Muschner et al. 2003, 2012; Krosnick et al. 2013).

In addition, species of P. subg. Astrophea have the following diagnostic characters: entire leaf blades (most Passiflora species have deeply lobed leaves), with two small glands at the distal portion of the petiole or at the base of leaf blade base; tendrils absent or modified into spines; small bracts subtending the flower or inflorescence; solitary flowers, in pairs or in inflorescences, white with a yellow corona or pink to purplish, or red to orange hypanthium; erect and membranaceous operculum; unilocular berry-type fruits (Feuillet and MacDougal 2007; Mezzonato-Pires et al. 2018, 2020).

According to phylogenetic analyses, P. subg. Astrophea diverged much earlier from the other lineages, what would explain the more distinct characteristics observed in this subgenus when compared to the others (Muschner et al. 2012). In several phylogenetic analyses, the sister groups of Passiflora (Adenia Forssk., Paropsia Noronha ex Thouars) indicate that the ancestral state would be represented by erect woody plants, with shrubby or tree-like species (Muschner et al. 2003, 2012; Hansen et al. 2006; Hearn 2009; Krosnick et al. 2013), which indicates that this is an ancestral condition in Passiflora, present in P. subg. Astrophea.

Recent taxonomic and morphological studies have contributed to expanding the knowledge about P. subg. Astrophea, which is still an understudied group within the genus. Mezzonato-Pires et al. (2015, 2017) conducted palynological and seed analysis within the subgenus, contributing to the identification of species with delimitation problems as well as to the revision of the subgenus. Mezzonato-Pires et al. (2020) also published a taxonomic review of P. subg. Astrophea in Brazil, in which 31 species and 2 varieties were recognized and included in the sections P. sect. Botryastrophea (Harms) Killip, P. sect. Capreolata J.M.MacDougal & Feuillet, P. sect. Leptopoda Killip ex Feuillet & Cremers, and P. sect. Pseudoastrophea (Harms) Killip.

Some studies in the literature address the morphology and leaf anatomy of Passiflora, such as Farias et al. (2016), who studied the leaf anatomy of some species belonging to P. subg. Decaloba; Pérez and d’Eeckenbrugge (2017), who performed a morphological characterization of some species of the genus Passiflora; Chitwood and Otoni (2017) analysed the morphometric characteristics of the leaves of different species of the genus Passiflora in order to identify the different patterns of leaf vascularization; Mezzonato-Pires et al. (2015, 2017) conducted studies on the importance of pollen and seed morphology in the taxonomy of P. subg. Astrophea.

Despite advances in morphological, palynological, and phylogenetic studies, the anatomy of stems in Passiflora remains relatively unexplored in a comparative and comprehensive manner. Available studies are sporadic and limited to a few species, which makes it difficult to access evolutionary patterns of growth forms in the genus, especially in P. subg. Astrophea. The following studies are mainly descriptive, searching for diagnose features: Beraldo and Kato (2010) conducted a brief characterization of the morphoanatomy of the leaf and stem of P. edulis Sims; Zerpa and Gómez (2014) compared the characteristics of the stems of three Passiflora species (P. edulis, P. quadrangularis L., and P. ligularis Juss.); Chinniah and Thiagarajan (2015) studied the stem and leaf of P. incarnata L.; Pereira-Sühsner et al. (2016) who characterized the stem and leaves of P. caerulea L.; Rajput and Baijnath (2016) analysed the stem anatomy of five Passiflora species aiming to characterize the anatomical variation between species. More recently, Pereira-Sühsner (2023) conducted an extensive study of species from Paraguay in the subgenera P. subg. Decaloba and P. subg. Passiflora, covering genetic resources, anatomy, morphology, and cytogenetics, contributing to the taxonomy, genetic improvement programs, and conservation of the genus Passiflora.

Considering the shift from woody forms to herbaceous vines observed in most derived lineages of Passiflora, we present a descriptive anatomical comparison of the stems of 12 species of four out of five recognised subgenera, with the exception of P. subg. Tetrapathea, a small group restricted to Oceania. This study aims to test the existence of structural characteristics to differentiate stems of P. subg. Astrophea from stems in the other, more derived subgenera, P. subg. Passiflora, P. subg. Decaloba, and P. subg. Deidamioides.

Passifloraceae is a Neotropical family in which the climbing habit predominates, and Passiflora is among the ten genera richest in climbing species (Sperotto et al. 2023). Here, we discuss adaptive aspects of the woody habit, predominant in P. subg. Astrophea, and of the herbaceous climbing habit, common in the other Passiflora subgenera, identifying the modifications that took place in this woody to herbaceous transition within the genus.

The secondary xylem of vines and lianas frequently shows numerous structural modifications making the stem more flexible and voluble, assisting anchorage to the support and growth toward the canopy while avoiding severe structural damage during this process caused by the twisting of the stem associated with the climbing process (Ewers et al. 1991; Rowe and Speck 2005; Angyalossy et al. 2015; Isnard and Feild 2015; Cunha Neto 2023). Among these modifications typical of climbers are large-diameter vessels combined with narrow-diameter vessels (called vessel dimorphism), an increase in axial and radial parenchyma concomitant to a reduction in fibres, and vascular variations associated with non-cylindrical stems geometries (Angyalossy et al. 2015; Leme et al. 2021; Cunha Neto 2023). These traits compose the “lianescent vascular syndrome”, associated with higher hydraulic conductivity and greater stem flexibility at the cost of reduced investment in supporting tissues (Angyalossy et al. 2015). In this study, we observed some of these traits, such as different stem geometries, large and narrow vessels, and a high percentage of parenchyma in the stem, mainly in the form of a well-developed pith, although vascular variants were not detected in the branches studied.

Cross-sections of stems in the group studied show a wide diversity of geometries, ranging from circular to deeply winged, in P. subg. Decaloba and P. subg. Passiflora, while species of P. subg. Astrophea and P. subg. Deidamioides have only an elliptic-circular outline. Pereira-Sühsner (2023) also observed a variety of geometries in her study of the same subgenera (P. subg. Passiflora and P. subg. Decaloba), and it is worth noting that she analysed younger stems cross sections, with very little secondary growth, meaning that these different geometries are prior to the arise of secondary growth. In self-supporting trees and shrubs, the circular cross-section of stems develops from the typical activity of a vascular cambium that produces secondary phloem outward and secondary xylem inward, in the same proportion in all directions (Carlquist 1991; Spicer and Groover 2010). In climbing plants, the deeply modified geometries of the stem in cross-section arise, in many cases, from vascular variants, either from the primary or secondary vascular system (Isnard and Silk 2009; Luna-Márquez et al. 2021; Cunha Neto 2023; Kaçamak et al. 2025), or at least from some differences in cambial activity which produces unequal deposition of xylem and phloem (Rajput and Baijnath 2016) or even by irregular division of cells outside the vascular system, causing profound changes in the cross-sectional geometry of the stem (Soffiatti and Rowe 2020). In any case, in climbers, these geometrical changes will contribute to stem flexibility and survival from injuries, optimising the growth of the species throughout the climbing process (Carlquist 1991; Ewers et al. 1991; Speck and Rowe 2003; Isnard and Silk 2009; Soffiatti and Rowe 2020). In the Passiflora species studied, these geometries appeared before the emergence of any cambial variants and could be a product of the primary vascular system configuration or different patterns of cortical cell divisions, influenced by mechanical constraints (Bassel et al. 2014).

Yang et al. (2025) also reported different geometries and cambial variants in adult stems of several species of Passiflora but notably, the presence of cambial variants was not always associated with non-circular geometries and not all species had cambial variants (Yang et al. 2025). Rajput and Baijnath (2016) also recorded some differences in adult stem geometries, such as lobbed stem for P. foetida L., and a cambial variant type “shallow phloem wedges” in adult stems of P. edulis and P. vesicaria L. (all P. subg. Passiflora). The authors attributed both the differentiated geometry and the cambial variation observed to the unequal activity of vascular cambium, which produces the vascular components of phloem and xylem in an irregular manner. In our study, cambial variants were not detected, either in woody or herbaceous plants. The literature shows that it can vary in Passifloraceae and it does not occur in all species (Ayensu and Stern 1964; Rajput and Baijnath 2016; Pereira-Sühsner 2023; Yang et al. 2025). Besides, as noted by Ayensu and Stern (1964), for some species of Passiflora, the cambial variants only appear at very old stages of development, which are not usually included in herbarium collections, which mainly consist of young branches.

Considering diameters of vessel elements, the Passiflora species studied presented relatively small diameters when compared to those reported for other Passiflora (Ayensu and Stern 1964; Rajput and Baijnath 2016; Yang et al. 2025), although not too different from the values obtained for P. coccinea Aubl. (Ewers et al. 1990). These differences observed in relation to the information in the literature are somewhat anticipated, as we used samples collected from branches, which typically have smaller vessel diameters than those of the main stems, as vessel diameter scales with stem size and tapers from base to top (Anfodillo et al. 2006; Olson and Rosell 2013; Olson et al. 2014). It is worth noting that, from species of P. subg. Astrophea analysed, the ones growing as scandent shrubs or lianas had the smallest mean vessel diameters (P. longiracemosa, P. mansoi, and P. pentagona; Fig. 7A; Table 3), while species growing only as climbers had larger diameters, especially those of P. subg. Passiflora, and also P. rhamnifolia and P. haematostigma, both lianas from P. subg. Astrophea having also among the largest vessel diameters. Because lianas possess extremely long stems but often smaller stem cross-sectional area than self-supporting species, increased conduits diameter can compensate for smaller transverse stem area (Ewers et al. 1990; Leme et al. 2021; Kaçamak et al. 2025). The other two species representing P. subg. Decaloba, characterised by smaller vines, and P. subg. Deidamioides, which can be more woody, exhibited values that that were close to those of P. subg. Passiflora and P. subg. Astrophea, respectively. Functionally, lianas and vines have large-diameter vessels (Ewers and Fisher 1989; Carlquist 2013; Rosell and Olson 2014; van der Sande et al. 2019), a pattern that becomes clearer in comparative studies within the same genus where transitions between lianescent and erect self-supporting habits occur (Leme et al. 2021). By contrast, vessel element lengths recorded here do not differ from those reported for other Passiflora species (Rajput and Baijnath 2016; Yang et al. 2025).

The subgenera differed in the amount of mechanical (lignified) tissues, reflected both in the quantity of secondary xylem and in the abundance of fibres or sclerenchyma, allied to a thick and well-developed peridermis. Passiflora subg. Astrophea stands out by having a more developed vascular cylinder and a larger amount of mechanical tissue than the species belonging to P. subg. Decaloba, P. subg. Deidamioides, and P. subg. Passiflora. Studies comparing taxa with lianescent and erect habits report increased percentages of cross-sectional area occupied by lignified tissues in the erect taxa, which affords them greater support (Leme et al. 2021). Woodiness is ancestral in angiosperms (Doyle 2012), and changes towards herbaceousness occurred multiple times across different lineages (Klimeš et al. 2022; Luo et al. 2023). Herbaceous forms are considered phylogenetically derived, although many shifts back to woodiness is also observed (Carlquist 2013; Lens et al. 2013; Zizka et al. 2022). In Passiflora, the evolutionary trend towards specialized lighter architectures is evident, and the basal lineage, represented by P. subg. Astrophea, exhibits remarkable woody robustness, characterised by a more developed secondary xylem cylinder and an early onset of a thick periderm.

Our findings demonstrate that the woody Passiflora subg. Astrophea is characterized by well-developed vascular cylinders, the occurrence of great amount of lignified tissue (sclerenchyma) besides a well-developed peridermis. These characteristics confer on these species a more robust and rigid structure than that observed in herbaceous vines of Passiflora. The underlying anatomical traits that accompanies the shift from woody to herbaceous are the reduction in mechanical (lignified) tissues (wood and sclerenchyma) and delayed periderm formation in more recent lineages, which may have facilitated the widespread emergence of lightweight herbaceous climbers. The robustness of P. subg. Astrophea contrasts with the persistent epidermis and a more parenchymatic structure present in the tendril-climbers clades. Future studies with larger samples from all subgenera, including adult stems, will certainly contribute to more in-depth discussions about growth forms diversification patterns and inherent anatomical changes in Passiflora.